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TABLE 2 Consumer surplus estimates for increases in flow for section 3 of the North Fork Feather River in 1981. Consumer surplus Average flow Total (initial): $108,465 20 cfs increase: $109,923 100 cfs increase: $114,137 200 cfs increase: $117,605 strongly significant and all coefficients are of the expected sign. The CREEL and TRAVCOST variables are significant at the 5 percent level, while INCOME and IN- Marginal change Net change per cfs $1,458 $72.90 $5,672 $56.72 $9,140 $45.70 TERCEPT are not significant in the TSLS regression. All coefficients estimated for equation (4) are significant at the 5 percent level. BENEFITS OF ADDED INSTREAM FLOW Net economic benefits, or consumer sur- plus, to the anglers are calculated using the TSLS estimate of equation (3). The area un- der this demand curve between the TRAV- COST at the initial level of trips and the maximum observed TRAVCOST (taken as the vertical intercept of the demand equa- tion), is the net willingness to pay, or con- sumer surplus. This integral is approxi- mated through a numerical technique programmed into LOTUS 123. Since the creel census summary for the Rock Creek section (section 3) of the North Fork Feather River estimates that 4,721 to- tal angler trips were taken to that section in 1981, a sample blowup factor of 15 (total angler trips/ estimated angler trips) is used to adjust estimated sample trips and esti- mated sample consumer surplus up to the level of total actual visits and consumer surplus of the site. Table 2 shows the total consumer surplus under existing flow con- ditions is $108,465. This translates into a consumer surplus per trip of $23.00. Table 2 also shows the new consumer surplus when the seasonal average ob- served rate of flow in 1981 (101 cfs) is in- creased by 20 cfs, 100 cfs and 200 cfs. These new benefits are calculated by increasing the FLOW variable in the CREEL equation in Table 1 to predict the new level of CREEL. This new level of CREEL is then inserted into the TCM demand equation to predict the new higher level of trips per capita. The area under this shifted TCM demand equation for a 20 cfs increase is $109,923. Thus, the 20 cfs increase during the season adds $1,458 to angler benefits. This translates into a value of $72.90 per additional cfs. As can be seen in Table 2, the bigger the increase in flow, the larger the total benefits. However, also notice that the value of an additional cfs diminishes as flow is increased more and more. An- glers are willing to pay a great deal for the first increases in flow but less for each in- crement as flow increases. Some functional forms of the demand or value function might result in extremely high flows hav- ing a negative value to anglers. CONCLUSION AND QUALIFICATIONS Our analysis demonstrated that a simple bio-economic system could be estimated using angler origin data. The results in- dicated a statistically significant relation- ship between flow and catch. Given that the angler's demand function is partially a function of fish catch, we derived benefit estimates for changes in streamflow. We think this is an important result because it is based on relating actual visitation data to an actual fish catch-flow relationship. The economic value of instream flow re- ported in this paper is the value to current anglers from the effect of increased flow 'N 28 Rivers • Volume 1, Number 1 January 1990