Stream channels in the American Southwest are misunderstood because they have been understudied and overgeneralized. This misunderstanding has serious consequences for environmental policy, particularly in defining the scope of jurisdiction under the Clean Water Act. While the current status of regulations defining the scope of jurisdictional waters is in flux, the currently-codified regulations require that jurisdictional tributaries must have a physical, chemical, or biological connection to downstream traditional navigable waters. The lack of consistent flow in arid stream channels causes misinterpretations of physio-chemical indicators that are commonly used in perennial streams of humid environments. Here we overview the use of riparian vegetation as a biological indicator of stream channel presence and connectivity in arid environments. Based on a thorough literature review and synthesis of arid region studies, we found key spatiotemporal patterns of arid riparian vegetation that could potentially be used to determine hydrologic connectivity. Much of the vegetation along arid stream channels is well-adapted to water scarcity and varies in response to differences in geomorphology, hydrology, and land use across multiple scales. Riparian vegetation in arid environments ranges from hydroriparian to xeroriparian and can include, or be made up exclusively, of upland species. Regardless of species type, plants near stream channels tend to be denser and larger than upland plants. Access to reliable water results in denser, larger plants and more water-dependent species. To demonstrate the potential of riparian vegetation as an indicator of connectivity, we conclude with a case study of Little Sycamore Wash in northern Arizona. We used the Normalized Difference Vegetation Index to (1) distinguish between riparian and upland vegetation, and (2) assess connectivity of patches of riparian vegetation. By understanding the spatiotemporal variability of riparian vegetation along arid stream channels, we can make better decisions on their regulation and management.
Nicholas Institute for Environmental Policy Solutions