We present a Cost-Bene¯t Analysis (CBA) of environmental change, in which theory and empirical work are closely knit together. The theoretical framework is used to derive a cost-bene¯t rule for projects that a®ect wild salmon survival. In contrast to many similar studies, we use this dynamic cost-bene¯t rule in structuring the contingent valuation study. Data comes from an ongoing project regarding a potential salmon passage- hydropower con°ict in the northern Swedish river UmeÄalven and its largest salmon producing tributary VindelÄalven. Daily water °ow data are com- bined with daily data on the number of salmon (1974-2000) that pass the hydropower plant Stornorrfors. Detailed ecological studies are used to build the contingent valuation scenario and to study the opportunity costs of re- leasing more water to the potential bene¯t of salmon upstream migrants. We present results from pilot-studies on the value Swedes place on increasing the amount of wild salmon in this particular river.