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it (3) depth, velocity, and substrate are independent in their influence on habitat selection by fishes (this assumption allaws one to calculate the composite weighting factor as the product of individual weighting factors); (4) the stream can be modeled on the basis of one or more representative sample read; and (5) there is a positive, linear relationship between weighted usable area and fish standing stock or habitat use. Major criticisms of MWIN have concentrated on the lack of evidence that fish biomass responds to the weighted usable area (WUA) component in the model (Mathur et al. 1985). If this is true, population responses in terms of standing crop or biomass cannot be predicated based on flow alterations (Orth 1987). Due to the complexity and irregularity of natural stream ecosystems, inability to make accurate predictions using a model should not be surprising (Behnke 1987). Attests to apply HMBSIM to quantify flow requirements of various life stages of Colorado squawfish have not met with great success. A major problem is that habitats used by Colorado squawfish are difficult to model. Backwaters, embayments, and eddies are slackwater or reverse-current habitats. These habitats have to be included in the modeling process for meaningful predictions to be made. This has not been done because of the high cost of placing multiple transects at a large number of sites based on habitat use patterns for various life stages at different times of year. Perhaps the main objective of applying Instream Flaw Incremental Methodology (IFIM) should be to quantify flows needed to maintain specific habitats important to Colorado squawfish at critical times of the year. The HUNBSIM model of IFIM should be used with caution. The WM calculation assumes that various ccubinaticns of depth, velocity, and substrate provide adequate habitat regardless of the habitat type in which they are located. For large streams, Moyle and Baltz (1985) recommended that field data be weighted to reflect proportional habitat composition of the stream reach. Because Colorado squawfish utilize a high percentage of zero velocity water at shallow depths, WUA is often m xua zed at extremely low flow levels if only depth, velocity, and substrate variables are used without some type of habitat qualification. A given amount of WUA in the main channel is not necessarily the ecological equivalent of the same area in backwater habitat - even if depth, velocity, and substrate are the same. These problems relate to this study because the IFIM site at Goverment Bridge does not represent the high percentage of backwater and embayment habitats used during winter months. In addition to the above difficulties in recommending flows, complex problems created by different forms of river ice negate the possibility of standard application of PMBSIM techniques. Winter discharge measurements recorded at the USGS gaging station new Maybell, Colorado, were used during winter 1 to provide a record of flow conditions during the study period. Close examination of these records indicated that these data were not accurate erg to establish a reliable stage-discharge relationship for the purposes of this study. The relationship between gage height and discharge established during open water conditions is eompr mmised during the winter. Accuracy of discharge readings at gaging stations is decreased considerably because the stage discharge relationship becomes te. Various forms of ice at downstream controls, including 58 I' t t 1 t '- 11 1