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EXECUTIVE SUMMARY As a part of the areawide water quality planning process, nonpoint source pollution was identified as having a significant impact on water quality. One sou rce of such pollution, agricu ltu ral retu rn flow, may contribute nutrients and sediments to the extent that stream standards are violated. The St. Yrain - Left Hand Water Conservancy District was designated as the management agency for controlling agricultural non point pollution in the St. Yrain Basin. As a result of this designation, a study was conducted in 1982 by DRCOG in conjunction with the St. Yrain - Left Hand Water Con- servancy District on part of the St. Yrain Creek to determine the relative magnitude of agricultural nonpoint pollution and to what degree this source of pollution is degrading water quality. Withdrawal of water from St. Yrain Creek for agricultural and municipal water supply amounts to 90 percent of the water in the basin. Withdrawing this much water decreases the fishery potential and may result in increased loads of pollutants from irrigation return flow. In addition to investigating the water quality impacts, this study investigated the hydrology of the St. Yrain and examined the potential of maintaining a cold water fishery. In this report, Best Management Practices (BMPs) for controlling the quality of irrigation return flows were investigated, and a cost/benefit analysis of the BMPs and recreational benefits of providing a fishery are discussed. Results from this study indicate that the water quality of the St. Yrain through the reach studied may be capable of supporting a cold water fish- ery but that much more water quality data is necessary before a specific conclusion can be made. Findings of this study indicate that: 1. Water qua.lity of the St. Yrain at Lyons, Colorado, is good, and meets the State Stream Standards of a Class 1, Cold Water Fishery. 2. Water quality data through the study reach (Highway 36 to Hygiene Road) is virtually non-existent. In predicting the water quality through the reach, it appears to be capable of supporting a Class 1, Cold Water Fishery. Before any determination is made as to whether or not a Class 1, Cold Water Fishery can be maintained, additional water quality information is needed. 3. The water quality through the study reach is not seriously impaired by irrigation return flows. This may not be the case further downstream, where the majority of the streamflow consists of irrigation return flow water. 4. Water quality impacts of irrigation upon the hydrology of the stream. dilution of irrigation return flow. return flows are directly dependent High streamflows provide greater 1