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INSTREAM FLOW ~ 97 to the river. The historical size of the combined spawning runs forthe San Joaquin River was about 50,000 fish per year. They used a contingent value method survey instrument to estimate benefits for a hypothetical fall run of 15,000 anadromous fish. They7 also estimate gross benefits of over $2 billion per annum for the partial restoration of the run. The implicit average value of a single spawning fish is about $138,000.7 The range of estimated amenity values spanned by instream flow non- market valuation studies for fish is, then, quite large. Much of the variance is accounted for by the inclusion or omission of off-site benefits estimates. Studies of instream flows that embody the estimation of a streamflow versus fish production relation are problematic. Accurate depiction of the social payoffs and costs of various water allocation proposals is the principal goal of instream flow studies that impute nonmarket values for streamftows. Douglas and Johnson32 list the special assumptions that might induce biased empirical results obtained with the production function approach. For example, Johnson and Adams28 neglect the possibility that river water might have a negative marginal physical product in fish production during spring runoffs. Harpman33 developed a technique for overcoming this difficulty in his doctoral dissertation. But Harpman's33 fish production model incorporates assumptions about the temperature versus fish production relation that may only be applicable to stream reaches located below bottom release dams. This may sharply limit the generality of Harpman's33 finding that instream flow has a positive marginal value for fish production in the winter, but not in the summer. The point about t~e difficulty of estimating fish production functions may be stated more broadly. Estimating a derived demand curve for a factor input from data on the demand for the final good involves a complex intermediate step. This step can be dispensed with if adequate data on prices and quantities for the factor input are available. Economists who claim that embodying the fish production relation in their. estimation procedure represents a sharp improvement need to appeal to gains in the statistical reliability of their results to validate the claims. WATER MANAGEMENT ISSUES The preceding discussion focused on gross rather than net benefits. The estimation of net benefits is complex because the net social benefits provided by instream flows varies with the relevant set of alternative water uses. Early estimates of instream flows did not always examine a particular water allocation issue. In certain cases, the authors suggest that the allocation of water is inefficient, irrespective of the conflict between market and nonmarket uses. For example, Walsh34 noted that on certain Colorado streams. the socially optimal reservoir release policy could sharply dimin- ish the agricultural use versus angling use water allocation conflict. while increasing nonmarket recreational benefits. Spring runoff water has a low marginal value prod- uct for fish production and irrigated crop production. Increasing the quantity of water stored during the spring runoff and releasing it during the warm, dry, summer months would augment agricultural output and produce greater instream flow benefits for anglers.34 Ward35 also used data on in stream flow benefits to address a water management issue. The City of Albuquerque releases water from municipal reservoirs on the Rio Chama during the winter in order to minimize evaporation losses. He showed that gross instream flow recreational benefits from whitewater boating and angling were