Threats to Aquatic Environments - Sediment—good And Bad

Erosion of river and stream banks brings dirt into waterbodies. Once in the water, this dirt is known as silt or sediment. Most of these particles settle to the bottom. However, sediment is easily stirred up by the movement of fish and other aquatic creatures, many of which spawn or lay eggs at the bed of their habitats. The dirt that remains in suspension in the water is said to make water turbid. The measure of the dirtiness (lack of clarity) of a waterbody is called its turbidity.

Freshwater aquatic creatures are sensitive to turbidity levels and choose their habitats based in part on their sediment preferences. Some fish prefer waters with large amounts of sediment. It provides cover that prevents predator fish from seeing them. Other species prefer clean waters with low turbidity levels. Excessive sediment may clog their gills or smother their eggs. (See Figure 4.10.)

Sediment levels in a water system can be drastically changed by deforestation of banks and nearby lands through forestry or agricultural practices. Excessive grazing of livestock along riverbanks can strip vegetation and permit large amounts of dirt to enter the water. Likewise, timber harvesting and crop production can expose loosened dirt to wind and rain that carry it into water bodies. Dams and diversion structures trap sediments behind them, interrupting the natural downstream flow of sediments that takes place in moving waters.