7884 - Laserfiche WebLink

Home Browse Search

Evaluation of Diversion Structures The structures that are the most permanent have the least effect on the local aquatic habitat and aquatic biota. The structures that are rebuilt annually have the most effect on the local aquatic habitat conditions. The amount of disturbance relative to the entire river is relatively small. The annual disturbance at each structure typically appears to be less than one or two channel widths in length. The structures near river islands have the highest amount of disturbance. This was due to the blocking of both channels around the island to control the river flow. Examples of this situation include the Marshall-Roberts, Williams, Walker and Cary diversion structures. The disturbance is seasonal and does not appear to cause significant long term effects except at very localized areas immediately adjacent to the structures. The most biologically significant structures with regard to Colorado squawfrsh migration during low flow are the Maybell Canal and Patrick Sweeney/K. Diamond diversion structures. ' Both of these structures are well inside designated critical habitat and known Colorado squawfrsh populations migrate past these structures annually, if flow conditions allow. Both of these structures are the most permanent in the river since they are constructed from large rock. The temporary structures constructed from river cobble and gravels have less lasting effects on the Colorado squawfrsh and other resident fish species. PROBLEMATIC DIVERSION STRUCTURES The evaluation of diversion structures is summarized in Table 3-1. Each structure considered in the evaluation is listed in this table, along with the level of analysis to which it was subjected and an assessment of the degree to which the structure poses a problem from a fish passage and habitat disturbance perspective. Notes are provided for a few structures. The following section attempts to distill the overall evaluation process down into a set of ' recommendations regarding problematic structures. These recommendations are based primarily on the field studies, but also consider other secondary information made available in the course of the evaluation. While the recommendations identify specific structures as being ' problematical, it is not known precisely how Colorado squawfrsh behave at these structures (even the detailed studies of the City of Craig's structure relied on surrogate fish species), at other structures not examined in detail, and at natural obstructions such as the cataracts of Cross ' Mountain Canyon. Structures Within Designated Critical Habitat Pump Diversions Between Juniper and Cross Mountain Canyons ' While these diversion structures were only viewed from the air, many of them displayed a common characteristic--namely, an extensive shallow braided channel downstream of the structure caused by instream construction activities at the structure itself. In many cases these alluvial berms extend only part way into the river, but in some cases they extend most of the way across the river. This feature, in conjunction with the poor approach channel characteristics, probably make some of these pump diversions difficult for fish to navigate ' during low flow conditions. Accordingly, we believe they merit further, more detailed evaluation. 3-7