We test the robustness of visitor dichotomous choice willingness to pay (WTP) for maintaining instream flow by challenging respondent's affirmative answers. This is followed by rephrasing the per trip WTP question into an annual dichotomous choice WTP question and reasking the question. About 5% of visitors revised their yes answer to no. Estimated WTP changed from $12.81 per trip to $11.96 per trip. Using the method of convolutions, this is an insignificant difference. To further evaluate the robustness of contingent valuation method estimated WTP, these values are compared to WTP derived from the revealed preference travel cost model of the same visitors. No statistical difference was found.¿ 1997 American Geophysical Union