Variation of Hyporheic Exchange Metrics along an Urbanization Gradient

Hyporheic exchange is well established as being important for stream ecosystems and water quality. Urbanization is known to greatly alter hydrologic and geomorphic conditions as well as water quality in streams, leading to extirpation of aquatic organisms and degradation of ecosystem function. Yet the impact of urbanization on hyporheic exchange and function is poorly understood. We utilized metrics that quantify the potential for hyporheic exchange to characterize ten 1st - 3rd order streams that span an urbanization gradient near Washington, DC. Sites were also chosen to minimize variation in other relevant dimensions like parent geology, degree of agriculture, channel slope, and stream order. Metrics of hyporheic potential included horizontal and vertical hydraulic conductivity of sediments (measured via falling head tests), vertical channel thalweg complexity (measured via topographic surveys), and channel planform sinuosity (measured via aerial orthophotos). Degree of urbanization was characterized via both local conditions (e.g., riparian buffer width) and contributing watershed conditions (e.g., percent impervious). Resulting data were first analyzed narratively, by noting the impact of local urbanization stresses (e.g., individual stormwater outfalls) on metrics of hyporheic potential at an individual site level. Data were subsequently analyzed more globally, by regressing metrics of hyporheic potential against various metrics of urbanization within the contributing watershed. Preliminary results reveal a complex picture, where urbanization exerts broad control on hyporheic exchange potential, but individual site circumstances exhibit considerable ability to locally modify such larger landscape trends. For example, channel sinuosity generally decreases with increasing urbanization, reflecting increased constraints on channel meandering, but local riparian corridor widths can vary tremendously, imparting considerable scatter to the relationship. We present and interpret our preliminary results and discuss future research directions.