1. The Importance, Genesis, and Spatial Extent of Transient Islands within the Susquehanna River in the
Ridge and Valley Province of Pennsylvania
Brett M. Diehl and Matthew C. Ricker
Department of Environmental, Geographical, and Geological Sciences: Bloomsburg University, PA
The Chesapeake Bay watershed runs through six states including
Washington D.C. and encompasses approximately 165,800 km2.
The Bay’s fishery and seafood industry produces $890 million in
income and provides 34,000 jobs (Chesapeake Bay Foundation,
2014). The Chesapeake Bay has been undergoing prolonged
water quality degradation associated with watershed land-use
practices and many aquatic habitats have been adversely affected
(Chesapeake Bay Program, 2006). Currently, 4.7 million metric
tons of sediment enter the Chesapeake Bay per year and one-
quarter of this flux comes from the Susquehanna River
(Chesapeake Bay Program, 2013). In addition, the Ridge and
Valley Province of the Susquehanna basin has one of the highest
annual sediment yields, at 66.3 Mg/km2/yr (USGS, 2008).
Transient river islands in the Susquehanna River system may
play a significant role in improving downstream water quality by
acting as sinks for suspended sediment, contaminants, and
nutrients. However, to date river island genesis, spatial extent,
and sediment trapping capacities remain poorly understood. A
more in-depth understanding of these islands will produce useful
information for future watershed management efforts. Therefore,
the goals of this research were to (1) examine the potential
genesis of river islands using available geomorphic evidence, (2)
quantify the spatial extent of transient river islands of the
Susquehanna River throughout the Ridge and Valley province,
and (3) quantify sediment trapping potential of these forested
landscape features.
Introduction
Discussion
References
Study Area
On-Going Work
Methods
• The number and area (ha) of river islands within the Ridge and
Valley province of Pennsylvania were quantified manually using
recent aerial photographs obtained from Web Soil Survey (2015).
• Dominant soil series of forested islands (>0.4 ha, >10% tree
cover) were obtained via transects of the river using Web Soil
Survey (2015). Islands not previously mapped were delineated
using the area of interest tool.
• Aerial photo interpretations and soil survey data were used to
quantify potential island genesis, which includes development
from flow obstructions, glacial outwash, and bed load from
tributary creeks.
• Density of the islands was determined using measured river
length (km) and area of islands per branch (ha/km).
Fig. 1: Study
map showing the
North, West, and
Confluence
section of the
greater
Susquehanna
River watershed
in Pennsylvania.
Results
Fig. 2: North Branch
There are 51 forested islands totaling 460 hectares (ha),
with an average island area of 9.0 ha in the North Branch
of the Susquehanna River. The majority of the islands were
interpreted to contain glacial outwash cores, which have a
total area of 320 ha. Islands associated with flow
obstructions comprised 128 ha and there were only 12 ha
of islands associated with tributary creek mouths.
In addition, 43 percent of the islands are mapped on Web
Soil Survey (2015) as river wash (Rw)/Alluvial land (Ag).
0
200
400
600
800
1000
1200
1400
1600
Islands with
Bedload Cores
from Tributary
Creeks
Islands Formed
by Flow
Obstructions
Islands with
Glacial
Outwash Cores
Total Area (ha)
Hectares(ha)
Fig. 4: Island Area Below the Confluence
Fig. 3: West Branch
There are 42 forested islands totaling 434 ha, with an
average island area of 10.3 ha in the West Branch of the
Susquehanna River. Islands with glacial outwash cores are
the dominant island type with an area of 384 ha. There are
a total of 33 ha formed by flow obstructions, and
approximately 17 ha that are composed of bed load from
tributary creeks.
Currently, 34 percent of the islands in the West Branch
are mapped as Udifluvents/Fluvaquents on Web Soil
Survey (2015).
0
50
100
150
200
250
300
350
400
450
500
Islands with
Bedload Cores
from Tributary
Creeks
Islands Formed
by Flow
Obstructions
Islands with
Glacial
Outwash Cores
Total Area (ha)
Hectares(ha)
Fig. 2: Island Area of the North Branch
Fig. 4: Confluence
The confluence section had the majority of forested
islands, with 285 islands totaling an area of 1473 ha, with
an average island area of 5.2 ha. There are 873 ha of
islands that are interpreted to have glacial outwash cores.
There are approximately 572 ha of islands formed from
flow obstructions due to common bedrock outcrops in this
section. In addition, there are 27 ha of islands that were
formed from bed load at the mouth of tributary creeks.
Approximately 28 percent of the islands are mapped on
Web Soil Survey (2015) as Udifluvents, coal overwash (Uf),
which is the greatest soil series mapped by area.
0
50
100
150
200
250
300
350
400
450
500
Islands with
Bedload Cores
from Tributary
Creeks
Islands Formed
by Flow
Obstructions
Islands with
Glacial
Outwash Cores
Total Area (ha)
Hectares(ha)
Fig. 3: Island Area of the West Branch
Theories of Island Genesis
1) Islands formed from flow
obstructions
2) Islands formed from
glacial outwash
3) Islands formed from bed
load at tributary creeks
Flow obstructions include woody
debris, bedrock outcrops, and human
structures. As flow comes in contact
with these features, velocity
decreases on the downstream side
and suspended sediment is
deposited. Eventually, vegetation
stabilizes the sediment, forming an
island.
Glacial outwash consists of coarse
sediments that are deposited by
glacial meltwater. These gravel
materials have filled the
Susquehanna Valley with up to 12
meters of material and in some
cases the river reroutes around
these resistant deposits, forming an
island with an outwash core.
Stream bed load consists of coarse
sand and gravel. When high gradient
tributaries enter the main channel of
the Susquehanna River the bed load
is deposited. Slight downstream
migration of these deltaic materials
is common and colonization of these
deposits by vegetation forms islands
over time.
Our study focuses on the Susquehanna River in the Ridge and
Valley province of Pennsylvania (Fig. 1). Mean annual
temperature ranges from 12°C in lower elevations to
approximately 7°C in higher elevations (USGS, 2008). During a
normal year, the Ridge and Valley Province receives
approximately 1057 mm of precipitation (The Pennsylvania State
Climatologist, 2004). The Susquehanna River watershed is
predominantly forested (67% forest cover) with the remaining
land in agricultural (29%) and various urban (4%) land uses
(USGS, 2008). Agriculture, mining, and deforestation have
increased the rate of regional soil erosion (Jacobson & Coleman,
1986). This upland erosion in the past may have ultimately
resulted in the genesis of many river islands through deposition
of "legacy sediments".
N
• Field verifying initial island genesis interpretations
• Full soil descriptions on representative islands
• Measurements of forest stand structure and density
• Field and LiDAR measurements of island topography
• Calculating island-specific sediment trapping functions
Chesapeake Bay Foundation. 2014. It's more than just "the Bay." Retrieved March 31, 2015, from
http://www.cbf.org/about-the-bay/more-than-just-the_bay
Chesapeake Bay Program. 2006. Best management practices for sediment control and water clarity enhancement. Retrieved, March 31, 2015 from
http://www.chesapeakebay.net/content/publications/cbp_13369.pdf
Chesapeake Bay Program. 2013. Sediment loads and river flow to the bay. Retrieved March 31, 2015, from
http://www.chesapeakebay.net/indicators/indicator/sediment_loads_and_river_fl ow_to_the_bay
Gellis, A.C., Hupp, C.R., Pavich, M.J., Landwehr, J.M, Banks, W.S.L., Hubbard, B.E., Langland, M.J., Ritchie, J.C., and Reuter, J.M. 2008. Sources,
transport, and storage of sediment at selected sites in the Chesapeake Bay watershed. Scientific Investigation Report 2008: 1-95. Retrieved
March 31, 2015, from http://pubs.usgs.gov/sir/2008/5186/pdf/sir2008-5186rev1142011.pdf
Hupp, C. R., and D. E. Bazemore. 1993. Temporal and spatial patterns of wetland sedimentation, West Tennessee. Journal of Hydrology
141:179–196.
Jacobson, Robert B. and Coleman, Derrick J. 1986. Stratigraphy and recent evolution of Maryland piedmont flood plains. American Journal of Science
286: 617-637.
Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Web Soil Survey. Available online at
http://websoilsurvey.nrcs.usda.gov/. Accessed [03/31/2015].
The Pennsylvania State Climatologist. 2004. Williamsport local climatological data. Retrieved March 28, 2015, from
http://climate.psu.edu/data/city_information/lcds/ipt.php
The Pennsylvania State Climatologist. 2004. Williamsport local climatological data. Retrieved March 28, 2015, from
http://climate.psu.edu/data/city_information/lcds/ipt.php
N N
West
Branch
North Branch
Confluence
N
Fig. 5: Island density
Island area density for each branch of the Susquehanna River.
The North and the West Branch have relatively similar island
density at 3.7 and 3.8 ha/km, respectively. The confluence
section has the greatest island density at 15 ha/km. Overall,
there were 5.8 ha/km in the Ridge and Valley province.
• Although large islands
have the most area, there
are many more smaller
flow obstruction islands.
• Small islands have more
surface area and are lower
in total elevation.
• These factors indicate
that even small forested
islands may provide water
quality improvement. Fig. 6: Small island flooding
0
2
4
6
8
10
12
14
16
North Branch West Branch Confluence Total -
Susquehanna
River
IslandDensity(ha/km)
Fig. 3: Island Densities of the Susquehanna River
Branches
• “Legacy sediments” are deposits from upland erosion due to
initial European settlement, deforestation, agriculture, and
mining in the region. During precipitation events the legacy
sediment becomes mobilized and is likely a major source of
alluvium to form islands in the Susquehanna River. Many
obstruction islands (example: crib dams) are therefore
interpreted to be very young, perhaps only 100-300 years old.
• The Tennessee Ridge and Valley, has riparian sedimentation
rates ranging from 10 Mg/ha/yr to 26 Mg/ha/yr (Hupp and
Bazemore,1993). Therefore, in the similar Pennsylvania Ridge
and Valley total sediment trapping on forested islands could
range from approximately 24,000 to 62,000 Mg/yr, an
important ecosystem service for downstream communities.
• In addition, islands tend to flood more frequently than the
adjacent floodplain (Fig. 6) and the hydrologic connection
between the river channel and the islands provides greater
opportunity for floodwater reduction and sediment trapping.