Stream RestorationThe annual costs of water pollution due to sediment in North America alone approach $16 billion . According to the US Environmental Protection Agency (US EPA), sediment is the primary stressor for 31% of all declared impaired stream miles in the US. Excess suspended sediments reduce the diversity and abundance of aquatic organisms, reduce reservoir capacity, increase drinking water treatment costs, and serve as a carrier for contaminants such as phosphorus, bacteria, heavy metals, pharmaceuticals and pesticides.
While considerable resources have been directed toward reducing erosion from agricultural, silvicultural, and urban lands, stream channel degradation has only recently been recognized. Although streambank erosion is a natural process, important for the formation of channel planform and stable cross sections, human-induced changes, such as urbanization and channelization, can destabilize streams, causing accelerated channel erosion. Studies have shown that sediment from stream channels can account for as much as 85% of watershed sediment yields and streambank retreat rates as high as 7.26 m/yr have been documented . In addition to water quality impairment, streambank retreat impacts floodplain residents, riparian ecosystems, bridges, and other stream-side structures .
Riparian vegetation plays a significant role in bank retreat processes by modifying the local microclimate, altering soil moisture, increasing momentum diffusion within the channel, and reinforcing the streambank against hydraulic and mechanical shear stresses. While the importance of vegetation in streambank stabilization is widely acknowledged, the impacts are complex and have yet to be fully quantified [2, 4, 5].
Because riparian vegetation has a significant impact on stream stability and morphology, it has become an integral part of stream restoration designs. A recent synthesis of stream restoration projects within the US estimated on average $1 billion has been spent annually since 1990 on stream restoration , despite the lack of a thorough understanding of streambank retreat and the role of riparian vegetation in channel morphology. Currently, restoration designs are based on regional hydraulic geometry curves , regime equations , and/or stream classification systems , which do not permit the assessment of designs over a range of flows. Additionally, these models of channel form do not permit the long term evaluation of the impacts of future changes in hydrology or sediment supply, such as from landuse or climate change, on restoration designs. Existing models of stream morphology provide little assistance in the assessment of stream restoration projects because they do not consider the effects of vegetation . Ultimately, further research is necessary to analytically evaluate the impact of vegetation on stream morphology and to incorporate this knowledge into watershed models and university water resources programs for effective stream management .