Letter clarifying the origin of the EPA headwater estimate. This is a conservative estimate that may actually understate the number of discharges to nonnavigable waters. According to one reputable study, approximately 73% of the nation’s waters are nonnavigable. Luna B. Leopold, M. Gordon Wolman, J.P. Miller, Fluvial Processes in Geomorphology. W.H. Freeman and Company San Francisco 1964. The EPA estimate uses data on intermittent and ephemeral streams as proxies for nonnavigable tributaries. The EPA relies on such data because it can be correlated with data on discharge permits and water intake structures, resulting in a conservative approximation of the number of discharges and intake structures that would be found in nonnavigable tributaries. (accessed Jan. 10, 2006).