The recently promulgated Navigable Waters Protection Rule eliminates federal protection for ephemeral streams previously covered by the Clean Water Act in the USA, but the regulatory agencies assert that map limitations preclude any quantification of the scope and extent of affected surface waters. However, ephemeral stream channels, which only flow in direct response to precipitation events and are important contributors to water quality and aquatic ecosystems, have been the subject of extensive mapping efforts that do allow for basic estimation of the effects of the new rule. We use the National Hydrography Dataset and a simple headwater stream model derived from digital elevation data to conduct a rapid assessment of ephemeral stream extent. We estimate that, by length, 43 to 56% of conterminous USA stream channels are ephemeral and that ½ of these features will no longer be protected under the new rule.