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remained abundant in the Grand Valley throughout the early part of the century (Quarterone 1993). Additional water depletions did not begin occurring until the second development period (1942- 1953) when dams and transbasin diversions were built in the headwaters. Thus, since the fish were apparently still doing well under historic (though altered) conditions, we looked for an optimum flow among those that were equal to or less than the mean monthly flows of the historic period. To optimize adult habitat, our primary goal was to determine what flow level maximized the amount of the preferred habitat types. We assumed that non-preferred types were already under- utilized, and maximizing the area of those would do little to directly benefit the fish. We do, however, recognize that some habitats provide important indirect benefits that may be ignored by this methodology (areas of high food production, etc.). When more than one habitat type is preferred, additional weight must be given to those types more preferred than others. We therefore used the mean difference between habitat use and availability, our measure of preference, as a weighting factor. For each preferred habitat type, we summed the areas from each site to form a pooled composite of the four sites. The total absolute area within the four sites was then multiplied by the preference weighting factor. These weighted areas were then summed. That flow level at which the highest summed value occurred was considered best. Spring Flow Needs New information collected during this study is used to supplement the existing FWS spring flow recommendations provided by Osmundson and Kaeding (1991). In the earlier report, relationships between spring flows and certain geomorphic issues were discussed; these were based on casual observations made within the 15-mile reach over time but for which actual data were lacking. In the current study, stream bed cross sections monitored for the summer and winter habitat depth studies, described earlier, were also used to provide insights into sediment transport issues involving spring runoff. Specifically, we were interested in determining the minimum peak flows required to (1) maintain backwater depth through flushing the bed of accumulated fines (silt) and (2) cleanse within-channel substrates of accumulated fines via mobilization of coarse bed materials. Again, there were one or two transects established across either main or secondary channels within each study site and 2-3 transects established across a backwater if one was present within the site. A total of six channel cross-sections and six backwater cross-sections (two backwaters) were monitored each fall during 1990-1993. In fall of 1992, an additional transect was established at the mouth of each of the two backwaters; these were monitored again in fall of 1993 along with the other transects. Stream bed elevation was monitored along each backwater transect to determine degree of annual scour or fill. We assumed that scour occurs only during the high flows of spring and deposition occurs throughout the year: coarse and fine sediments are moved and redeposited in spring whereas fines can settle out at any time. Identification of spring flows that are sufficient or insufficient in magnitude was based on whether there was a net gain or net loss in depth, i.e., when scour exceeds deposition, spring flows were considered sufficiently high to maintain backwater depth. Scour and fill data for channel transects were useful in identifying spring flow levels capable or not capable of mobilizing coarse bed materials. 15