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1581 warm wa<er fish population is be" supported by a full reservoir rhrough July 15, followed by a rapid draw down between July 15 and August 15, The rapid drawdown allows colonizarion of the exposed substrate by annual species during the growing season, which contributes to reservoir food supplies when reservoir levds rise again. Given these conflicting demands, the overall reSOurce preference is to prevent draw down as long as possible in rhe spring, within the confines of opera<ional demands. B. Summary of Natural Resource Water Preferences by Individual Resource Values I. Water Preferences for Fish Populations a. Arkansas River The Arkansas River is no red for its exceprional brown trout fishery and for its devdoping rain- bow rrout fishery. Brown rrour were rhe focus of rhis smdy because of rheir prevalence in the river, because the popularion is sdf-sustaining, and because my given operational program will influ- ence rainbow trour in a similar manner. There are a nllmb~r of non-game fish species present in me Arkansas River drainage, primarily found between Canon Ciry and Pueblo Reservoir. This area was nor exrensivdy studied, but flows rhar protect and mainrain game species should also protect non- game species. Rare species, such as greenback cur- rhroar trour, Arkansas River darter, and redbeIly dace, are all found in the upper Arkansas River basin bur have nor been collected in rhe main- stem river or reservoirs. Habitat needs for brown trout and rainbow rroU[ were analyzed using the lnme:un Flow Incremenral :Vlethodology (IFIM). The twO mo" importanr physical variables affect- ing fish habirat on the Arkansas River are vdociry and depth. The further these variables are from rhe optimum value, the less likdy thar position is going [Q be occupied by a rrout. because brown [roU[ occupy positions in a stream that maximize net energy gain during foraging. The c:lIrying capaciry of a meam may be determined by avail- able habirar and number of foraging sites. Increosing flows frequenrly produce unfavorable habitat condirions in rhe Arkansas River, as ill us- crated in Appendix X. Increasing vdociry accounts for large drops in suitable habirat, particularly for small tlsh. For example, adulr brown erout prefer a vdociry of 1.3 feer per second for spawning, and vdocities ranging from 0.9 co 1.3 per second for orher activities, such as foraging. However, fry and juve- niles prefer vdociries of 0.3 [Q 0.7 teet per second. For all life "ages, increased vdocir;es nor only increase the merabolic cOst associared wirh forag- ing, but also create conditions chat reduce the capmre of drifting insecrs. As with velocity, increasing depth accounts for drops in suirable habirar, especially for small fish. Depths of 2.0 to 3.0 feer are oprimum for aduIr brown erour spawning, while the suirable range of deprh for spawning is 4.8 co 36.0 inches. Redds a<e generally found ar deprhs of 12.0 to 36.0 inches. However, juvenile brown troUt have opti- mum habirar depths ranging trom 0.9 co 1.7 feer. Finally, because brown crout are boaom-oriented, visual feeders, greater depth cre:lCes conditions thar reduce the captUre of drifting insects. In coldwater environments, troue growth is a good indicacor of the health of an aquaric ecosys- tem because it integrates all the biotic and abioric variables impacting organisms and reflecrs sec- ondary effecrs of chronic seress. Pre- and pOS!' runoff periods (April-May and July-Seprember) are critical for brown <rOut gtowth and survival, because there is a strong correlation between brown rrour growth and discharge in rhe Arkansas River. Warmer water temperamres and poor prey availabiliry make AuguS! and September particu' larly critical months for rroU[ growth. The nega- rive impacts trom higher flows are not offset by releases of cooler warer from Twin lakes in August and September, because rhese releases will not decrease Water temper:uure for my J.pprecia~ ble dist:U1ce downsrre:un. To oprimize the amounr of available brown erouc and rainbow trOUt habirar for all life stages on the Arkansas River from near leadville co Pueblo 29