Evaluating ecological function provided by bank stabilization in Box Canyon Reservoir, WA

Shoreline erosion is a dynamic natural process of alluvial rivers that humans often accelerate and then attempt to control. Traditionally, reduction of shoreline erosion is accomplished by structural means, such as vertical bulkheads and riprap/rock. However, hard bank stabilizing materials can radically alter ecological functions provided by native shoreline vegetation. Despite valid concerns regarding the use of riprap in bank stabilization, limited pre- and post-project monitoring make it difficult to assess a project\u27s ability to provide a healthy, functioning shoreline environment. Over {dollar}3 million has been spent stabilizing several miles of shoreline along the Pend Oreille River in Box Canyon Reservoir, Washington. My study was designed to evaluate the effectiveness of these projects in providing ecological function. The first step in my evaluation was to determine where and how stabilization occurs in the reservoir using GIS and existing mapped data. The second step was to compare erosion and vegetation among three different shoreline land-use treatments: 1) stabilized; 2) unstabilized, developed; and 3) reference. The three treatments were collocated in seven study sites for a total of 21 sites. Erosion at each site was assessed using a cumulative scoring index. Plant composition was visually estimated in 4 m² plots, and woody species in 50 m² belt transects stratified by bank geomorphology during August-September 2011. Erosion and vegetation were compared among treatments and across bank strata using general linear models (GLM), randomization tests, and post hoc Tukey\u27s tests. Riprap, the most common type of stabilization in Box Canyon Reservoir, was predominantly found along developed and forested shorelines in downstream reservoir reaches affected by reservoir operations and underlying geology. Reference sites were eroded just as much, if not more than developed sites. This suggests that although vegetation removal associated with development has the potential to destabilize banks, dams and boat wakes are likely having a greater effect on hydrologic and sediment regimes in Box Canyon Reservoir. Stabilization minimized erosion and enabled exotic herbaceous species to become established on the bank above the bank strata covered by riprap. However, little vegetation grew in the riprap itself, which replaced fringe wetland habitat comprised of native hydrophytic plant communities. Box Canyon Reservoir appears to be slowly widening as was evidenced by moderates rates of erosion and low vegetation cover along reference shorelines. At this stage in the reservoir\u27s adjustment, widespread stabilization has the potential to limit the long-term recovery ofbank stability and ecological function --Document