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to support the target species. Habitat capacity is defined as the level above which emigration stocks, i.e., increases in preferred habitat result in a concomitant increase in the capacity of a reach occurs (Mesick 1988: Bartholow et al. 1994). Unlike IFIM, this method actually measures changes in mesohabitat area within several study sites, rather than estimating changes in microhabitats from measurements of depth, velocity and substrate along several transects within one study site. Mesohabitats are defined as a discrete unit of habitat at the pool/riffle scale that has distinct hydrologic and biological characteristics (from Kershner et al. 1992); it is assumed that each mesohabitat type tends to behave similarly in response to discharge fluctuations (Bartholow et al. 1994). The mesohabitat approach used here eliminates the need to assume that depth, velocity and substrate are variables independent of one another and equal in importance in influencing micro- habitat selection by the fish, model assumptions for which IFIM has often been criticized (Patten 1979; Orth and Maughn 1982; Mathur et al. 1985). The mesohabitat mapping approach also allows the measurement of habitat heterogeneity, an environmental variable not addressed by IFIM. Retention of natural habitat interspersion and juxtaposition should be an instream flow consideration (Bartholow et al. 1994), adding an additional quality component to a method otherwise driven by habitat quantity considerations. In conjunction with maximizing preferred habitats, managing for habitat heterogeneity provides a hedge against the uncertainty of not knowing the importance of habitats that the target species does not prefer or use very much (Bovee personal communication). High habitat heterogeneity or diversity assures that these other habitats are interspersed with preferred habitats and are present to fulfill their respective functions. In this study, density of mapped habitat units was used as a measure of interspersion; it was treated as a secondary consideration to the primary objective of maximizing area of preferred habitat. Approach Used for Spring Recommendations In determining optimum flows for the spring months, Osmundson and Kaeding (1991) concluded that the greatest value of high flows, typical of spring, was the year-round benefits provided by the scouring and flushing action of the flood waters, i.e., channel maintenance, removal of fines from coarse substrates, control of encroaching vegetation, entrainment of organic debris into the system, control of non-native fish, etc. Thus, recommendations for spring flow levels were aimed more at maintaining and enhancing these effects than for optimizing rare fish habitat used during the spring months as was the case for the summer and winter periods. The exception to this was to assure that certain key habitats used by razorback sucker during spring were provided. This was because, unlike Colorado squawfish which spawn during summer, razorback suckers spawn in spring; thus, maintaining or enhancing appropriate habitats during this period could be critical to reproduction and survival of razorback young. For the reasons outlined above, this report does not employ for the spring period the approach used for recommending summer and winter flows, i.e., determining the flows at which preferred mesohabitats are maximized in area. The approach and rationale used in the previous FWS report on spring flows (Osmundson and Kaeding 1991) remains the most valid, and the existing recom- mendations for spring flows provided in that report are largely maintained here. That report is referenced in this report where appropriate and is included in its entirety as Appendix IX. In the current report, new information collected by FWS as well as that collected by others is used to 3