Streams themselves can be sources of nonpoint source pollution. In addition, some in-stream features can abate nonpoint source pollution. Stream bank erosion is a source of sediment and, potentially, nutrients. Channelized streams, due to more rapid water velocity, can deliver more pollutants than natural, meandering streams. Wetlands can reduce not only sediment but nitrogen and phosphorus. Features of streams and their effects on nonpoint source pollution and pollution control are discussed below.
Definitions of Wetlands.
(Most of the information on wetlands was taken directly from Osmond et al., 1995a.)
Numerous definitions exist for wetlands. Under Section 404 of the Clean Water Act, wetlands are defined as follows:
those areas that are inundated or saturated by surface or ground water (hydrology) at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation (hydrophytes) typically adapted for life in saturated soil conditions (hydric soils). Wetlands generally include swamps, marshes, bogs, and similar areas (40 CFR 232.2 r).
The U.S. Fish and Wildlife Service defines wetlands as follows:
lands that are transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water, and that have one or more of the following attributes:
Functions And Values of Wetlands.Wetlands play a critical role in regulating the movement of water within watersheds as well as the global water cycle. Wetlands store precipitation and surface water and then
Besides providing hydrologic flux and storage functions that reduce flooding, wetlands also provide biogeochemical cycling and storage that benefits water quality. Wetlands may be a sink for, or transform, nutrients, organic compounds, metals, and components of organic matter. Wetlands may also act as filters of sediments, pathogens, and organic matter. Beaver impoundments or created in-stream wetlands can be extremely useful in agricultural watersheds because they can retain up to 1000 times more nitrogen than streams that are not impounded (Whigham et al., 1988).
Another function of wetlands is biological productivity. Wetlands are among the most productive ecosystems in the world and they play an integral role in the ecology of the watershed (Mitsch and Gosselink, 1993). Wetland plants provide breeding and nursery sites for wildlife, resting areas for migratory species, and refuge from predators (Crance, 1988). Decomposed plant matter (detritus) released into the water is important food for many invertebrates and fish, both in the wetland and in associated aquatic systems.
Wetlands provide community structure and wildlife support to many mammals (beavers, muskrat), amphibians (frogs), reptiles (alligators, lizards), waterfowl, and insects. Wetland shape and size affect the wildlife community and the wetlands function as a suitable habitat (Kent, 1994; Brinson, 1993). The shape of the wetland varies the perimeter to area ratio. The amount of perimeter versus area has importance for the success of interior and edge species. Larger wetlands are necessary to support animals, such as black bear, that need larger and wider ranges.
Although some wetland systems are precipitation-dominated systems, most natural wetlands in the Coastal Plain of North Carolina are dominated by surface and ground water flow. Forested riparian wetlands, tidal freshwater marshes, and tidal salt marshes are examples of wetlands formed from surface and ground water flow.
Created In-stream Wetlands.
In-stream wetlands can be created on small streams by impounding or adding a
control structure to the stream. Mitsch (1993) observed that creation of in-stream wetlands is a reasonable alternative only in lower-order streams. Such wetlands are susceptible to reintroduction of accumulated pollutants in large flow events and may be unpredictable with regard to system stability. Construction of created in-stream wetlands is legal only if the U.S. Army Corps of Engineers has permitted the site under Section 404 of the Clean Water Act.
Construction or restoration of created in-stream wetlands provides an opportunity to control nonpoint source pollution, regulate water storage, and provide habitat for both aquatic and non-aquatic species. The creation of new or restoration of drained wetlands will create a marsh-like wetland (Gannon et al., 1995).
In order for created in-stream wetlands to function most effectively in controlling nonpoint source pollution, they must be located in optimal locations in the watershed. Created in-stream wetlands should be placed bordering agricultural fields and in the upper reaches of the watershed, along first- and second-order streams. Created in-stream wetlands can more effectively reduce runoff and erosion for an entire watershed if they are correctly positioned within the upper reaches of the watershed than could the same acreage of created wetlands put into large wetlands low in the watershed (van der Valk and Jolly, 1993). In addition, phosphorus retention and total nonpoint source pollution control is more efficient if the created in-stream wetlands are placed on the lower order streams (higher in the watershed) (Mitsch, 1993). Mitsch (1992) found that created in-stream wetlands controlled 63 to 96% of the phosphorus and 88 to 98% of the sediment.
Constructed wetlands, as opposed to created in-stream wetlands, are built outside of
the stream channel. They are engineered systems designed to simulate natural wetlands to exploit the water purification function of wetlands for human use and benefits. Constructed wetlands consist of former upland environments that have been modified to create poorly drained soils and wetland flora and fauna for the primary purpose of contaminant or pollutant removal from wastewaters or runoff (Hammer, 1992). Constructed wetlands are generally built for the express purpose of treating animal wastewater, on-site wastewater, or processing facility wastewater (Hammer, 1992). Constructed wetlands may also be built to control runoff from cropland or animal operation facilities.
Stream Bank Stabilization
Stream bank erosion is a natural process. Streams naturally erode their banks along the outer toe of a meander bend while depositing sediment along the inner meander bend, forming point bars. As more impervious surfaces are created in watersheds, the flow regime changes, routing more runoff to streams and less into the ground. The result is an increase in stream velocity and energy and a subsequent increase in stream bank erosion. Activities that cause stream bank erosion include agricultural practices that denude the bank of vegetation; cattle access to streams, which dislodges soils, destroys vegetation, and weakens soil structure; channelization and urbanization, which increase stream velocity and energy; and other activities(Thompson and Green, 1994).
Often the first step in restoring a riparian buffer is to stabilize the stream banks. This is especially true in lower order streams (stream order 1-3).
The ability of a stream bank to resist erosion is determined by eight factors, including
Typically, human activities accelerate stream adjustment and consequently accelerate
stream bank erosion. Therefore, it is of vital importance to identify the cause of stream
bank erosion before a solution is prescribed.
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