Instream flow is valuable to recreationists who rely on flows for fishing, boating, and other forms of river recreation. Instream flow is also valuable to many members of society, whether they visit the rivers or not, because flows maintain ecosystem stability and associated fish and wildlife habitat. This study estimates the economic value of these recreation and preservation benefits along the Big Hole and the Bitterroot Rivers in Montana. Valuation and participation information was obtained from recreationists who were interviewed along the rivers, and from households that were sampled using mail and phone surveys. Both dichotomous-choice and open-ended contingent valuation questions were used in these surveys to estimate the value of instream flow. In addition, methodological issues of additivity of preservation values and apportionment of total value into use and nonuse categories were investigated. Results indicate substantial economic value for maintaining instream flows above minimum levels, with most of the value attributable to preservation motives.