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96 A. 1. DOUGLAS AND R. L. JOHNSON per annum. The smallest value estimates of instream flows are tbose that address on-site benefits for one particular recreational activity.28 More recreationists can use a longer stream reach without crowding each other. Acre-foot values will increase per linear mile of streamflow.1 Daubert and Young12 noted that the marginal benefits of an acre-foot of instream flow increased as flows increased at very low flow levels. At high flow levels, the marginal value for an acre- foot of instream flows declined with increasing flows. Smaller streams generate larger acre-foot values than larger streams. Finally, the largest nonmarket instream flow benefits estimates are provided by studies such as Walsh et al.22 that combine off-site and broadly inclusive on-site benefits estimates. There are various factors underlying the wide variance in the value of water. Young and Haveman6list several physical and social forces that affect the value and allocation of water, including difficulties in precisely measuring the quantity of water consumed, problems in measuring the quality of return flows, the high costs of bulk transportation of water, and substantial temporal and spatial variation in supplies. Young and Haveman6 emphasize the fact that this ensemble of factors, in conjunc- tion with the paucity of water markets, tends to create large temporal and spatial variations in the private marginal value product of water. Large water sales among different regions would induce some uniformity in the private marginal value prod-. uct of water. Because this type of interregional transaction is rare, one expects to find substantial variations in the private marginal value product of water.6 Variation in the net social value of instream flows could easily be as large as the variation in the private marginal value product of surface water. Loomis,l in a review of instream flow methodology and analysis, brackets the range of estimates of benefits conferred from instream flows at $15-$74 acre-foot of water. His review discusses studies that use both the CVM and TCM to estimate in stream flow values, and the studies that rely solely on the CVM. Loomis1 does not provide quantitative evidence on the private marginal value product of water. He suggests that the social value of instream flows are competitive with diversionary uses of water. In any particular water allocation conflict, it is a matter of consider- able importance which diversionary uses of water, agricultural or municipal or industrial, is competing with instream flow uses for river waters. Reisner29 asserts that highly subsidized sales of irrigation water in the arid west lowers the marginal value product of water in agricultural uses. Municipal uses, on the other hand, seem to have high marginal values. Loomis et al.3o provide data that shows that the marginal value product of irrigation water in the San Joaquin Valley was less th~n $45 an acre-foot. In the late 1980's, municipalities in southern California were buy- ing water from various sources at prices in excess of $500 per acre-foot.3] Loomis'] statement about the limited range of estimated acre-foot values for instream flows studies may not be accurate for a more inclusive set of studies. This can best be appreciated by considering the problem of estimating the value of a sport fish. Johnson and Adams28 combined a production function approach with a contin- gent value method estimate of the marginal value of catching a fish. Their survey instrument estimated a marginal value for catching a steelhead salmon on the John Day River in Oregon of about $6.65 in 1986 prices.28 Johnson and Adams28 note that this translates into a marginal value per acre-foot of additional streamflow of $2.36. Thus, the value of the streamflow in providing extra fish for sport anglers is low; Johnson and Adams28 neglect boating and shoreline activities. If off-site benefits are relevant, the value of a "fish" can increase greatly. Loomis et al.7 estimated the nonmarket value of allocating water to a dewatered stretch of the upper San Joaquin River in California in order to restore anadromous fish runs