As demonstrated by the case of flooding around the National Mall, Washington, DC, those engaged in policymaking rarely confront a shared, single uncertainty in their differing capacities and deliberations. The range of these uncertainties is identified by asking three sets of questions. (1) Where is the uncertainty situated? This determines which people in what roles in the policy process have how much ability, if any, to manipulate the causes of uncertainty. (2) How much uncertainty is there? Are there clear indicators about outcomes and future conditions because there is low uncertainty or not because there is high uncertainty? (3) What is the nature of the uncertainty? Is it because of rectifiable imperfections in knowledge or because of inherent variability that cannot be reduced? Different paths for addressing uncertainties result from the answers to these questions. The uncertainties identified are associated with three drivers of potential harm from flooding: hazard, exposure and vulnerability. It is constructive to understand these drivers and their associated uncertainties because while flooding is the most prevalent natural disaster worldwide, it is also the one for which precautionary measures are most effective and for which ameliorative actions have the greatest potential.