Discontinuous ephemeral streams

Abstract

Many ephemeral streams in western North America flowed over smooth valley floors before transformation from shallow discontinuous channels into deep arroyos. These inherently unstable streams of semiarid regions are sensitive to short-term climatic changes, and to human impacts, because hillslopes supply abundant sediment to infrequent large streamflow events. Discontinuous ephemeral streams appear to be constantly changing as they alternate between two primary modes of operation; either aggradation or degradation may become dominant. Attainment of equilibrium conditions is brief. Disequilibrium is promoted by channel entrenchment that causes the fall of local base level, and by deposition of channel fans that causes the rise of local base level. These opposing base-level processes in adjacent reaches are maintained by self-enhancing feedback mechanisms. The threshold between erosion and deposition is crossed when aggradational or degradational reaches shift upstream or downstream. Extension of entrenched reaches into channel fans tends to create continuous arroyos. Upvalley migration of fan apexes tends to create depositional valley floors with few stream channels. Less than 100 years is required for arroyo cutting, but more than 500 years is required for complete aggradation of entrenched stream channels and valley floors. Discontinuous ephemeral streams have a repetitive sequence of streamflow characteristics that is as distinctive as sequences of meander bends or braided gravel bars in perennial rivers. The sequence changes from degradation to aggradation — headcuts concentrate sheetflow, a single trunk channel conveys flow to the apex of a channel fan, braided distributary channels end in an area of diverging sheetflow, and converging sheetflow drains to headcuts. The sequence is repeated at intervals ranging from 15 m for small streams to more than 10 km for large streams. Lithologic controls on the response of discontinuous ephemeral streams include: (1) amount and size of sediment yielded from hillslopes: (2) infiltration capacity of valley-floor alluvium that influences both the unit stream power available for upstream headcut migration, and the attenuation of flashy streamflow events by riparian vegetation in sheetflow reaches; and (3) cohesiveness of alluvium, which affects headcut and streambank morphology, and rates of arroyo extension, downcutting, and widening. Initiation of arroyo cutting may be too complex to be attributed to a single cause such as change in mean annual precipitation or grazing by livestock, but is most likely associated with a decrease in density of protective plant cover on hillsides and along valley floors. Relatively larger unit stream power makes downstream reaches more susceptible to initial entrenchment during floods than headwaters reaches, and favors persistent arroyos. Entrenchment continues until an equilibrium longitudinal profile is briefly attained. Then, channel widening occurs: streambanks are undercut and aggradation begins.