This paper reports on two years of ethnographic observation of the science and politics of flood risk in Colorado, as well as design research that examines citizen interaction with expert knowledge about flooding in the region. We argue that the 100-year floodplain standard that inform maps produced by the USA Federal Emergency Management Agency (FEMA)'s National Floodplain Insurance Program (NFIP) represent a problematic form of discursive closure of scientific understanding of flood hazard. We show that in order to meet the requirements of the NFIP, this standard acts as a closure that conveys a certainty that the underlying science does not warrant and foreshortens dialogue on disaster risk and public understanding of flood hazard. Engaging with literature in science and technology studies and human-centered computing, we investigate design opportunities for resisting closure and supporting public formation through encounters with the uncertainty and complexities of risk information.