Riparian Zone, Stream, and Floodplain Issues: Review


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Riparian Zone, Stream, and Floodplain Issues: Review

  1. 1. Journal of Hydrology, 150 (1993) 277 299 0022-1694/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved 277 [3] Riparian zone, stream, and floodplain issues: a review L.J. Bren* Forestry Section, Universityof Melbourne, Creswiek, Vic.3363, Australia (Received 3 May 1993; revision accepted 27 May 1993) Abstract In the last two decades, the effects of forest management on streams, riparian zones, and floodplains have become of much interest. In general, there is agreement that such areas should be maintained in a state approximating naturalness, although it is recognised that definition of this state is usually difficult or impossible. A diversity of management effects has been recognised and, in some cases quantified. For upland catchments, issues particularly relate to direct disturbance of the zone, changes in the flow of woody debris into the stream, or disturbance to the environment by effects generated upstream or down- stream. For many areas, a particularly important commercial aspect is the definition of a 'stream', as this can impose many expensive and severe restrictions on management of the land. For large rivers, a common issue is the effect of river management on flooding forests. In each case, the issues are complex, information is difficult to collect, and there are fundamental difficulties in going from anecdotal observation to data. Currently, most information appears to be at a relatively local level, and there is a very inadequate knowledge base to give a more holistic overview, although the concept of 'cumulative effects', with the effects accumulated over both space and time, has much potential value. There are many opportunities for work in this field. Introduction As forest hydrologists, we are intimately concerned with the management of riparian zones, streams, and floodplains. This paper examines current issues in the management of such zones. The underlying concept of the paper is that these areas should be managed 'naturally', and consideration is given to what this might imply and whether the concept is quantifiable. Streams and their environs have always been attractive for or necessary to humans, and hence the demands on such areas can be complex and varied. Perusal of forest hydrology literature suggests that formalised study of such areas is relatively new to forestry discipline; thus, for instance, the proceedings of the influential * Telephone: 615345 2405.
  2. 2. 278 L.J. Bren / Journal of Hydrology 150 (1993) 277 299 forest hydrology conference of 1966 (Sopper and Lull, 1966) make virtually no reference to such matters. Rigorous but simple definition of such areas is probably impossible, and this leads to many disputes. We can define the stream as a more or less permanent body of water moving to a position of lower energy. The flood- plain can be described as relatively level areas of sediment deposited by such streams and which are periodically inundated by the stream. The riparian zone is an area in close proximity to a stream or river, the environment of which is distinctly influenced by that proximity. As forest hydrologists, our interests probably change with increasing stream size. For small streams, forestry practices can have a direct effect on stream flows, floodplains (if any), and riparian environments. As the stream size becomes larger, forestry becomes one of many competing land uses, and the river or stream may be a major economic resource of the state. Forested floodplains may be extremely extensive on large rivers (e.g. the Atchafalaya River in southern Louisiana), and there may be many organisations and issues involved in the management of the river. Polemicists argue that such definitions, although convenient, are 'reductionism', and we will examine whether the concepts of cumulative effects may, in the future, allow a more holistic approach. Underlying most management decisions is a desire to have a 'natural' stream; however, it is difficult to define 'natural'. This implies a definite course that the overall development of the system would have taken without intervention by humanity. As we try to define this more tightly, difficulties become apparent (e.g. Anderson, 1991; Fairweather, 1993). A common one is the long history of environmental modification by indigenous societies. In many cases (e.g. Java), there is no definable 'natural stream' because of the long history of population pressure. In other cases (e.g. Australia), some riparian environments may have been extensively modified before the advent of European races by traditional land management practices including burn- ing. Even if we do not accept scenarios of climate change, the propensity of long-term climates to wander off on long excursions on one side or the other of their mean value (e.g. Mandelbrot and Wallis, 1969) gives difficulties in definitions based on a few decades (or centuries) of data or experience. If we do accept that long-term climate change is occurring, what is the value of 'natural management' that perpetuates a relic environment? Critics or com- mentators on stream management usually refer their criticism to an under- lying concept of what the stream should be, and that conception may be arguable or ill-founded, or based on differing personal views of nature. In examining issues we must also recognise many types of interest in such areas. To the limnologist, these areas may be an interesting or inspiring source of aquatic life, whereas, to a recreationist, they provide a site for a pleasant
  3. 3. L.J. Bren / Journal of Hydrology 150 (1993) 277 299 279 picnic. To a logger, the riparian area may be a major source of good logs, whereas, to a land manager, the areas are to be managed to maximise society's overall wealth. In steep country such areas may provide the only possible route for roads and railways (e.g. Kellerhals, 1985). For downstream water users the area must provide clean, potable water, but it may also be a potential site for a major water storage to ensure continued supply or to give flood protection. For river town dwellers the larger rivers must have adequate water for boating or navigation, leading to constant inundation of lower riparian areas. Irrigation farmers can, with justifiable pride, point to the food pro- duced from their holdings by regulation of an otherwise wild river. Values can be classed as tangible and intangible, and there is no widely accepted means (and possibly never will be) of trading off one against the other (see, e.g. Dan Tarlock, 1991). As forest hydrologists, it is argued that our interests must transcend the local interests of disciplines or land use needs and aim at developing a comprehensive knowledge of our entire stream, floodplain, and riparian zone resource, and perhaps balancing the conflicting demands of society. This demands both a detailed (mechanistic) knowledge and a wider 'overview' knowledge integrating biological, hydrologic, hydraulic, and land management issues with knowledge of the economic, ecological, social and political ramifications. Little streams and bigger streams The question of when a small catchment can be said to have a stream is a serious question in much land management, and one on which scientific literature gives little guidance. Figure 1 gives a view of a research catchment (Clem Creek, 46 ha), showing a small first-order stream commencing as the outflow of a spring. Research on this stream (Bren and Turner, 1985; Bren and Papworth, 1991) showed that the flow generally commenced at point A as a rather invariant spring outflow. Occasionally, during wet periods, flow would commence about point B. Observations showed that rarely, under the natural conditions, would flow occur upstream of this, although there was a definite stream bed. After clearing, in the wettest periods, flow would commence at point C (reflecting a generally wetter catchment), although in most circum- stances the flow still commenced at A. A number of questions arise in dealing with these stream-catchment systems in forest management. (1) The judgement of whether it is a 'stream' or lesser form ('drainage line', 'seep', etc.) varies with the perception and orientation of the observer and the recent hydrologic history. Indeed, an examination of forest practices around
  4. 4. 280 L.J. Bren / Journal ofHydrology 150 (1993) 277 299 Fig. I. A topographic view of Clem Creek Research Catchment in north-eastern Victoria (Australia). The path of Clem Creek is partly obscured by the catchment ridge. Under natural conditions flow in the stream appeared near point A. Under wetter conditions flow in the stream could be found at points B. After clearing, in wet conditions, flow would start appearing in the channel near points C, reflecting the greater catchment wetness. Should the sections between points B and C be accorded the full protection of a permanent stream? the world suggests that in many countries a catchment of this size would not be recognised as possessing a stream for regulation purposes. (2) The characteristics of the stream are changed by catchment actions. By planning, the flowing stream can be entirely protected during logging, but as catchment wetness increases because of decreased transpiration, hitherto dry areas suddenly become connected to the stream (e.g. the dry channel between B and C), and these may flow through logged areas, across roads, etc. This can be very testing of supervisors, although use of planning aids that link topo- graphy and hydrology (e.g. 'TOPOG', as described by Moore et al. (1988)) can directly avoid such situations. (3) In mild hydrologic environments the usually dry sections of streams are important. These may be extremely common in many forest areas. They may have some rudimentary properties of stream channels but do not carry normal stream biota, reflecting their very ephemeral or occasional flow. Often, they
  5. 5. L.J. Bren / Journal of Hydrology 150 (1993) 277-299 281 provide accessible paths to upper slopes in steep country. Questions arise as to why they must be demarcated and protected (how might the use of such an area for forest harvesting lead to downstream degradation?). Conventionally, the answer is that flow might occur, leading to erosion, but this is hard to demonstrate under the normal hydrologic regime. Similarly, one can argue that there is a cumulative effect on downstream resources, but again this is hard to demonstrate, and almost certainly varies according to the weather Table 1 Examples of current wording used for stream definition in Australian states (because of topography there is little problem in South Australia; in each case there were qualifications and elaborations attached) State Stream definition Distance of edge of reserve from stream New South Wales A filter strip.., where the catchment area Generally 20 m; lesser exceeds 100 ha; where erosion hazard is protection for high... 40 ha ephemeral streams In plantation areas: Streams: Streams: if flow occurs for at least 2 months Large (> 20 m width after normal summer rains at top of banks): 30 m If not a stream: Medium (10-20m Gullies: cannot be traversed easily by wheeled width): 30 m vehicles Small (< 20 m width): 10m Gullies and waterways: 5m In native forest: as designated Guidelines only: first- to third-order - - 30m fourth-order -- 75 m fifth-order -- 200 m Class 1: 40 m Class 2:30 m Class 3:20 m Class 4:10 m (provi- sion for machinery movement) Queensland Western Australia Tasmania Victoria Waterways if traversable In native forest: as designated All streams, permanent and ephemeral.., to be protected by riparian zones; timber harvesting to be excluded from all riparian zones Class 1: rivers and lakes Class 2: catchment area exceeds 100ha Class 3: watercourses carrying running water most of the year between the points where their catchment is 50-100 ha Class 4: all other watercourses carrying water for part or all of the year for most years Permanent: flow all year Temporary: flow at wetter times only Variable, but typically 20 m along permanent streams, and 5 m filter strip along temporary streams
  6. 6. 282 L.J. Bren / Journal of Hydrology 150 (1993) 277-.299 after harvesting. Although one does not wish to see a cavalier attitude towards stream protection, it must be recognised that regulation of land use on the basis of numerous ephemeral streams can make management actions onerous or impossible. Examples of the wording and buffer protection used by most Australian states in such matters are shown in Table 1. These were provided by officers from each of the states. In discussion, these officers stated that conscious attempts are made to modify prescriptions to individual streams and drainage lines, and that although there may be wording difficulties the actual prescriptions adopted work well. Equally, however, in each state one can find independent observers critical of the stream protection given for specific cases. The issue would be a fruitful field of research provided some agreement on the values being traded off could be reached. However, there is a direct trade between priced and non-priced goods and services, with both involving an anthropocentric view. This, of course, is a major issue in environmental philosophy (e.g. Fairweather, 1993), and it is unlikely that there will be any agreement on such trade-offs for a long time. As forest hydrologists, it is hoped that our role will be in managing such conflicts rather than as active participants. The stream channel Central to all of this is the stream. This body of water moves in a container formed by the rock, earth, and organic matter of the surrounding forest. In most forestry cases the stream receives water from the surrounding land, but sometimes it may pass water back to the catchment by bank recharge (Cooper and Rorabough, 1963). The stream is usually characterised by a well-developed armoured stream-bed which may show sequences of pools, riffles, and drops, distinct substrate environments, a characteristic and often distinctive biota, and usually a pleasantly distinctive noise and appearance. Hawkins (1975) has shown that streams dissipate a small portion of their hydraulic energy as low-frequency, atonal noise, and this also provides a unique (and little explored) characteristic of riparian environments. In high- energy streams the noise may be associated with water spray, helping further to create a distinctive riparian environment. Most issues relate to interaction between the stream channel and forest management, and the cumulative effects of disruption of streams. Many such controversies either reflect or show our paucity of quantitative knowledge of the stream environments that we manage. Probably the most visually distinctive and difficult characteristic of streams
  7. 7. L.J. Bren / Journal of Hydrology 150 (1993) 277 299 283 is their temporal and spatial variability, and the absence of developed, natural geometric measures that assist in defining change, complexity, degree of similarity or uniqueness, or other valued attributes. If the stream channels of many natural streams in mountain country are viewed as containers, they could be reasonably said to have difficult and non-homogeneous properties. These channels seem to have no natural scale or simple characteristic measurement that can be used as direct indicators of their properties (a characteristic shared by clouds and coastlines -- see Feder (1988) for a discussion of this). The small-scale structure is difficult to quantify and can make nonsense of many textbook parameters. For instance, the definition of hydraulic radius usually requires the ratio of the wetted area to the wetted perimeter (Streeter, 1971). The former is definable, but the latter increases as the measurement step size decreases, leading to the concept of fractal dimen- sion developed by Mandelbrot (1982), but making our definition of hydraulic radius useless unless we care to nominate a basic distance of measurement below which we will not consider. Study of actual small streams suggests that the concept of a thalweg is convenient fiction. Furbish (1985) noted that the flow structure of a mountain stream consists largely of an alternating series of backwater spillage sites and that the thalweg wanders across the channel essentially independently of bed form. The coarseness of the bed and bank materials effectively damps inertial effects so that 'the distance at which local conditioning outweighs flow structure derived upstream is relatively short'. Fractal dimension provides an attractive analytical method for quantifying such variation but is difficult to apply in specific cases, and it would seem that the concept of self-similarity is, at best, a dubious approximation in stream channels. However, the development of such techniques does show that abstract thought can lead to new and better natural geometries which can be used to tame otherwise intractable forms, but these concepts can only be made useful in practical terms by solid field measurement work. They can then allow us to define new questions and to develop comparative methods in much the same way that use of dimensionless ratios (Reynolds number, Froude number, etc.) stimulated and changed hydraulics a century ago. It is hoped that forest hydrology studies will, in the future, develop an array of such measures that will allow better quantification of the properties of our forest stream channels. In most cases, the stream channel is viewed as a more or less fixed container. This ignores the role of large woody debris in this container, perhaps the role of trees in stabilising the sides, and the influence of unusually large flows in forming it. It is easy to show that the presence of organic debris has a number of direct physical effects. First, it provides a flow obstruction, creating swirls and turbulence. These may lead to local redistribution of material (e.g. Fig. 2,
  8. 8. 284 L.J. Bren / Journal of Itydrology 150 (1993) 277 299 Fig. 2 Change in the longitudinal profile of a small alluvial-bed stream associated with a piece of large woody debris (from Beschta and Platts 1986). taken from Beschta and Platts (1986)), but have been shown to provide microhabitats favourable to some biota. Second, organic material represents an input to the stream ecosystem, thereby providing food material for stream biota, and this may be a strong influence on water quality parameters. Third, the obstructions created trap and hold sediment (Mosley, 1981). Fourth, depending on the range of flows, organic debris may represent a major source of stream stability or instability. The stability or instability comes from the massive nature of trees in many upland environments, and the sheer volume of such material. In many cases, the most probable range of flows is inadequate to move such material. Instability is not commonly manifested, but in very high flow periods the water depth may be adequate to move the material, leading to dramatic changes in the riparian stream environment and damage to human property values. In such cases, limnologists will refer to the stream as having been 'blown out', reflecting the relative violence of the changes. For instance, the December 1964 storm in the US West Coast is known to have completely re-formed stream channels, stripped away streamside vegetation, and left very different stream channels compared with those before the storm (Stewart and LaMarche, 1967; Lyons and Beschta, 1983). In many cases, allegations will be made against land management agencies that this reflects a non-natural regime, although there is much anecdotal evidence that such non-continuous change is entirely natural. There is a body of anecdotal evidence and observation on the role of 'episodic events', but we do not have much information to allow individual cases to be viewed in a perspective
  9. 9. L.J. Bren / Journal of Hydrology 150 (1993) 277-299 285 of long-term change, nor even to make good estimates of how often such instabilities may occur (in this regard, see the comments of Klemes (1986) on our abilities to estimate higher flow). What is well known is the damage caused by the accumulation of debris on bridges, etc., and the demands to 'do something to stop it happening again'. This can lead to measures which, in hindsight, are viewed as ill considered (e.g. the removal of large pieces of stream debris from otherwise natural streams to avoid such change). One area of concern has been the drive to modify river channels for improved hydraulic conveyance or navigation (e.g. Walker and Thoms, 1991). Structures have been built, large pieces of woody debris have normally been removed, and a distinctly non-natural turbulence regime has been devel- oped. Resultant declines in fishing and biotic values have sometimes led to an appreciation of the loss, although it seems rare that management has attempted directly to reverse the situation (e.g. Cadwallader and Lawrence, 1990). The need is for us, as forest hydrologists, to derive a good theoretical knowledge of forest stream environments, the natural woody debris regime, and the effects of land and stream management, and to ensure that good practice is used in the management of the channel. Such work must be tied to the occurrence of extreme and episodic events, with due recognition given to these. In this, as in other matters, the question of how much we should bow to economic requirements is moot and should be continually discussed; on one hand, we must be realistic about meeting needs of our society, but, on the other hand, we must strive to protect natural values. The riparian zone The riparian zone is in intimate connection with the stream. A strict con- struction of the term might include only vegetation along a water course, but the term has come to include vegetative areas associated with some non- flowing waters, including lakeside wetlands. Most developed countries have extensive bodies of riparian law and water rights aimed at protection of various conceptions of 'public good' (e.g. Lamb and Lord, 1992). The zone is important for human recreation, water quality, wildlife habitat, and physical productivity of vegetation. The trees in this zone have direct access to the stream water or groundwater flowing either to or from the stream. They shade the stream. Sometimes they fall into it. Their debris provides organic matter for the stream; if they are 'natural' species (e.g. eucalypts in Australia) this is 'good', but if they are introduced species (willows) this is 'bad'. The roots may provide mechanical reinforcement of the banks and form 'nick- points' -- points of stability of the channel. With willows, the masses of roots
  10. 10. 286 L.J. Bren / Journal of Hydrology 150 (1993) 277 299 may give completely non-natural channels. Because they are close to both the stream and the groundwater the trees tend to be unusually well-favoured, having faster growth rates and better form than more remote trees, and this means that such areas have higher commercial value. The areas provide repositories for sediment (Lowrance et al., 1986), serve as nutrient sinks for surrounding watersheds (Cooper et al., 1987), and may improve the quality of water leaving the watershed (Karr and Schlosser, 1978). The riparian zone may coincide with the floodplain, but equally (particularly in mountain country) it may be separate. Intuitively, there is an assumption that the condition of the stream and the condition of the riparian zone are intimately linked, but this is hard to quantify. We can quantify some aspects of the stream-riparian zone relationship. First, Elmore and Beschta (1987) demonstrated that one of the benefits of improved stream rehabilitation is bank storage in the riparian zone, which increases the sustained low-flow capacity of the stream, and this is claimed for other areas (e.g. Savory, 1988). Second, we can show reasonably well that tree removal along streams may increase the insolation load reaching the stream, and thus lead to increased daytime stream temperatures and changes in oxygenation of the water. Beschta and Taylor (1988) found that stream temperatures remained elevated for several years after major storms when riparian zones and bars remain partially devegetated. Third, the importance of riparian vegetation in providing seston and allochthonous vegetation, and in stopping the passage of sediment pollutants into flowing water is self-evident. In each of these cases, however, it is hard to put an observed change in a small catchment or a small reach of stream into the context of a larger catchment over an extended time-scale, and this must be a direction of future work. The riparian zone may also contain sections of older stream channels (cut- off meanders, oxbow lakes, or 'billabongs'), and these may be important sources of food for organisms able to take advantage of them. These cut-off sections have, at best, an intermittent connection to the main channel at flood times. Many stream organisms have evolved mechanisms which appear to maximise their ability to take advantage of this periodic access to the resources of the floodplain system. Boon et al. (1990) suggested that these areas have an important but unappreciated role in the maintenance of biotic diversity of stream systems, and that good riparian management should both ensure their preservation and a natural flow regime that allows periodic connection of such areas to the main flow channel for the health of both. The issues in riparian zone management generally involve, on the one hand, desire to utilise some commercial values of the site, and, on the other, desire to protect the stream environment. It is not hard to show examples of riparian
  11. 11. L.J. Bren / Journal of Hydrology 150 (1993) 277 299 287 degradation caused by poor forestry practices (particularly roading), mining, cattle grazing, or drainage. It is hard to quantify the offsite effects of all of these except in unusually bad cases. The riparian degradation may include destruction of visual values, sediment input (from roads and machine activity areas), input of polluted water (particularly turbidity pollution), introduction of inappropriate vegetation, and inappropriate modification of the vegetation. Often dramatic cases of damaged riparian values (particularly as a result of roads) can be relatively easily cured by relocation, rehabilitation, and time. The more subtle and difficult-to-answer questions relate to management of the vegetation and protection from offsite influences. Thus, for instance, skilled forestry management may result in lower-land vegetation being harvested and the area restocked with virtually no detectable effect on measurable stream variables at the time. However, this process will, ultimately, change the prob- ability of accretion and the type of debris in the stream channel over time, perhaps leading to corresponding changes in the biota of the stream. It is difficult to determine whether this is an important or trivial change. First, it is subtle. Second, the reach affected by an individual coupe management is small, and there are many other such reaches (but possibly at any given time these are undergoing similar types of change). Third, the entirely natural environment could hardly have been static at all. The utilisation produces marketable goods and services which are important to the economy. The problem is as old as the concept of ecology and economics, and seems no closer to being resolved now than in the past. More difficult to deal with can be damage to riparian zones induced by off site or unappreciated changes. Thus Conner and Toliver (1990) have described how damage to riparian swamp cypress forests was caused by flood control, navigation, and agricultural activities many kilometres from the site of the forest. Factors that lead to changes in water levels similarly alter groundwater levels in the riparian zone. Thus backwater effects from down- stream weirs may completely alter riparian vegetation although the area is not flooded. Erosion from downstream change (river straightening, swamp clear- ing, etc.) has often been known to propagate upstream (sometimes for many kilometres), leading to simplified streams, degraded bed levels, and lowered groundwater levels, thereby changing riparian zone soil moisture conditions. In Australia, a common cause of degradation has been straightening of streams to hasten drainage of water from the floodplain of agricultural land. In one case, in Tasmania (E. O'Loughlin, personal communication, 1993), this led to an increase in the erosion potential of the stream of 200% compared with the undisturbed channel. The stream responded by headward erosion which passed into the adjoining forest area, damaging the riparian zone and leading to unfounded allegations of logging damage. Harvey and
  12. 12. 288 L.J. Bren / Journal ofttydrology 150 (1993) 277 299 Bencala (1993) suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography. Thus factors that reduce these, such as siltation, 'river manage- ment' to improve hydraulic conveyance, bed degradation, or loss of large organic matter components to the stream, may alter the bank recharge discharge relationship, leading to changed soil moisture conditions in the riparian zone and possibly to change in the vegetation. This may also help explain the observations of Elmore and Beschta (1987) noted above concerning improved bank recharge after stream rehabilitation. In summary, anticipation of possible effects and vigilance that such effects do not occur seems to be essential. The riparian zone as a stream buffer There is a growing appreciation of the utility of a small belt of riparian forest to protect the stream against agricultural pollution. Thus Phillips (1989) found that all riparian forests provided significant protection but that there was a wide variation in buffer effectiveness, and suggested a range of widths of 15-80m for different situations. Clinnick (1985) reviewed the use of buffer strips in Australian forestry, and showed that there was considerable variation from state to state; from Table 1 it can be seen that 20 m is about a median figure for well-defined streams, and perhaps 5 m for ephemeral streams. One can also find many variations in practice relating to personal definitions of such features as 'the edge of streams' or 'drainage lines'. There seems to be little rational design of buffer strips at present, and it is hoped that this will be remedied by good research in the near future. Although it is possible to conceive, for a given situation, of a minimum buffer width that gives satisfactory stream protection, buffer strips are regarded by biologists as having many other values for wildlife preservation and as wildlife corridors (e.g. Triquat et al., 1990). Attempts to reduce buffer widths below current levels would be strongly resisted on non-hydrologic grounds even if they did meet hydrologic criteria. Floodplain management Many of the forest hydrology issues in floodplain management are the same as those of riparian land management. There is one issue, however, that is proving very challenging, and that is the issue of flooding forests on major, regulated rivers. These depend on a periodic flooding for their survival, and generally on a periodic drying out for regeneration. Inadequate or excessive
  13. 13. L.J. Bren / Journal of Itydrology 150 (1993) 277-299 289 flooding will change the nature of the forests. Typically, the vegetation has oxygen transport mechanisms that allow survival under flooding. The follow- ing are examples of forests affected by major induced change in flooding frequency and duration; many other examples could be detailed: (1) Taxodium forests including T. distichum in the southern USA. These need dry periods to regenerate, but many of the forests have become perma- nent wetlands because of river works, incursions into bayous for oil drilling, etc. (Conner and Toliver, 1990). Thus, for instance, Finn and Rykiel (1979) found that construction of a small sill on the Suwannee River raised the water level in the major Okefenokee Swamp by ll0mm, with a major change in hydroperiod over 28% of the wetland area. (2) River red gum (E. camaldulensis) forests along the River Murray in Australia. These extensive forests grow in a sub-humid to semi-arid zone, and require flooding from the river to have adequate water for transpiration (Dexter et al., 1986). Regulation for irrigation, navigation, and water supply purposes has reduced the frequency and duration of flooding, and in some cases substantially changed the season of flooding, reducing growth and leading to long-term structural changes in the ecology of the areas (e.g. Bren, 1992). (3) Ash (Fraxinus angustifolia) and other forests along the Danube River. Modification of the river for drainage and navigation and, more latterly, development of hydroelectric potential are leading to major changes in the hydrology of these areas (Wenger et al., 1990). Jackson (1990) listed many other examples. In most cases, the importance of large waterways for meeting national needs has affected the floodplain ecology. In the examples listed above, the species involved have physiological adaptations which make them suited to the environment. Similarly the (short- term) economic value of the installations to be protected or built is far in excess of any economic valuation of the forest areas in any commercial sense. The changes may be attributable to very large structures or river works, hundreds of kilometres away, and often the linkage is substantially unappreciated. In each case, the situation is complex and the land manage- ment may involve very many interested parties with interlocking interests. Invariably, sound management will require: (1) Information on the 'natural' hydrologic environments of the particular ecology, including timing, depth, and duration of inundation, the quality of the water, and the mechanism by which water reaches and leaves particular areas. The hydraulic head of the water may be of critical importance in ensuring adequate water penetration into large blocks of forest. There may also be demands from neighbours that their private property should not be flooded at the same time.
  14. 14. 290 L.J. Bren / Journal of Hydrology 150 (1993) 277 299 (2) Information on alternative sources of water; e.g. can the trees survive on groundwater? Can water from this branch be used to water that area of forest? (3) Information on trade-offs of 'naturalism'; e.g. 'We know that the nor- mal flooding is in winter and spring but we can give you the same extent of flooding for the same duration in summer. Will that be OK?' (4) Mechanisms for implementing change or management options. The techniques of ensuring maintenance of flooding may be complex and very 600 - ~- 400 E2 E ~' 200 ~z Post-Hume and Dartmouth . . . . Natural River L .......... " - ~ I I I [ 1200 1600 Estimatedmonthly flow, GL 2o'oo (a) Pre Hume PostHume Jan ;-;___ o o o o o o Jan ,_........ , ............ o Feb " o o o Feb ;"; ..... ~ o o Mar ;o a ~ oo. Mar ;""'-; ........... April : o o o o April ; ~ ~ o May 2"--; . . . . ° ~ o o May : o o ~ o .............................................. June ,_........... :____~___i ....... '.................. June ............... July ............. ; ............................. ; .... July ....................................................................................................... ....................................................................... Aug ................ ,................. *_......... ~_.... Aug ........................................... ............................................................ Sept ......................... t_............. _*...... ~_ - Sept ........ , ..................... ~: ............. ,_-- Oct .............. Oct ; ............................ ; ................. ; Nov ................. ' ......... ; ....... ' ............ Nov ............................................................................................. Dec ;; .......... ; .................... Dec C ........... ; ......... ; ..................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.0 .'0 6'0 8.0 16o ; 2; ;0 6'0 8'016o Fraction of forest flooded (%) (b) Fig. 3 (a) Frequency distribution of monthly flows for 'natural' and the 'current regulation" case of the River Murray. (b) Box and whisker plots showing the statistical distributions or the percentage P of the forest flooded in each month for the pre-Hume (1895 1930) and post-Hume (1935 84) periods. In these representations the 'box' contains the distribution median ( + ) with its limits defined by the upper and lower quartites (l), the "whiskers' (dashed lines) show the effective range of the data, and the outliers (*) and exteme outliers (O) are shown individually. Reproduced with permission of CSIRO (Bren, 1987).
  15. 15. L.J. Bren /Journal of Hydrology 150 (1993) 277-299 291 difficult to implement, given limited information and resources. These include structural modification of levee banks and inverts of stream channels passing through the levees, earthen walls across the forest to allow water to be ponded, large channels to ensure hydraulic conveyance of water to specific points, groundwater recharge, pumped water, and dam release policy changes. Figure 3 illustrates some of these changes for a 30000ha river red gum forest along the River Murray. Figure 3(a) (from Bren, 1991a) shows fre- quency polygons of monthly River Murray flow for the 'natural' and the 'currently regulated' river. The latter case includes the effects of one major dam and minor dams. The dramatic change in the lowest flows experienced in Fig. 4 Changes in a natural plain-red gum forest boundary reflecting changes in flooding frequency. Each photograph shows an area of 1.2 x 2 km. The left photograph, taken in 1945, shows a clear, stable tree-grass plain boundary. The right photograph shows exactly the same area in 1985. The aggressive growth of young red gum and the transition to forest can be seen. The change apparently reflects siltation on the flood plain, reduced winter-spring flooding and increased summer flooding. Under the natural regime the grass plain was too wet in winter and too dry in summer for the trees to invade, but the changes dues to river regulation appear to have favoured the tree growth. Reproduced with permission of Blackwell Scientific Publications (Bren, 1992).
  16. 16. 292 L.J. Bren / Journal of Hydrology 150 (1993) 277 299 FFFF FFFFIFF /FIF!FF FImmI[IFFFFIFF FI I!FFFIIFF FIII!IFFFFIIIF FFIIII!IFFFFIIFF 1945 (Actual) FFFF FFFF FFFF FFFFFFF FFIIIFF FFFIIFI VFFFFFF FFFIFFF FFFFFFF FFFFIFFFFFFIF FIFIFFFIFFFFF FIFIFFFFFFFFF FFFIIIFFFFIIF FFFIFFFFFIIIF FFFFFFFFFIFIF FFFIIIFFFFFIIF FFFFFFFFFFFFF,I FIFFFFFIIFFFFII FFFFFFFFFFFFFIIF FFFFFFFFFFFIFFIF FFFIIFIFFF!NFFFI FFIIFFFFFIIIFFImI ~ FFFIIFFIFFIFFFmII FFIFIIFIFFIFIFFII FFI~LIFFFIIIFFFI I FFFIIFFFIFFFFFFFF FFFFIIFFIIFFFFFFF FFFFI FFFIIFIFFI IF F!FIFIFIFFIIIFIFF~ FIIFI~FFFIFIFFFF~IF FFFFFImIFFIIIIFFIIIF IFFFFFFFFFFFIFIFIIFF IFFmFFFFFFFFFFFFFFFF FIFIIIFIIF~II!IFFFFI FFFFFIFIFIIIFIFFFIFI IFFFFFIFFFF!FFFFFFFF FF!IFIIIIFII IIIFFFFI FFIFFFIFIFIFmlIFFIFIFI FFFFFFIFIFFFIFFFIFFFII FFIFFIII~FI IIFFFIII IFFFFINFIFIIIFFFFFFIFF IIIFFFIFIFIIIFFFFFFFFF FFFIIFFII~IFIIIIIIFFIFI FFIIIFFFFFIFFF IFFFFF~FF IFIIFIFFFFIFFFFFFFFFFFI FIIIIIIIImIIIIFIFFIIIFF FFFFIIFIIFIIIIFIFIIFI!I FFIFIFFIFFFIIFIFFIIFFFF FFFFFFFFFFFIIFIFFFIFFIF FFFFI~FF~IFIFIFIFF~FF FFFFIIIFFFIFFIIFIFIFFIFF FFFFFIIIFII~IIFFIIFFFIIF FIFFFFFFFFF~IIFFFFFIFFIF FIFFF~IFFIIFFIFIFI~IIF IFFFFII~FIFFFIIFFFFFIIIF IFIIIINIFFFFFFFFIIFIIFFI FIFFF!IIFVFFFFFFFFFFFIFF FFIIFIIFFIFFFFFIFIFIFIFF VFFFFIFFFFFFFFFIFIFFFFFF FFIIII~FFIF[~IFIIIIIIF IEFIFIIFIIVFIIFFIIFIII FFFNFIFI!IF~FFNIFII ~ IFIIIFFFFFIF~IFIFVFFIF FIIIIIFFFIFFFFIFIIFFFF FFFiFFFFFFFFFFFFFFIIIF FFFIIFIFFIIFFFFIFF~F IFFIIFFIIFFFFIFIIFFFF FFIIFI IFFFFFF IFFFF IFII~FFFFFFFFI~FIFFI~IFFFFFFFIIII FIIIIIFFFFFmFIFIFIF FIFIIFIIFIFIFFIF FFFIIFIFFIFFFF!I II~IFIIFFFFFIF~I FFIIFIFFFFFFIIIF FIE FIFFFFFFIIIFI~FFIFFFFFFFF FFF~IFFFFIFFFFIIFIIIFFIFIIFFFFF IIFIIFFFFFFFII FFIFIFFFFFFFFI FIIF FFFI FFFI 1985 2030 2075 (Ac~al) (Es~mated) (Es~mated) F Forest : Intermediate II Plain Fig. 5 Mapped progression of vegetation on the plains of Fig. 4 from 1945 to 1985, and distribution of the vegetation in 2030 and 2075, as predicted using a Monte-Carlo model. Each letter represents a grid cell of 200 x 200m. F means the cell contains only forest, P means only plain, and I ('Intermediate') means an intimate mixture of forest and plain, it can be seen that the large expanses of open plain will cease to exist, apparently because of river-regulation changes. Reproduced with permission of Blackwell Scientific Publications (Bren, 1992). summer months and the tendency of river managers to keep the flow at their preferred level of about 400 G1 month-1 can be seen. Not so obvious are the reductions in frequency of the less commonly occurring higher flows. Figure 3(b) (Bren, 1987) gives box plots showing the estimated range of forest flood- ing before and after the construction of the first (Hume) major dam. Clearly shown are the advent of smaller 'unseasonal' summer floods in January- March, and the reduction in size and duration, and increase in the variability of 'winter-spring flooding'. Bren and Gibbs (1986) showed that, in this environment, the plant communities are distributed more or less in accord with the flooding frequency and duration, and hence a change in this presum- ably changes the associated forest ecology. Figure 4 shows a possible effect of such change, in that a hitherto stable forest is invading a natural grass plain; a Markov chain model examining this (Bren, 1992) showed the more or less complete elimination of the latter in the near future (Fig. 5). In this environ- ment, a number of changes immediately can be shown: (1) under the natural flooding regime, the areas with the highest flooding
  17. 17. L.J. Bren / Journal of Hydrology 150 (1993) 277 299 293 frequency and duration are flooded only in winter. Under the managed river regime, substantial flooding of these areas can occur all year, but with a reduced winter-spring flooding frequency and an increased summer flooding frequency. (2) The forest flooding is both reduced and more variable. Many of the areas with a lower flood frequency will hardly be flooded at all, leading to changes in ecology and reduced site quality. (3) Although one can demonstrate historic change and extrapolate it into the future (as in Fig. 5), one can never be dogmatic about the exact cause and effect relationship between flooding and vegetation development. Forest water managers may also be faced with hard decisions on how much they care (or dare) to emulate nature. For instance, in this area of variable Australian streamflows, natural ibis breeding seasons were often interrupted by flood recession. The fledgling ibis would be abandoned to die (Chesterfield et al., 1984). Such emulation of nature by river managers withdrawing water from the forest by a major reduction in river levels during the breeding season would be an act of political courage because ibis breeding is perceived as 'natural'. Maintenance of water levels to allow the fledglings to mature forces a distinctly modified hydrologic regime onto the vegetation, leading to a corresponding decline in certain vegetation types. The actual hydraulics of flooding in channel-rich, large, flat plains is both complex and little explored in the literature. It becomes of importance in attempts to implement a water management scheme to counter river regula- tion changes. First, the elevation of the inverts of the water offtakes through the river bank and the conveyance of the channels control the river stage at which water enters the forest and the time at which backwater effects began to restrict outflow from the river. It is easy to show that these have been arbi- trarily modified many times and for many reasons, but there are rarely records of such changes. Second, the hydraulic conveyance capacity of the channels is linked to the accumulation of debris, vegetation growth, and actions which modify channels (road construction, bridge construction, etc). Third, because of the flatness of such areas, the resultant changes in water levels may propa- gate over many kilometres. Fourth, such areas commonly have complex branching networks of distributaries, with water being able to reach a given point by many different paths. It can be shown (in laboratory flows) that water passing through a branch is unstable in which branch it will favour, and that small changes in the turbulence will cause the water to switch branches (see Streeter, 1971). Somewhat similarly, in braided river channels it can be shown that complex feedback mechanisms exist that cause flow to change sponta- neously from favouring one side of a branch to another (Ferguson et al., 1992). Observations in channel-rich floodplains suggest that a somewhat similar switching mechanism operates; thus a tree falling into a channel
  18. 18. 294 L.J. Bren / Journal of Hydrology 150 (1993) 277 299 may cause the main flow to switch suddenly from favouring the left-hand to the right-hand branch. In principle, perhaps, one could set up large complex simulations on hydraulic models but the infinity of detail required is too great for any practical case. Fifth, in the Australian environment at least, it is thought that changes in burning frequency associated with European settle- ment led to the death of large areas of reed beds, which then made the forest much more freely draining, again changing the hydrology. The upshot of all of this is that the hydraulics of these large, flowing wetlands is complex and sensitive to many small changes. They can probably never be modelled as deterministic systems but, rather, are better viewed as an exercise in statistical mechanics. This has many implications for the way in which we think about management of these areas, and is a subject that will require much experimentation in the future. Ideally, the information to help answer such questions should be systematically collected over many years by the manage- ment authority. In many cases, the urgency of the political process is such that specific answers are needed over short time-spans that preclude long-term data collection, and when the urgency fades the will to collect information also fades. There are no simple answers to any of the questions raised. Modelling in such situations has some applicability, but adequate verification of models is almost impossible. A fundamental question is the description of the natural hydrologic variation, but often the river systems had been grossly changed before any data were ever collected; in some cases, it is arguable that there has never been a period in the last millennium when the river could have been called entirely 'natural'. In Australia, there has been a growing appreciation of how indigenous populations actually modified floodplain inlets and outlets to trap fish. It can be easily shown that such changes can be a powerful modify- ing agent for at least the distribution of small floods. The questions raised are amenable to resolution in the long term, and the development of optimal solutions gives fascinating insights into our allocations of resources and necessary trade-offs of values. Rehabilitation and enhancement of riparian, stream, and floodplain resources Rehabilitation (also called reclamation) involves returning areas to some- thing reminiscent of their pristine state. Let us consider the following examples: (1) the modification of grazing practices to allow development of meadow vegetation along streams emanating from overgrazed semi-arid land (e.g. Elmore and Beschta, 1987). (2) The rebuilding of wildland streams to develop habitats for anadromous
  19. 19. L.J. Bren / Journal of Hydrology 150 (1993) 277 299 295 fish, including the placing of logs to make pools, and other riparian habitats (De Bano and Schmidt, 1990). (3) Management to increase or restore depleted populations of beavers in North American streams, with the resultant backwater effects and riparian modification by the beaver dams. (4) Demolition of old or inappropriate dams to recover the riparian values and remove obstacles to fish. This seems to have been more discussed than undertaken for larger dams. An example is the Hetch-Hetchy Dam, built to provide San Francisco with water and power, which occupies a valley close to and similar to that at Yosemite (Wilkinson, 1991). About 1987, suggestions were made that the dam be demolished to recover some of the pristine river and scenic values foregone by its construction. The proposition of the demoli- tion has indicated some of the unknowns. First, is there a replacement water and power supply? Second, once the structure is gone, how long would an attractive riparian environment take to form on the sulphide-rich beds of deposited sediment? Third, would the deposition of sediment have irreversi- bly destroyed the valley form? It is likely that forest hydrologists will be faced with such issues in the near future. (5) Replanting of denuded stream banks in agricultural land to approxi- mate the natural vegetation condition. The common thread in each of these is the recognition that our streams are a fundamental resource to be guarded, nurtured, and restored to a more natural state. In each case, the practitioner is forced to make significant and difficult judgements with little direct information base. Riparian habitat enhancement has been defined by Platts and Rinne (1985) as 'returning the riparian/stream habitat to a more productive condition by natural or artificial means'. This has been undergoing a wave of popularity in parts of the USA (De Bano and Schmidt, 1990). These projects basically create near-perennial streamflow by providing a more consistent year-long supply of water, or supplying more water to an area. Typically, channel structures are installed which trap and store sediments and create artificial ponds. Traditionally, the technology has been used for restoring habitat in degraded areas, but there has been a tendency to use the same methods to improve the abundance of certain fish species in streams that are already arguably natural. Although not wishing to decry the value and excellence of the work, the ethical questions posed by making a natural stream 'more natural' or 'more productive' are interesting and worthy of philosophic debate and research. Information needs: local and holistic A basic yet often unmet need in forest management is simple characterisa-
  20. 20. 296 L.J. Bren / Journal of ttydrology 150 (1993) 277 299 tion of our stream, riparian zone, and floodplain environments in terms of the variables of length, area, and geographic extent, and examination of their changes over time. Further, there is a need for classification of distinct hydraulic environments and guarding of them, and development of techniques such as those of Grant (1988) for assessment of possible change. We cannot pretend that all such environments can be retained indefinitely, but any change made should rely on a thorough knowledge of values foregone. As forest hydrologists, we must further develop our knowledge of protection of these zones, whether it be by planning laws, legal sanctions, or better tech- niques in forestry (Lamb and Lord, 1992). Realistically, there will be continued pressure on these areas to meet all sorts of demands, and we must continue to evolve workable mechanisms that protect such areas but also help to meet the demands of our society. More difficult is the question of putting it all together to give a 'holistic' view (holism: tendency in nature to form wholes that are more than the sum of the parts by creative evolution -- Concise Oxford Dictionary). The concept of cumulative effects offers the chance of developing a holistic view provided adequate information on the linkages between separate ecosystem compo- nents can be obtained, but the information needs are formidable. A cumula- tive impact is 'the impact on the environment which results from the incremental impact of the action when added to other past, present, and reasonably foreseeable future actions... Cumulative impacts can result from individually minor but collectively significant actions taking place over a period of time' (Reid, 1992). Thus, for instance, Potter (1991) showed that the effects of many small improvements in riparian habitat and agricultural practices were apparent in measured hydrographs from a catchment of 221 miles2 in Wisconsin, although the individual projects were small. It must be recognised that no one person or organisation can ever have an 'overall view' or knowledge of where any decision taken may ultimately lead, and know- ledge is generally inadequate to define these linkages. Work such as that by Leibowitz et al. (1992) is showing the way towards developing usable quanti- tative approaches involving concepts of cumulative effects for riparian issues, but there are many difficulties. It is to be hoped that, at least for specific river basins or areas, a knowledge of the cumulative effects of the land management decisions on key environmental variables will be developed to help guide the way to more sustainable long-term management. In particular, this need will probably lead, ultimately, to a return to multiple catchment experimentation with measurement continued over a much longer time-span and with more complex experimental layouts to give information on assessment of cumula- tive effects. In some cases, it may lead to a reopening of long unmeasured multiple catchment experiments (e.g. Stednick, 1991) to determine the effects
  21. 21. L.J. Bren/JournalofHydrology 150 (1993) 277 299 297 of forestry treatments over much longer time-spans than traditionally envi- saged, and to attempt to view them in a much less restricted frame (spatially and temporally). Conclusions The science or knowledge of these areas is not well developed, but is growing fast. In general, the needs appear to be good fieldwork to allow quantification of otherwise vague concepts, development of new and better methods of measurement and of formalising information, and reconciliation of the needs of environmental management with economic and social needs. Because of humanity's liking for streams and riparian areas, it is likely that conflict about the optimal use of these areas will increase in the future. It is hoped that, by development of measurement techniques and collection of information, forest hydrology will be able to help resolve conflicts and protect environmental values. Acknowledgements The concepts in this paper owe much to discussion with Professor Bob Beschta of Oregon State University, Pat O'Shaughnessy of Melbourne Water, and Dr. Emmett O'Loughlin of the CRC on Catchment Hydrology. References Anderson, J.E., 1991. A conceptual framework for evaluating and quantifying naturalness. Conserv. Biol., 5:347 352. Beschta, R.L. and Platts, W.S., 1986.Morphological features of small streams: significance and function. Water Resour. Bull., 22(3): 369 379. Beschta, R.L. and Taylor, R.L., 1988. Stream temperature increases and land use in a forested Oregon watershed. Water Resour. Bull., 24(1): 19 26. Boon, P., Frankenberg, J., Hillman, T., Oliver, R. and Shiel, R., 1990. Billabongs. In: N. Mackay and D. Eastburn (Editors), The Murray. Murray-Darling Basin Commission, Canberra, pp. 183 200. Bren, L.J., 1987.Flooding in the Barmah Forest and its relation to flow in the Murray Edward River system. Aust. For. Res., 17:127 144. Bren, L.J., 1991a. Modelling the influence of River Murray management on the Barmah river red gum forests. Aust. For., 54:9 15. Bren, L.J., 1991b. The contribution of River Murray tributaries to the flooding of Barmah Forest. Aust. For, 54: 23~ 29. Bren, L.J., 1992.Tree invasion of an intermittent wetland in relation to changes in the flooding frequency of the River Murray, Australia. Aust. J. Ecol., 17: 395-408. Bren, L.J. and Gibbs, N.L., 1986. Relationships between flood frequency, vegetation, and topography in a river red gum forest. Aust. For. Res., 16:357 370.
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