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The Straight River, in north-central Minnesota, is a trout stream having cold, clear water. The 75-square-mile Straight River watershed contributes flow to the stream. The watershed is underlain by highly transmissive surficial and confined-drift aquifers. Ground-water discharge from these aquifers sustains flow in the Straight River, and the cold water supports a population of trout. Water withdrawals from these aquifers are increasing in response to changes in land use from dry-land to irrigated fanning. Degradation of the stream's habitat for trout could result from the following: a decrease in ground-water discharge to the stream caused by ground-water withdrawals for irrigation, an increase in ground-water temperature resulting from percolation of irrigated water to the ground-water system, and introduction of agricultural chemicals to the stream through ground-water flow or runoff. Physical data indicate a hydraulic connection between the stream and the surficial aquifer. Discharge of the Straight River increases from about 25 cubic feet per second at the outfall from a reservoir near the headwaters to about 51 cubic feet per second near the mouth. The rate of streamflow gain during summer decreases downstream, possibly as a result of ground-water withdrawal for irrigation. The water table and potentiometric surface of the uppermost confined-drift aquifer generally slope to the southeast and locally toward rivers and lakes; gradients decline to about 5 feet per mile from spring to summer. Daily fluctuations of stream temperature are as great as 15 degrees Celsius during the summer, primarily in response to changes in air temperature. Ground-water discharge to the Straight River decreases stream temperature during the summer. Results of simulations from a stream-temperature model indicate that daily changes in stream temperature are strongly influenced by solar radiation, wind speed, stream depth, and ground-water inflow. Results of simulations from ground-water-flow and stream-temperature models developed for the investigation indicate a significant decrease in ground-water flow could result from ground-water withdrawal at rates similar to those measured during 1988. This reduction in discharge to the stream could result in an increase in stream temperature of 0.5 to 1.5 degrees Celsius. Nitrate concentrations in shallow wells screened at the water table, in some areas, are locally greater than the limit set by the Minnesota Pollution Control Agency. Nitrate concentrations in water from deeper wells and in the stream are low, generally less than 1.0 milligram per liter. Introduction The Straight River, in Becker and Hubbard Counties, Minnesota (fig. 1), is a trout stream having cold, clear water. According to State officials (Dennis Ernst, Minnesota Department of Natural Resources, written commun., 1990), the stream is one of the most productive trout-fishing streams in Minnesota. The stream is an important recreational resource partly because it is located in the north-central part of the State where trout streams are uncommon. Streams designated as trout streams in Minnesota are regulated by the Minnesota Department of Natural Resources (MDNR) to help protect the quantity and quality of the water. The Straight River is similar to many streams in the upper midwest that flow through and receive ground-water discharge from glacial outwash. Ground and surface water are closely related in the Straight River watershed. Infiltration through the permeable soils is rapid when the soils are free of frost, resulting in substantial recharge to highlytransmissive surficial aquifers that are hydraulically connected to the stream. The surface drainage to the Straight River, about 75 square miles in area, is underlain by highly transmissive confined and surficial sand and gravel (drift) aquifers. Flow of the Straight River is sustained by discharge from these aquifers, especially during periods of no rainfall. Brook trout (Salvelinus fontinalis Mitchillj were common in the Straight River as late as the 1940's, but now are absent (Dennis Ernst, oral commun., 1990). In addition to adequate stream discharge, brook troutrequire cold, clear water with a silt-free, rocky substrate (Raleigh, 1982). The temperature of streams is critical because brook trout do not thrive in water where the temperature exceeds 20°C (degrees Celsius) for extended periods. The Straight River presently is too warm a habitat for brook trout. Brown trout (Salmo trutta) were introduced into the StraightRiver in the 1940's and are abundant in the stream today (Dennis Ernst, oral commun., 1990). The stream produces some of the largest brown trout in Minnesota. Brown trout require stream conditions similar to brook trout. Like brook trout, themostimportantenvironmental factor that determines suitable habitat for brown trout is water temperature. Brown trout, however, can survive in warmer water than brook trout (Raleigh and others, 1986). The maximum, near-lethal water temperature for brown trout is 27°C, and optimum temperatures for growth and survival range from 12 to 19°C. During the summer of 1988, temperature and streamflow in the 95'25' 95'15' 95-05'

Stream-aquifer interactions in the Straight River area, Becker and Hubbard counties, Minnesota