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SCA Survey -Major Problems identified in tributaries of the Anacostia- |
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* Maryland State's SCA Survey Protocols were used for this explanation.
Channelization refers to the once common practice of dredging, straightening and/or widening stream channels in an attempt to reduce flooding or to lower the ground water table. While channelization can be partially effective at reducing flooding or lowering the ground water table in an area, it can also have a variety of negative environmental impacts. Channelized steams often have poor instream habitat for aquatic organisms. They can be a barrier to fish migrations, and in areas where the riparian buffer has been removed, the water in the stream can be heated by the sun during the day reducing its oxygen holding capacity and raising water temperatures above the tolerance limits of some fish species. In addition, while channelization may be able to reduce flooding in one specific stream reach, often it increases flooding downstream.
Erosion is a natural process and necessary to maintain good aquatic habitat in a stream. Too much erosion, however, can have the opposite effect, destabilizing stream banks, destroying in-stream habitat and causing significant sediment pollution problems downstream. This often occurs when land use in a watershed changes. As a watershed becomes more urbanized, forest and agricultural fields are developed into residential housing complexes and commercial properties. As a result, the amount of impervious surfaces in a drainage basin increases, which in turn causes the amount of runoff entering a stream to also increase. The stream channel will adjust over time to the new flows by eroding the stream bed and banks to increase its size. This channel readjustment can extend over decades during which excessive amounts of sediment from unstable eroding stream banks can have very detrimental impacts on the stream’s aquatic resources.
Exposed pipes are any pipes that are either in the stream or along the stream’s immediate banks that could be damaged by a high flow event. In urban areas it is very common for pipelines and other utilities to be located in the stream corridor. This is especially true for gravity sewer lines which depend on the continuous downward slope of the pipeline to move sewage to a pumping station or treatment plant. Since streams are located at the lowest points in the local landscape, engineers often build sewer lines parallel to streams to collect sewage from adjacent neighborhoods. While the pipelines are stationary, streams can migrate and over time can expose previously buried pipelines. When this occurs, the pipeline becomes vulnerable to being punctured by debris in the stream. Fluids in the pipelines can then be discharged into the stream causing a serious water quality problem.
Fish migration barriers are anything in the stream that significantly interferes with the upstream movement of fish. Unimpeded fish passage is especially important for anadromous fish which live much of their lives in tidal waters but must move into non-tidal rivers and streams to spawn. Anadromous fish species, including American shad, white perch, yellow perch, blueback herring and alewife migrate from the Chesapeake Bay into Maryland rivers and streams in early spring to spawn. Unimpeded upstream movement is also important for resident fish species, many of which also move both up and down stream during different parts of their life cycle. Without free fish passage, some sections in a stream network can become isolated. If a disturbance occurs in an isolated stretch of stream, such as a sewage spill on a small tributary, some or all fish species may be eliminated from that isolated section of stream. With a fish blockage present and no natural way for a fish to repopulate the solated stream section, the diversity of the fish community in an area will be reduced and the emaining biological community may be out of natural balance.
Forested stream buffers are very important for maintaining healthy streams. Forest buffers help shade the stream, preventing excessive solar heating, and the roots stabilize the stream banks. Forest buffers remove nutrients, sediment and other pollutants from runoff, while the leaves of trees are a major component of the stream’s food web.
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