Levick, LR; Fonseca, J; Goodrich, DC; Hernandez, M; Semmens, DJ; Leidy, RA; Scianni, M; Guertin, DP; Stromberg, J; Tluczek, M; Kepner, WG
This report represents a state-of-the-art synthesis of current knowledge of the ecology and hydrology of ephemeral (dry washes) and intermittent streams in the American Southwest, and may have important bearing on establishing nexus to traditional navigable waters (TNW) and defining connectivity relative to the Clean Water Act. Ephemeral and intermittent streams make up approximately 59% of all streams in the United States (excluding Alaska), and over 81% in the arid and semi-arid Southwest (Arizona, New Mexico, Nevada, Utah, Colorado and California) according to the U.S. Geological Survey National Hydrography Dataset. They are often the headwaters or major tributaries of perennial streams in the Southwest. This comprehensive review of the present scientific understanding of the ecology and hydrology of ephemeral and intermittent streams will help place them in a watershed context, thereby highlighting their importance in maintaining water quality, overall watershed function or health, and provisioning of the essential human and biological requirements of clean water. Ephemeral and intermittent streams provide the same ecological and hydrological functions as perennial streams by moving water, nutrients, and sediment throughout the watershed. When functioning properly, these streams provide landscape hydrologic connections; stream energy dissipation during high-water flows to reduce erosion and improve water quality; surface and subsurface water storage and exchange; ground-water recharge and discharge; sediment transport, storage, and deposition to aid in floodplain maintenance and development; nutrient storage and cycling; wildlife habitat and migration corridors; support for vegetation communities to help stabilize stream banks and provide wildlife services; and water supply and water-quality filtering. They provide a wide array of ecological functions including forage, cover, nesting, and movement corridors for wildlife. Because of the relatively higher moisture content in arid and semi-arid region streams, vegetation and wildlife abundance and diversity in and near them is proportionally higher than in the surrounding uplands. In the rapidly developing southwest, land management decisions must employ a watershed-scale approach that addresses overall watershed function and water quality. Ephemeral and intermittent stream systems comprise a large portion of southwestern watersheds, and contribute to the hydrological, biogeochemical, and ecological health of a watershed. Given their importance and vast extent, it is concluded that an individual ephemeral or intermittent stream segment should not be examined in isolation. Consideration of the cumulative impacts from anthropogenic uses on these streams is critical in watershed-based assessments and land management decisions to maintain overall watershed health and water quality.
