Temporary Rivers

Temporary rivers are those that do not flow continuously through time along their entire length. The phrase temporary rivers primarily came into use during the first decade of the twenty-first century. Other terms used for such rivers include ephemeral, intermittent, IRES (intermittent rivers or ephemeral streams), and non-perennial. Conversely, permanent or perennial rivers reliably have year-round flow. Although definitions vary, ephemeral typically indicates rivers that flow relatively briefly after precipitation and runoff, and intermittent indicates rivers that dry seasonally resulting in longitudinally discontinuous surface flow, with downstream alternations between flowing and dry portions. The phrase temporary rivers encompasses both ephemeral and intermittent rivers. Temporary rivers occur in every region of the world, even areas that receive high amounts of precipitation. Headwater streams are more likely to be temporary in wet regions, but human consumptive demand has been progressively drying even some of the world’s largest rivers, which now no longer flow continuously to the ocean. Examples of human-induced drying occur on the Colorado River and the Rio Grande River in the southwestern United States and Mexico, the Indus River, the Yellow River (Huang He) of China, and the Murray-Darling River in Australia. Temporary rivers constitute over 30 percent, and likely more than half, of the total length of the global river network. Historically, the physical and ecological processes in temporary rivers have been considered less important than processes in perennial rivers, but a substantial body of research indicates that temporary rivers are vital components of river networks in terms of surface and subsurface hydrological connectivity, sediment transport, biogeochemical processes and nutrient uptake, habitat diversity, and biodiversity. Studies of channel form and sediment dynamics in temporary rivers date back several decades, but ecological studies of temporary rivers grew rapidly in number starting in the late 1990s. Together, these studies demonstrate how temporary rivers contribute to all the fundamental ecosystem services provided by perennial rivers. As scientific recognition of the importance of temporary rivers has grown with time, so has the realization that these rivers need legal protection and careful management. Temporary rivers are far less likely to be legally protected with respect to human activities that affect water quality or biotic communities, for example, than are perennial rivers. This article reviews works that underpin contemporary understanding of physical and ecological processes in temporary rivers, works that explore how diverse human actions affect temporary rivers, and works that examine regulatory and management issues surrounding temporary rivers.