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Final Report 3-41 September 2000 An important component of shoreline complexity is backwater habitat; this comprises areas oflow or no flow velocity that serve as important nursery habitats for young fishes (Chapter 4). After the 1987 spring peak, remote sensing was used to examine trends in the size, total area, and numbers of backwaters over a range of flows (Pucherelli et al. 1990); the total area of backwater habitat in Reach 2 was maximized at flows between 37 and 55 m3/s (Figure 3.16). The relationship to flow at the two study areas within Reach 3 was less clear, but the gage data used to determine this relationship probably did not accurately reflect actual flow at the study areas. Bell (undated) used aerial photography to measure the amount of backwater present at Jensen and Ouray in Reach 2 in October 1993 and August 1996 and at Mineral Bottom in Reach 3 in October 1993 and August 1996. Flows on these dates were similar (46 and 48 m3 Is, respectively, at the Jensen gage and 57 and 63 m3/s, respectively, at the Green River gage). For comparison, Bell (undated) presented the amount of backwater area in 1987 as determined by Pucherelli et al. (1990) at comparable flows (46 m3/s at the Jensen gage, 79 m3/s at the Green River gage). Despite the similarity in flows at the time of photography, the area of backwater habitat differed considerably among years (Figure 3.17). Bell postulated that differences in annual peak flows could have produced the observed differences. Rakowski and Schmidt (1999) concluded that establishing a single target flow that is intended to maximize habitat availability every year is inappropriate because bar topography, and therefore habitat availability, changes annually in response to the passage of peak flows. They placed the magnitude of flood peaks into three categories: (I) very low peaks that do not inundate the bar tops but rearrange sediment along the bar margins, (2) low peaks that inundate the bars but do not overtop the banks, and (3) large floods that overtop the banks. Although the channel responds rapidly to changes in discharge, the imprint of antecedent conditions on the low-flow channel form (for example, the relative elevation of the bar tops and the distribution of sediment within the channel) survives flood passage, especially the passage of low-magnitude floods. Thus, the availability of nursery habitat for the endangered fishes during low-flow periods depends on the channel form that has resulted from recent floods and antecedent channel conditions (Table 3.13; Rakowski and Schmidt 1999). Detailed measurements of a sand bar in 1993 and 1994 were used to determine the inter-annual changes that occur to habitat availability as a result of flood passage in Reach 2 (Rakowski and Schmidt 1999). Spring runoff was much higher in 1993 (566 m3 Is) than in 1994 (331 m3/s), and the topography of the study bar during low flows as well as the configuration and availability of nursery habitats differed between those years (Rakowski and Schmidt 1999). During both years, habitat availability was maximized at flows much greater than the target flows identified in the 1992 Biological Opinion. In 1993, the amount of nursery habitat was highest at flows of about 140 m3/s; in 1994, the greatest amount of habitat was available at 120 m3/s. The difference between the two years in the relationship between flow and habitat availability was so great that the flow that produced the maximum amount of habitat in 1993 produced no habitat in 1994.