As rivers and streams flow, they naturally carve their course through the landscape - carrying a certain amount of sediment downstream. In natural conditions, a river will alter its course over time, as it creates natural meanders and floodplains that slow its flow during storm events, dissipating energy and reducing erosion. But in urban environments, large areas are covered with impervious surfaces. As a result, water cannot infiltrate into the ground, and instead drains into stormwater systems, and then creeks and rivers, much faster than it naturally would.
This rapid drainage and increased quantity of runoff results in high peak flows in waterways. The matter is made worse by the fact that many urban waterways have been "channelized" or straightened in order to maximize develop-able land, reducing the natural capacity of the waterway to slow down and dissipate the water’s energy. The result is severe erosion of stream banks, scouring of stream beds, and excessive sedimentation. Sediment loading is recognized as one of the greatest threats to the Rivanna River and the Chesapeake Bay; sediment carries pollutants that have bonded to it into waterways, suspends in the water column and blocks sunlight from aiding in the growth of submerged aquatic vegetation, clogs the gills of fish (sometimes suffocating them), and eventually destroys aquatic habitat in streambeds when it settles.