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r I .! 20 The 18 mile reach is also important for larval Colorado sguawfish because it has an abundance of backwaters in close proximity to potential spawning areas in both the 15 and 18 mile reaches. Larval mortality likely increases with the distance to a backwater (Tyus and Haines, 1991), and -. downstream of the 18 mile reach backwaters are relatively rare. Larval Colorado squaw fish have been found in this reach almost every year since 1986 (Doug Osmundson, personal communication). .-.. .j ., . .".~ .-." i~~~ I J j 1 ~J .J J J ] J j J STUDY SITES Within the 51 kilometer (32 mi) study reach, three sites were established to study the geomorphic changes in backwater habitats, and four sites were established to model flow and sediment transport. In addition, main channel cross-sections were surveyed every mile in order to quantify the downstream hydraulic geometry of the reach, and put the seven sites in a broader context. Of the three backwater study sites, one was established in the 15 mile reach, and two in the 18 mile reach (Figure 2.1). These sites were chosen because they were recommended as potential endangered fish habitat by the USFWS, and they represent different types of backwaters. The side channel at site 1 is filled with fine sediment and vegetation, site 2 is primarily gravel-bedded, and site 3 is intermediate. The morphologr. of the three backwaters is partially controlled by thick riparian vegetation on at least one bank (all sites), bedrock (site 2), or dikes (site 3). Aerial photographs taken in 1937 show that backwater-type features existed at these sites at that time, thus even though their morphology has changed, these three backwaters have persisted in more or less the same location. A total of 25 cross-sections were surveyed across these backwaters.