Towards a Rationale Basis for Planning Rehabilitation of Lost Aquatic Habitats of in the Rio Grande and Colorado River Watersheds

Authors: John C. Schmidt

Year: 2010 (xlii)

Abstract

Although the demise of many endemic fish species in the Rio Grande and Colorado River watersheds is also related to biological and hydrologic factors, transformation of the physical template of the aquatic habitat is clearly important to the threatened or endangered status of many species. Planning for the rehabilitation of lost habitats requires four key elements. First, such planning necessitates a clear understanding of the pre-disturbance fluvial geomorphology of these watersheds and of the relation between channel conditions and watershed runoff and sediment yield. Most of these streams carried naturally large suspended sediment loads, and their hydrology was determined by snowmelt in the Rocky Mountains and Utah High Plateaus, or by summer and fall rains. Dams and diversions affect runoff and sediment yield patterns differently depending on where in the watershed the dams and diversions occur. Secondly, rehabilitation planning necessitates understanding the present, transformed condition of the region’s rivers, because such analysis allows quantification of the difference in channel form and process between the processes in which the target species evolved and the conditions in which these species persist today. Thirdly, rehabilitation planning requires undertaking the field and experimental science by which the tradeoffs between effort and species recovery can be understood. In other words, to what degree is species recovery dependent on returning the stream flow or sediment supply regime to pre-disturbance conditions? Do stream channels need to be shifted to the wide, shallow condition characteristic of the early to mid-20th century? How much effort, in terms of non-native woody vegetation removal or channel reconfiguration, needs to be undertaken to achieve these goals? Fourth, rehabilitation planning requires a watershed perspective with which to guide decisions of where to work and how much effort to undertake? Is it acceptable to accept partial restoration of each part of the drainage network or is full restoration to pre-disturbance conditions required for some streams? If so, then which streams? Are there dams whose existence fundamentally jeopardizes the recovery of target species? Are there streams so transformed that restoration ought not to be attempted? This four-step approach for planning rehabilitation of channel geomorphology is illustrated using results from a 20-year geomorphology research program describing channel change in the Colorado River watershed and in the Big Bend region of far west Texas.