Lake shorelines and riverbanks are areas of dynamic energy. The powerful forces of waves, currents and ice move soil particles toward, away from, and along the shoreline. Rivers are continually downcutting into their valleys carrying sediments downstream. The current moves from side to side, undercutting banks and causing the stream channel to meander. The ice of frozen lakes can expand shoreward with a force of many tonnes per square foot, moving most obstacles in its path including shoreline soil. Masses of ice put in motion by winds or currents can scour the banks of lakes and streams.
Even in small inland lakes, breaking waves and nearshore currents can dislodge sediments. Headlands and points usually have relatively high erosion rates because the waves, currents, and ice attack from all sides and eventually transport the sediments to bays where they are deposited. Bays are usually the most erosion-resistant areas. Erosion and the transport and deposition of sediments is a natural process along shorelines which usually proceeds very slowly so the plants and animals that live along the shoreline can adjust to these slow changes, maintaining a stable, healthy, productive ecosystem. When some catastrophic natural or human disturbance causes this equilibrium to be upset, accelerated erosion can result. Examples of natural disturbances include large trees uprooted by a windstorm, or a flood resulting from a torrential rainstorm. Human disturbances include vegetation removal, dredging, filling, or construction on or near the shoreline.
A study of shoreline impacts of climate change at Chicago found that if the lake level was reduced by up to 1 metre over the next 50 years, the impacts would not be sever and much of the $100 million cost could be offset by adaptive measure taken during normal maintenance and replacement of infrastructure over the period. However a reduction of 1.5 metres or more, over the same period, could cause sizeable impacts costing between $3 and $35 billion to offset.
Signs of serious erosion problems include: Large areas of bare soil on a steep, high shoreline bank; Noticeable recession of the shoreline over a period of time; Leaning or downed trees with exposed roots on the shoreline; Large patches of muddy water near a lakeshore, or unusually muddy streams during periods of high water or following a rainstorm; Excessive deposits of sand or other sediments on the stream bed, or very wide, shallow areas in a stream.
The U.S. has 3.5 million miles of rivers. The 1992 National Water Quality Inventory of 642,881 miles of these rivers stated that only 56 percent supported multiple uses, including drinking water supply, fish and wildlife habitat, recreation, and agriculture, as well as flood prevention and erosion control. In the remaining 44 percent sedimentation and excess nutrients were the most significant causes of degradation.