A procedure for measuring option value and other preservation values of water quality is developed and applied to a case study area in the South Platte River Basin, Colorado. Benefits from water-based recreation activities are the focus of the study. The results provide an empirical test and confirmation of Weisbrod's proposal that option value and other preservation values represent important social benefits, and should be added to the aggregate consumer surplus of recreation activities to determine the total benefit of environmental amenities to society. In the absence of such an estimate, insufficient resources would be allocated by society to preservation of unique environments such as pristine mountain streams where mineral and energy development may irreversibly degrade water quality.