Definition: Erosion is a necessary and natural process of streams and rivers. When in equilibrium, or a natural state, the rate of erosion is relatively slow and is balanced with the rate of deposition of material. Erosion rates differ by soil type and stream type, but generally streambank erosion occurs on the order of inches or less per year. Accelerated or excessive erosion results from changes in stream power, bank strength and stream channel geomorphology.
Actual Ecosystem Stress: Excessive erosion often causes stream beds to widen and become more shallow over time, which in turn reduces fish habitat and may lead to warmer water temperatures. Erosion and routing of fine sediment (sand, silt) from the bank increases sedimentation, which in turn can suffocate fish eggs, reduce sensitive macroinvertebrates, and fill up spaces in the substrate (stream bottom) where aquatic insects and small fish live and hide.
Sources: There are many sources of excessive stream bank erosion, and they are often complicated. Changes in hydrology upstream of the project site from increased stormwater runoff can increase stream power and cause stream bank and stream bed erosion. Dam operations that artificially raise and lower stream levels can cause erosion. Channelization, filling in floodplains, erecting stream channel walls, installing rip-rap or changing the natural configuration of a stream channel all change energy vectors within the channel and can cause a stream to erode as it seeks to return to its original, more stable shape. Removing gravel or adding material to a stream bed can alter stream power as well and cause similar instability and erosion. All of these changes to the basic shape, sediment balance, and power of a stream can cause widespread erosion that can extend miles from the actual source of the disturbance.
On a more localized level, removal of riparian trees and bushes reduces bank strength and almost always results in accelerated erosion rates. Livestock trampling banks and eating riparian vegetation are causes of erosion as well.
Left: eroding bank resulting from weak bank strength due to removal of riparian trees.
Right: a combination of increased stream power from channel constriction at undersized bridge crossing and weak bank strength due to removal of streamside trees.
Mitigating the threat: The key to successful bank stabilization or reducing excessive erosion entails
Eroding banks were graded back at 1.5:1 angle, and planted with fast-growing willows and dogwood bundles and poplar and sycamore seedlings (3 years progression).
|Left: willow bundle (fascine) when partially buried with moist soil will sprout in the spring and become large bushes in several years.|
|Middle: a formerly eroding bank stabilized with logs and willow bundles 2 years after planting.|
|Right: tree saplings planted in riparian zone and protected with plastic tree guards from deer browse.|
Links to Restoration Examples:
Fencing out livestock and planting vegetation (see TU’s Potomac Highlands Project)