Allegheny River Headwaters Watershed Conservation Plan

Allegheny River Headwaters Conservation Plan
Chapter 2. Land Resources
Landslides occur throughout Pennsylvania and are heavily concentrated in southwestern
Pennsylvania. Within Landslide Hazards: A National Threat, the Allegheny River headwaters were
identified as being located with a region of the U.S. that has a moderate to high potential for the
occurrence of landslides (USGS, 2005). Landslides cause damage to transportation routes, utilities, and
buildings. They can create travel delays and other side effects. The threat of landslides should always be
assessed when planning any development project. Proactively avoiding a landslide is cheaper than the
clean-up, repair, and time that is required after a landslide. If development within a landslide-prone area is
ultimately chosen, additional precautionary measures should be taken during development, such as
additional drainage features and proper site planning, which are essential to minimize the risk of a
landslide (Delano & Wilshusen, 2001).
Subsidence
Subsidence is the downward movement of surface material involving little or no horizontal
movement. Occurring naturally due to physical and chemical weathering of certain types of bedrock,
subsidence usually takes place locally as a result of underground mining, excessive pumping of
groundwater, or subsurface erosion due to the failure of existing utility lines. Subsidence usually occurs
slowly over long periods, but also can happen rapidly. The development of a sinkhole occurs when the
support of the land is gradually removed, causing the land surface to sag and finally collapse, leaving a
hole or cavity. Although subsidence is not common in the watershed, the potential exists; especially in
areas that were previously mined (Kochanov, 1999).
Erosion and Sedimentation
Erosion is the transfer of soil particles through air or water. The relocation of these particles is
sedimentation. Erosion and sedimentation are natural earthmoving processes, but the extent of this
movement can be greater than normal due to poor land-use practices. Erosion and sedimentation are very
serious issues with the potential to cause significant degradation to an area’s waterbodies.
Erosion is common along streambanks, steep slopes, and ridgetops. Streambank erosion occurs when
the banks of a creek or river erode and deposit sediment into a waterway. Typically, erosion is caused by
improper land uses and a lack of vegetation along the streambank. Vegetation anchors soils in place,
preventing it from washing away during high stream levels or heavy rains. However, if the vegetation is
removed or inadequate, the soil is easily washed into the waterbody. A lack of vegetation also leaves soils
vulnerable to high winds, which can induce erosion.
An increase in sediment in a waterbody is itself a cause for
concern, as it alters native aquatic habitats. Excessive
sedimentation clouds the water, which reduces the amount of
sunlight reaching aquatic plants. It covers fish spawning areas and
food supplies and may clog their gills. Other pollutants attached to
soil particles are deposited in waterbodies with the sediment.
Sediment deposits can significantly alter the channel and flow of
the stream.
Improperly built and managed dirt
and gravel roadways increase the
amount of sediment entering streams
via runoff following a precipitation
event (Source: Upper Allegheny
Watershed Association)
Erosion can negatively impact a stream’s water quality. Soil
eroded from construction sites, timber operations, or agricultural
operations exacerbates sedimentation problems in streams. Dirt
particles from poorly designed and managed dirt and gravel
roadways enter waterways by direct runoff or are deposited in
waterways as dry deposition being transported in a cloud of dust.
In an effort to reduce erosion and sedimentation, the
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