Reengineering Banklines for Stabilization: A Biological View from the Pueblo of Santa Ana's Reach of the Rio Grande

摘要

Ten years of river restoration at the Pueblo of Santa Ana have included projects focused on bioengineering, native plantings, and habitat creation for the federally listed endangered Rio Grande silvery minnow (Hybognathus amarus). When a natural river meander threatened to erode bankline adjacent to ageing cottonwoods and flank an instream grade control structure in the Rio Grande, the Pueblo and Bureau of Reclamation collaborated to create an ecologically based solution. A modified bendway weir project was implemented which included a series of 13 earthen-buried weirs. Earthen material was used to reestablish 2,000 linear feet of bankline from a 10 feet vertical height, 50 feet in length, to meet the water's edge. During construction, a large scour pool associated with the vertical bank was filled. Additionally, the channel was moved toward the opposite bank and a spoil pile berm was removed that increased floodplain overbank inundation. Both scour pools and floodplain overbank inundation are important habitats during the silvery minnow's life history leading the Pueblo to ask "how did reestablishing the bankline change the river channel at the project site and what were the short-term effects on river habitat and the fish community? Localized geomorphic and hydrologic changes were identified using channel cross-section surveys before (2008), immediately following (2009), and after construction (2010, 2011). The Pueblo used relative comparisons of the project site to reference sites for habitat change analyses based on ecological criteria described for the silvery minnow. Channel characteristics (thalweg location, habitat type, flow, depth, and sediment type) were evaluated for short-duration effects from project implementation. Fish community survey data was collected three times per year (March, July, November), before and after construction. Minnow catch per unit effort, fish species richness and diversity were analyzed using non-parametric statistics to identify differences. Lastly, the five most abundant fish species were compared for dominance before and after construction. The Pueblo's analyses indicated that the project produced localized changes to the river channel resulting in a two year shift of the river's thalweg. This resulted in some generally positive changes related to the silvery minnow's preferred habitat and overall fish community. Despite these changes, the silvery minnow population was relatively unaffected leading the Pueblo to determine that the project successfully met the bankline reengineering objectives while minimizing disturbance to the species.