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December 29, 2014
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Muddy Waters; Fish sing the blues

Visible in the foreground is a slug of sediment-laden water as it enters a clear body of water.
Photo courtesy Wayne County (PA) Conservation District

By Faith Zerbe

Natural habitats like our forests and native meadows hold valuable soil in place. But when these habitats are disturbed and removed, leaving bare soils, as they are during construction activities, that soil poses a major threat to our local streams and the aquatic life that lives there. Erosion and sediment transport in streams is a natural process, but with so much disturbance (via habitat fragmentation, conversion of natural habitat to housing developments, shopping malls and other human activity), sedimentation in our streams and rivers has become the number one pollutant by volume, nationwide.

What does sediment do to our water bodies? As excess sediment runs off the land and into a stream, it brings with it pesticides, nutrients and whatever else was hitching a ride with the soil particles. Increased sediment in streams creates turbid or cloudy water (turbidity), which means that sunlight cannot penetrate the river and photosynthesis may be limited, thus reducing the growth of food available for aquatic organisms.

Sediment also acts as a heat trap, absorbing sunlight and often increasing stream temperatures. As a result, dissolved oxygen needed for a healthy, diverse aquatic life is diminished, because warmer waters hold less oxygen. Suspended materials in the water can physically clog fish gills, reducing their resistance to disease, lowering their growth rates and negatively affecting development of eggs and larvae. As sediment drops out of the water column, it smothers important riffle habitat, filling spaces between the rocks on the stream bottom and covering habitat, while also smothering the macro-invertebrates and fish eggs that live there that cannot move out of harm’s way.

Streams are dynamic systems, and flooding and bank erosion are natural processes; without these natural events, streams would not meander and change course (this is why it’s never a good idea to build or alter floodplains and instead keep healthy riparian natural buffers and undeveloped flood plains to protect water quality and keep people out of harm’s way). But stream banks themselves are more vulnerable to erosion as we alter and pave over watersheds, causing more stormwater runoff that enters and overwhelms streams with very high flows.