Land Cover - Its Impact on Water Quality
Map of land use in the Spring Creek Watershed in 2014.
By clicking on the map above, you will open it as a pdf image that you can enlarge.
Rain falls everywhere on the Spring Creek watershed. Where it falls, whether on the forested ridges, the agricultural valleys, or the built up and paved over neighborhoods, it has a profound impact on the water quality of its streams. The Centre Regional Planning Agency's estimate of land use in the watershed in 2014 was 38% forest, 29% agriculture, 26% developed and 6% vacant (quarries, etc.) as shown in the pie chart below.
Rain falling on undisturbed land percolates into the soil until the surface is saturated. The degree of soil infiltration versus runoff and erosion depends on factors such as soil texture (particle size) and compaction, which vary among soils in the watershed. Soil texture of fine sand and silt is most erodible. Texture improves with the presence of the smallest particles in soil, clay, which bind soil particles together, though a preponderance of clay creates an impermeable surface.
Vegetation provides a canopy that shelters soil from the force of falling rain and slows the velocity of runoff. Its roots hold soil in place and dead plant material becomes the soil's organic matter, which further stabilizes the surface and increases permeability (Erosion and Sediment Control...2012). Cleared, bare land is most vulnerable to erosion.
Beneath the soil, the limestone bedrock of the Spring Creek watershed's valleys is slowly dissolving, resulting in the formation of sinkholes and small openings that catch some of the excess soil water and deliver it to the creek's subsurface watershed. This groundwater replenishes wells and emerges in seeps and springs. The flow that's not captured rushes down slopes carrying surface debris and soil from the forest floor and the agricultural fields, eroding the land and the stream banks and enters the stream.
Erosion and stream pollution increases where development has covered the land with buildings, roads, and parking lots that render the surface impervious, directing large volumes of water over small areas of permeable land, intensifying the flow which washes along materials such as motor oil, road salt and miscellaneous trash from the roads and sidewalks. Some flow is collected by sinkholes and the rest goes into the stream, introducing turbidity and chemical pollution detrimental to aquatic life.
Chemical pollutants such as heavy metals, solvents, and pesticides can be toxic to amphibians, fish, and macroinvertebrates (insect larvae and other small life in the water). There have been some well-known incidents of toxic spills into Spring Creek. In 1956 sodium cyanide from a university lab killed 147,000 trout in the Benner Spring hatchery and unknown numbers of wild fish in Spring Creek. In 1982, kepone and mirex leaked from the lagoon of a pesticide factory, poisoning the water and everything in it. A no-harvest regulation was instituted at that time because fish consumption was toxic to humans. Today, levels of the contaminants are no longer a health concern and the trout population is doing well (Robert Carline, personal communication).
In addition to toxins, aquatic organisms suffer when the oxygen dissolved in the water is inadequate to support them. This condition can be due to some cleaning agents, fertilizers, and septic tank and sewage treatment plant discharges that increasenutrients, causing a flush of algal growth. The algae are short-lived and their bacterial decay consumes much dissolved oxygen, suffocating other life in the water. Levels of dissolved oxygen are also reduced as water temperature rises, so removing trees from stream banks or diverting storm water runoff from impervious surfaces into streams disturbs or destroys the stream habitat for many species (Benthic Macroinvertebrate Monitoring 2012).
As the Spring Creek watershed becomes more urbanized, development brings more impervious surfaces, which are directly related to increased erosion, increased rates of storm water runoff, and the biological integrity of the streams is compromised. For example, thresholds of as little as 10% impervious land cover in a watershed produce detrimental effects on trout populations (Scheuler 1994). In 1996 the proportion of impervious surface in the whole Spring Creek watershed was 12%. By 2002, it had reached 19% in the upper Spring Creek watershed (Centre County Planning Office 2003), yet Spring Creek still supports wild trout populations. This is likely due to substantial amounts of groundwater that enter the creek via springs (Carline et al. 2011).
Because all kinds of earth-moving activities in both construction and agriculture cause erosion and sedimentation in the watercourses, the State of Pennsylvania has developed numerous regulations covering these activities to protect natural resources, water quality, and carrying capacity. These regulations require the implementation of Best Management Practices (BMPs) developed to minimize erosion. For the Spring Creek watershed, the Centre County Conservation District administers these regulations. (For more information, see the Centre County Conservation District website).
A long term study supported by the U.S. Department of Agriculture, Conservation Effects Assessment Project, was conducted on two tributaries of Spring Creek, Cedar Run and Slab Cabin Run, to determine the best approaches for assessing BMP effectiveness and promoting adoption of these practices (Brooks et al. 2011). These two sites were chosen because they suffered from excess sedimentation due to agriculture. Many of the findings of this study are important to refining the methodology of research on erosion and sedimentation. For the purposes of this article, only observations about landscape responses to BMPs will be mentioned.
Implementation of the BMPs, which included stream bank fencing, stream crossings (such as bridges and gravel-lined ramps) and bank stabilizations, substantially reduced total suspended solids, resulting in improved water quality. Macroinvertebrate species diversity increased, and the fish community remained stable while the population of older brown trout increased.
Where a section of Slab Cabin Run flowing through Millbrook Marsh receives heavy runoff that had deepened the channel bottom, researchers elevated the stream surface to promote flooding of the wetland during storm flows. They installed cross vanes (V-shaped structures of rocks or logs) in an effort to reduce non-point source pollution by filtering suspended pollutants in the wetlands. Results from two storm events indicated that cross vanes reduced sediment loads by 19.2% and 5%, respectively (Brooks et al. 2011).
The Clearwater Conservancy is a local independent conservation organization focusing on stream restoration and land conservation, and targeting impaired stream sites that have been identified by the Pennsylvania Department of Environmental Protection. In cooperation with landowners, mostly farmers, Clearwater projects may involve installing rock-lined stream crossings so that cows can cross without roiling the water, or finding and restoring the natural flow path of streams that were previously straightened, an old practice that accelerates erosion, formerly undertaken to turn wetlands into farmland. Clearwater also creates streamside buffers by planting and maintaining trees in the flood plain to anchor the soil as shown in the before and after photos below.
The left photo (before) shows the eroding stream bank along Buffalo Run,
and the right photo (after) shows restoration by the ClearWater Conservancy's Riparian Buffer Program.
The Centre County chapter of Trout Unlimited, dedicated to protecting cold water fisheries, also conducts stream side buffer projects. To enhance fish habitat, members install structures in the stream to provide feeding locations, cover where fish can hide from predators, and gravel necessary for spawning.
Sedimentation and pollution are the two major sources of impairment to the health of streams in the watershed. Land use can cause or mitigate these threats. As the human population in the Spring Creek watershed expands and associated development increases, these changes will be reflected in the health of life in the streams. Managing land use to protect the waters is an important challenge for local government and conservation organizations.
Betsie Blumberg studied anthropology and soils. Before retirement she was a science reporter for the National Park Service.
Benthic Macroinvertebrate Monitoring. 2012. The Water Resources Monitoring Project's 2012 State of the Water Resources Report. Available at http://www.springcreekwatershed.org/images/stories/pdf/WRMP/AnnualReport2012_2.pdf
Brooks, R. P., S. E. Yetter, R. F. Carline, J. S. Shortle. J. A. Bishop, H. Ingram. 2011. Analysis of BMP Implementation Performance and Maintenance in Spring Creek, an Agriculturally Influenced Watershed in Pennsylvania. U.S. Department of Agriculture. Project Number PENW 2006-03984
Carline, Robert, R. L. Dunlap, J. E. Detar, B. A. Hollender. 2011. The Fishery of Spring Creek – A Watershed Under Siege. Pennsylvania Fish and Boat Commission Technical Report Number 1.
Centre County Planning Office. 2003. (personal communication, R. Jacobs)
Erosion and Sediment Control Program Manual. Technical Guidance Number 363-2134-008. 2012. Bureau of Waterways Engineering and Wetlands. Pennsylvania Department of Environmental Protection.
Scheuler, T. R. 1994. The Importance of Imperviousness. Watershed Protection Techniques 1(3).