DYNAMICS OF EPHEMERALLY-FLOWING STREAMS: CHARACTERISTICS AND CHALLENGES (Invited Presentation)

Non-perennial rivers range from those that are dry for short periods or seasonally to those that only flow occasionally. Ephemerally-flowing streams in drylands pose particular challenges for both measurement and understanding of processes and impacts. Flows are infrequent and therefore long periods are needed to gain data. Flows tend to occur as flash floods and are highly variable in both time and space, often generated by localised, intense storms, falling on sparsely vegetated and low infiltration surfaces. These are also particularly hazardous events, with lack of warning and rapid intensification. Thresholds for occurrence of flows of differing magnitude need to be identified for specific environments. The morphology of these channels is also spatially and temporally variable, with many being highly dynamic, though some characteristic types can be identified.

Strategies and methods of instrumentation and data collection are reviewed and experience in implementation is discussed. Evidence is provided from a range of channels of differing lithology and morphology in southeast Spain of the flow dynamics over a period of 25 years. The processes and impacts of flows of differing magnitude in the various channels are outlined and trajectories of morphological change shown to have varied between sites. Low thresholds for sediment mobility in sand and gravel reaches produce high sediment fluxes and connectivity. Measurements associated with an extreme event resulted in calculations of record sediment fluxes. The contributing conditions and hydraulic characteristics are assessed. Events and sequences of conditions, including vegetation dynamics, are shown to have complex feedback effects. The highly variable and episodic dynamics of these channels and rapidity of responses pose major challenges for management and for protection of communities. Use of conventional strategies such as check dams are evaluated and more sustainable methods are advocated.