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BIOLOGICAL REPORT 90(5) 29 Comparison of Instream Flow Methods for Western Virginia Donald J. Orth by Department of Fisheries and Wildlife Sciences Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061 and Paul M. Leonard EA Engineering, Science, and Technology, Inc. Sparks, Maryland 21152 Reliable methods are needed for recommending minimum instream flows to protect aquatic life be- cause conflicts between instream and offstream uses of water will increase in the future, even in some water-rich regions. Water resource planning can help avoid these conflicts only if we know the ac- ceptable magnitude and duration of low flows to maintain current instream values. For basinwide planning purposes, simple methods, which require little or no field investigations, are required. While some of the existing methods tend to be conser- vative, the degree to which they protect habitat for fish is seldom determined. We applied physical microhabitat models (pHABSIM) for nine target fish species in four streams in the upper James River basin, Virginia, to (1) identify optimum flows to pro- tect the fish fauna, (2) investigate the relation be- tween optimum flow and average discharge, and (3) compare our findings with recommendations based on simple discharge methods. Microhabitat availability for riffle-dependent spe- cies was most limited at low flows, whereas micro- habitat availability for pool-dependent species was most limited at high flows. At each study site there was a rapid increase in riffle habitat as discharge increased above zero. Optimum flow maximized habitat for the most critically habitat-limited species or life stages. The recommended optimum flows in- creased with increases in stream size, but the slope was not constant; as stream size increased, a lower proportion of average discharge provided optimum habitat. The recommended flows were related to the average discharge (AD) with a power function: op- timum flow (m3/s) = 0.583 ADo.746. ~ Water resource managers can develop flow recommendations for other streams in the upper James River basin based on the average discharge- assuming water quality is not a limiting factor. Aquatic Base Flow recommendations (Le., Septem- ber median flow) provided varying-but a reasonable degree of-habitat protection. The Montana method (10% average discharge) recommendations correctly identified degraded or poor habitat conditions, and the 30% recommendations corresponded to near optimum habitat in small streams but greater than optimum flow at the large-stream site. Seven-day, 1-in-10-year low flows (7Q10) provided very limited amounts of physical habitat for riffle-dwelling fishes. The results of our study provide a basis for making preliminary flow recommendations in this region from readily available data. Studies will be needed, however, to test the generality of the findings in other basins and the assumptions of the methods employed. Information Sources Gore, J. A., andJ. M. Nestler. 1988. Instreamflow studies in perspective. Regulated Rivers 2:93-101. Leonard, P. M., and D. J. Orth. 1988. Use of habitat guilds of stream fishes to determine in stream flow require- ments. N. Am. J. Fish. Manage. 8:399-409. Leonard, P. M., D. J. Orth, and C. J. Goudreau. 1986. Development of a method for recommending in stream flows for fishes in the upper James River, Virginia. Va. Water Resour. Res. Cent. Bull. 152. 122 pp. Lobb, M. D., III, and D. J. Orth. 1988. Microhabitat use by the bigmouth chub Nocomis platyrhynchus in the New River, West Virginia. Am. MidI. Nat. 120:32-40.