DEFINING HYPORHEIC ZONES: CHALLENGES IN SPACE AND TIME

Hyporheic zones have become the focus of intense study in the past 50 years. In many streams and rivers hyporheic zones are important locations for hydrologic, biogeochemical, and biological processes. However, because of the diverse scientific interests in hyporheic zones, there are numerous guidelines that exist to delineate them. One commonality among all definitions is that the hyporheic zone is noted to be a mixing zone between surface water and groundwater. Often the temporal and spatial scales over which we seek to define hyporheic zones constrain our ability to do so. For example, from the hydrogeologist's perspective, the hyporheic zone may be very large in small mountain basins, where losing stream reaches may provide water to aquifers that contributes to gaining reaches several km downstream. In this presentation, we seek to: 1) clarify concepts/definitions of the hyporheic zone and hyporheic exchange; 2) provide an accurate conceptual picture of the structure of the river-hyporheic zone continuum, within the context of the geomorphic, hydrologic and ecological continua of streams; 3) discuss various methods for delineating the hyporheic zone and quantifying hyporheic exchange rates, emphasizing the validity of the methods in the context of definitions; and 4) present a general and versatile definition of hyporheic exchange applicable to the need for well-posed, site-specific definitions in individual studies.