Fluvial Processes and FloodingSummary of Important Concepts • Stream erosion is a very important force shaping the earth’s surface. A stream’s ability to erode is related to its velocity and its discharge. • Streams transport sediment particles. Streams that end at standing bodies of water (oceans or lakes) deposit this sediment to form deltas. Intermittent streams in arid mountain areas deposit coarse sediment to form alluvial fans. • Flooding is the main hazard associated with streams. Flooding is measured using hydrographs -- graphs which plot water level (or discharge) over a period of time associated with the flood. • Upland floods (upstream) occur in the higher areas of a drainage basin. These are sudden floods (“flash floods”) that move quickly through narrow valleys. • Lowland floods occur in the lower areas of a drainage basin. These are slower but longer- lasting floods that spread out over broad areas of the stream’s floodplain. • Urban development increases flooding because buildings, roads and parking lots decrease the infiltration of rainwater into the ground, and increase the speed with which rainwater runs off into channels.Water on Earth is Recycled:The Hydrologic Cycle•>97% of earth’s water is contained in the oceans and roughly 3% is fresh.•Of that three %, MOST of earth’s fresh water is tied up in ice (~2%), then groundwater (next chapter),lakes, ice and streams.•Weather patterns determine the amount and location of precipitation, and the amount and time over whichprecipitation occurs is not constant.•Evaluation of Precipitation –Area over which the rain falls –Duration of the rain –IntensityThe Hydrologic Cycle The Fate of Precipitation Most water evaporates off the ocean (large volume) and enters the atmosphere. Through precipitation of snow, rain and ice, water falls either directly back into the ocean or on the land. Water that falls onto land enters streams by infiltration through the ground or by runoff. Infiltration •The movement of water into rocks or soil through cracks and pore spaces. Runoff •Water that flows over the land.The Hydrologic CycleHow Water Enters a StreamA cup of land bounded by areas of high relief is called the DRAINAGE BASIN. Any precipitation that falls within the ‘cup of land’ flows to the stream as either runoff or by infiltration. Infiltration capacity of the soil is controlled by: –Slope of the land (steeper slopes cause water to flow as runoff rather than infiltrate). –Soil texture (we will see with groundwater, properties of soils may dictate how easily water will infiltrate—think about sand versus clay). –Nature of the vegetative cover (plants can remove water with roots and transpire the water back into
the atmosphere). EX. The United States is divided, so to speak by the Appalachian Mountains on the East and the Rockies in the Midwest. All water falling in between those two mountain ranges is in the Mississippi River Drainage basin.StreamsA body of running water that is confined in a channel and flows under the influence of gravity.Channel width may vary from a few cm’s to several km’s.The term “stream” in geology refers to any water flowing in a channel, and so includes “rivers”, “streams”,“creeks”, “brooks”, “washes”, “arroyos” and other related features. Throughout this lesson we will use theterm stream to include all these types of features.In general, streams begin at the headwaters at higher elevations, and discharge into other streams andlakes (relative base level) that will eventually reach the ultimate base level (sea level).Longitudinal Profile - Elevation changes of a stream from source to mouth.Gradient - the vertical drop of a channel over a horizontal distance.The headwaters are the upper part of the stream near its source in the mountains. Upstream regions. –Steep Gradient.Lower reaches of a stream are referred to as downstream regions. –Shallow Gradient.The mouth is the place where a stream channel terminates and enters the sea, a lake, etc.Base level is the theoretical limit to which the stream can erode. It is, in effect, the elevation of the streams mouth.The ultimate goal of a stream is to erode the land down to sea level!•Streams BEGIN at headwater regions and either DISCHARGE (flow into) into other streams, into lakes orinto the ocean.•The limit to which a stream can downcut the bottom of its channel (erode) is its BASE LEVEL. The ultimatebase level for fluvial processes is the ocean (weather all land to sea level). Many streams have a localizedbase level.Stream Gradation•Headwater streams are located at higher elevations with steep slopes or gradients (change inelevation/distance). These streams usually have more ‘work’ (breaking down the mountains) to do in terms ofweathering away land and carrying sediments downstream. These streams are termed ‘youthful’ becausethey are just beginning to carve the land’s surface (DEGRADING STREAMS). It is essentially a collectingsystem for water and sediment.•As we move further downstream, the land begins to level, slope gradient begins to become more gentle andthe water velocity slows down (GRADED STREAMS). It is essentially a transporting system for water andsediment.•Ultimately a stream will discharge into a large lake or ocean. As the stream approaches its ‘base level’, theland is relatively flat, water is flowing extremely slow and sediments are deposited into many landforms(AGGRADING STREAMS). It is essentially a dispersing system for water and sediment.In order to examine features of these three streams, we need to look at the relationship between streamvelocity and how much ‘stuff’ a stream can carry (i.e. sediment).
•The VOLUME of sediment a stream can carry is called CAPACITY. Think of the capacity of sediment astream is capable of carrying. Larger streams (having a larger volume of water has a higher capacity totransport stuff).•A stream’s COMPETENCE is the MAXIMUM grain size a stream can transport. This is directly related tothe velocity of the stream. Faster moving water has a higher competence because it can move larger sizedmaterials.The ability of a stream to erode (competence and capacity) relates to two things: 1. Velocity -- the speed of the water, generally measured in feet per second. 2.Discharge -- the total amount (volume) of water carried by the stream. Discharge is generally measured in cubic feet per second, or cfs.Stream VelocityFriction along the bottom and sides of a stream (wetted perimeter) decreases the velocity there. Thefastest moving water is therefore, in the center of the stream at the surface.Stream Discharge•Stream discharge is the VOLUME of water that passes a given point along a stream in a given amount oftime.•Discharge (Q) depends on two things—how fast is the water moving (velocity, v) and cross sectional area (A)of the stream (to determine volume of water).•Q = A*v where velocity is measured in feet/sec and area is ft2. ft/sec*ft2= ft3/sec or cubic feet persecond or CFS.•For example, if we have a lot of precipitation, stage level in the stream rises, increasing the cross sectionalarea of the stream (i.e. more water). Increase A and multiply it by v, will increase the discharge Q.•The higher the velocity, the faster the water moves, the more sediments can be transported, and thehigher erosive capabilities of the stream.Stream Gradation•Okay, considering what we have learned, we can examine the features of the different types of streamsmentioned earlier. These three types of streams are associated with specific geologic landforms and arelocated at certain places on land.•DEGRADING STREAMS are streams that are young, located near headwater areas at high elevations.•GRADING STREAMS are located down stream of degrading streams (getting closer to sea level)—are saidto be in equilibrium (I’ll address this later).•AGGRADING STREAMS are located closest to base level at the lowest gradients (very gentle slopes sothey move slow).Degrading Streams•Degrading streams are found in headwater regions with steep gradients. On a map view, the streams appearstraight as water is flowing STRAIGHT down hill.•Steep slopes contribute to faster water velocity and erosion predominates, especially at the bottom of thestream (downcutting).•The velocity is so rapid that the stream’s sediment load is less than the stream’s capacity to carry load(water can transport more sediments than it does). Fast moving water results in a high competence.•Excessive downcutting results in a V shaped valley with steep valley walls (over time, the valley walls willmass waste into the stream channel, widening the stream—working towards a graded stream system).
•These streams are characterized by rapids and may be associated with water falls. These streams DO NOThave floodplains (flat areas of land adjacent to the stream channel). Degrading streams at headwaterregions effectively erode mountains at higher elevations. Water velocity is the fastest (steep gradient) anderosion most intense. Load<capacity. Associated with these streams are v shaped valleys, steep valley wallsand falls. Streams are straight (not meandering). They are considered ‘youthful’ as they are beginning theerosion process at higher elevations.Graded Streams•As erosion of degrading streams continue, the valley walls are widened and deepened—and the landsurrounding the stream is less ‘steep’.•The gentler gradient of graded streams translates into slower velocity of water flow and a shift in capacityto carry sediment– typically load equals the capacity to carry sediments so the stream is said to be inequilibrium (overall transporting sediment load). Load equals the capacity. Sediments are deposited alongpoint bars and eroded at cut banks (I.e. deposition = erosion).•Stream energy is directed laterally, where the stream slows down, meanders (an S shaped pattern on a mapview)—erosion is lateral not at the base of the stream channel.•Graded stream valley shape is wider, as a result (U shaped).•Geologic features associated with graded streams include, point bars, cut banks, FLOODPLAINS, levees,oxbow lakes, etc. (View the next few slides).Stream channels tend to meander more and more over time. The reason is because streams erode on theoutsides of curves (fastest water flow), and deposit sediment on the insides of curves (slowest waterflow).The eroding outside part of the curve is called the cut bank. The inside part of the curve where sediment isdeposited is called the point bar.Eventually graded streams meander to such an extent that its easier to just cut off the meander. A long-term result of erosion of cut banks (outsides of bends) is that a stream may eventually cut through the neckof a tight meander, abandoning part of its channel, and forming a feature called an oxbow lake. The cutoffhorseshoe shaped body of water is no longer part of the stream.Eventually the lake, being stagnant, will fill in with vegetation. Organisms will proliferate and suck theoxygen out of the water (sediments will turn what color?). As it fills in it will become swampy and thenfinally land when the water dries/evaporates.Aggrading Streams•Aggrading streams are considered Old Age.•Landscape is very flat as we approach sea level.•The water velocity decreases tremendously at the gentle slopes so that the sediment load exceeds thecapacity of the stream to carry it whereby deposition of sediments occurs.•Massive deposits of sediments produce geologic features called channel bars (filling of the stream channelwith sediment) producing a BRAIDED stream pattern on a map view.•The channel shape is extremely wide and extremely shallow due to the filling in of sediments.•Generally, the main stream valley branches out as it approaches the ocean or large lake—these are calleddistributaries.•Downstream near the mouth of the stream, gradients are very low. Load>capacity so much of the sedimentsare deposited. Water moves very slowly. As sediments deposit, stream channel becomes shallow (sedimentsfill in). Braided streams heavily loaded with sediment result from channel bars that are deposited. The mainchannel is lost. Eventually the stream will branch apart into distributaries, right before it discharges into
another body of water. These are considered OLD age streams simply because the land is essentially erodeddown to base level.The End of the Line•Ultimately streams will discharge into a larger body of water (lake or ocean).•Sediments deposited into the ocean produce a large fan shaped deposit called a delta.•New Orleans is built on the delta of the Mississippi River—technically, the delta will prograde (grow) out tosea as more and more sediments are deposited in the Gulf by the Mississippi.•Unfortunately, the large volumes of sediment in this local area cause the weight of the lithosphere to bevery heavy—this results in isostatic subsidence as the lithosphere sinks into the asthenosphere—NewOrleans is not below sea level due to a RISE in sea level, but because it is being PUSHED down under sealevel!•If the stream is ephemeral (short lived) as in desert, mountainous areas, sediments are deposited at thebase of a mountain producing an Alluvial Fan (similar shape to a delta).DELTAS: Occur at the ends of a stream. Deltas form when a stream discharges into a lake or ocean basin.Because water velocity decreases rapidly (I.e. into stagnant water such as the Gulf of Mexico or a lake), thesediment is deposited. Deltas move when streams migrate, that is, streams can move side to side.ALLUVIAL FANS: Similar to deltas, alluvial fans are sediments deposited by ephemeral streams (shortlived) in mountainous areas. Material is deposited rapidly at the base of the cliff resulting in coarse, angulargrains. Alluvial fans, too appear like a triangle.Alluvial fans can grow very large in size. Material originates from the steep walled valley (top center ofpicture) and spills out in a fan when deposited. After heavy rains, water charges down the canyons as flashfloods, carrying large amounts of sediment as debris flows. As the debris flows slow down, this sediment isdeposited to form alluvial fans -- broad sloping sheets of coarse sediment at the mouths of mountaincanyons.HydrographStream flow over time, including during flooding, is measured using hydrographs: graphs which plot waterlevel (or discharge) over a period of time. Hydrographs are derived from instruments that measure dischargeand/or stream height at key locations along the stream.Reminder!When water falls within a stream’s drainage basin it enters the stream by infiltration/base flow and byrunoff contributing to the streams discharge.The relationship between precipitation and the effect it has on a streams discharge can be shown ingraphical form using a hydrograph.The lag time indicated on the graph is the time between peak precipitation and peak discharge(flooding).High infiltration = high residence time in the soil before entering the stream as base flow: results in a longlag time and low peak discharge.High runoff = water that falls to the ground enters the stream quickly; results in a short lag time and a highpeak discharge.Urban development increases flooding, because buildings, roads and parking lots decrease the infiltration of
rainwater into the ground, and increase the speed with which rainwater runs off into stream channels. Afterurban development, floods tend to peak earlier after a rainstorm, and have higher peak discharge!Flooding - an excessive discharge.Flooding is the main hazard associated with streams. The behavior of a flood, and the type of damage itcauses, vary depending on where in the drainage basin the flood occurs.Upstream floods occur in the headland areas, where the valleys are narrow and steep-sided (degradingstreams). Upland floods are sudden floods of relatively short duration that move quickly through narrowvalleys. They are commonly called flash floods because they occur so quickly. These floods cause damagemainly from the force of the rapidly moving water.Downstream (Regional) floods occur in the lower areas of a drainage basin, where the valleys are wide andgently sloping, with broad floodplains (graded and aggrading streams). Lowland floods are slower but longer-lasting floods that spread out over broad areas of the stream’s floodplain. These floods typically causedamage from extensive wetting and deposition of sediment, rather than from the shear force of the movingwater.Types of FloodsUpstream Flash Floods•Caused by locally intense rainfall (covering only one or two tributaries) over a short period of time.•Steep Gradient.•High and Rapid runoff•Short lag time and high peak discharge.•Waters rise quickly.•Narrow V-shaped channels. No floodplains or levees.•Q = A X V. Increased discharge increases water velocity resulting in an increase in erosion.•Unpredictable (because it is a function of weather conditions), high loss of life.•Floods rapidly, recedes quickly.•Downstream unaffected.•Covers a smaller area of land.Downstream Regional Floods•Caused by large amounts of rainfall over an extended period of time over a large portion of the drainagebasin.•Shallow gradient.•High infiltration (Natural)•Long lag time and broad discharge curve.•Water rises slowly.•Wide, deep U-shaped channels with floodplains and levees.•Q = A x V. Waters rise as soil becomes saturated. Water spreads out onto the floodplain (large cross-sectional area).•Predictable, great property damage because it covers a large area of land and flood waters remain high forlong periods of time.•Recedes slowly.•Floods affect tributaries.As a stream rises prior to flooding, its increased velocity and discharge allow it to carry more and moresediment. When the stream crests its banks and spills out onto the floodplain, the water slows down,
depositing ridges of sediment along the banks called levees. The levees are often the highest places on thefloodplain.When a stream floods and overtops a levee it quickly erodes a gap, leading to massive flooding of thefloodplain in the region of the broken levee.Urban development increases flooding, because buildings, roads and parking lots decrease the infiltration ofrainwater into the ground, and increase the speed with which rainwater runs off into stream channels.After urban development, floods tend to peak earlier after a rainstorm, and have higher peak discharge!Conclusions•Streams are extremely dynamic geologic processes that continually erode and reshape the landscape withthe goal of weathering and transporting all continental rocks below sea level (of course tectonics keepspushing the rocks up).•Streams have various geologic structures associated with them as a result of changes in topography(gradient), velocity, and competence and capacity to carry sediment load.•Flooding differs greatly upstream regions versus downstream regions— upstream floods are unpredictable,triggered by locally intense rain storms, quick to rise and fall and result in high loss of life. On the otherhand, downstream floods have larger drainage basins—here precipitation over a large regional area willeventually make its way downstream resulting in long lived, slow to rise and fall floods—predictable so lowloss of life, but over a large area and large time, much economic damage.•Urbanization can increase flooding merely by reducing infiltration of water in soils and causing precipitationto reach a stream much quicker.