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5. Pollutants in the St. Vrain associated with irrigation return flows are salinity, nitrates, and sediment. Periods of low streamflow are probably the limiting factor in terms of sus- taining a fishery. Conclusions on the fishery potential show that: 1. Natural low flow conditions exist on the St. Vrain Creek and the South Branch of the St. Vrain during the months of November through March. This period of low flow may prohibit a sustained fishery. 2. Minimum streamflow to promote and maintain a fishery on the St. Vrain can be achieved through releases from the proposed Coffintop Reser- voir. The minimum streamflow would need to be available throughout the entire study reach before being used for irrigation. 3. The South Branch of the St. Vrain would need to be regulated to ensure that a constant amount of water diverted from 'the St. Vrain provides a continuous minimum flow. Best Management Practices for controlling irrigation return flows are avail- able, and the potential exists for reducing pollutant loads by implementing these BMPs. However, return flows through the study reach do not seri- ously impair the St. Vrain and therefore may not be necessary. In down- stream reaches of the St. Vrain. BMPs may be necessary if a similar fishery potential is proposed. Recreational benefits of the St. Vrain as a result of the improved fishery potential would be valuable. The cost/benefit analysis of providing instream habitat improvements indicated that a positive benefit is realized and that the initial investment would be recaptured in seven years through an increase in the value of having a quality fishing stream. Although not discussed in this report, this study may prove to be useful in evaluating water quality problems and the fishery potential of other streams in the region. Methods are demonstrated on how to evaluate the impact of irrigation return flows on receiving waters along with methods for determin- ing if stream standards are being violated. Frequency of streamflow dis- tribution is discussed, which provides an accurate method of assessing annual streamflow variations. These methodologies could be applied to simi- lar streams in the region to determine if water quality problems exist and define potential uses of the stream. The findings of this report can be used, in part, to define a water quality management program for this reach of the St. Vrain. Potential uses of the St. Vrain are identified and measures to control agricultural nonpoint source pollution are defined. if neered. It appears that a Class 1, Cold Water Fishery can be maintained if minimum streamflow is provided and habitat improvements are made. 2