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IN STREAM FLOW .93 make-their willingness-to-pay-for the provision of a hypothetical nonmarket amenity. The contingent value method is widely used to estimate nonmarket instream flow benefits. 12 The CVM and TCM have been used to estimate benefits conferred at the same site.13 Cummings et al. 13 report that in these cases the two techniques often provide similar estimates. In certain noteworthy examples, such as the Colorado River below Glen Canyon Dam, the estimates are quite disparate.4.11 Bishop et aI.4 estimated smaller values for net benefits conferred by angling par- ticipation days at the Lee's Ferry site on the Colorado River than Richards and Wood.ll Bishop et al.4 used the contingent value method, while Richards and Woodll used the micro-data variant of the travel cost method. Rough estimates of benefits per angling day for Richards and Woodll can be obtained from data on the number of 1983 angling days at Lee's Ferry given in Bishop et al.4 Richards and Woodll estimate that the Lee's Ferry site generated nonmarket aggregate angling benefits of $4.9 million per annum for late 1982 participation rates. The survey instrument was administered in late 1982 and early 1983. Assuming that participa- tion rates were about the same in 1982 as for 1983, they implicitly estimate angling benefits to be about $94 per day. Bishop et al.4 estimate 1985 angling day benefits to be $35. Richards and WOOd'Sll estimates are similar to regional TCM angling par- ticipation day estimates reported in Martin et al. 14 To estimate benefits conferred by instream flows with the travel cost method, one must use much more data than is needed to estimate streamflow benefits with the contingent value method. The reason is that with a travel cost method study a demand curve must be estimated for each of several prevailing flow levels at the site. The variations in the flow levels should persist long enough so that one may assume that they systematicaHy affect the demand for trips to the site. For these reasons, the CVM and TCM are often combined (see Amirfathi et aI.15) to estimate benefits conferred. The study by Loomis and Cooper16 may be the only study that relies solely on the TCM to estimate instream flow nonmarket benefits. The technique involved the esti- mation of a separate TCM demand curve for each of five prevailing mean flow levels for five years at sites on the Feather River.16 The estimation of streamflow benefits with the TCM multiplied five-fold the amount of data needed for the statis- tical analysis. Data requirements for the estimation of benefits conferred by streamflow improvements with the CVM are modest, and entail no more than the insertion of a few additional questions into an on-site or mail-out questionnaire.4,7 The streamlined data requirements of the CVM is the reason that it is used more widely than the TCM to estimate benefits conferred by changes in flows. The estimates produced by the travel cost method must satisfy relatively stringent criteria used to judge the empirical reliability of statistical demand curves. These criteria include large multiple correlation coefficients, and high t-values for the travel cost variableP Contingent value method estimates of aggregate willingness-to-pay also employ regression techniques. Bishop and Welshl8 note that actual markets have been set-up for the purchase and sale of licenses and other commodities in order to test the validity of contingent value method estimates. Daubert and Young12 were among the first economists to use a CVM survey instrument to estimate instream flow benefits. They queried three types of recre- ationists on their willingness-to-pay for marginal increments in instream flows for the Cache La Poudre River in Northern Colorado. A key innovative element in the work of Daubert and Young12 is that they estimate benefits for a change in the qual- ity of a good or service, rather than the quantity of the good. The quality of a trip to