TRANSIENT FLOW BEHAVIOUR IN THE HYPORHEIC ZONE RESULTING FROM CHANGES IN STREAM STAGE

The hyporheic zone describes the saturated pore space in sediments beneath and lateral to a stream channel that is strongly influenced by the interchange of ground and surface water. The hyporheic zone is regarded as an ecologically important ecotone that defines the extent to which nutrient-rich surface waters penetrate the shallow subsurface in the immediate vicinity of a flowing surface water body to provide essential life support for a distinct and often dynamic community of invertebrates or ‘hyporheos' that permanently or temporarily inhabit the region. Previous research has attempted to explain the spatial development of the hyporheic zone in terms of the flow of water through stream bed riffles under conditions of steady-state flow. This approach helps to explain the incursion of surface water into the sub-surface immediately beneath the stream bed, but does not account for the depth to which surface water penetrates the sub-surface or for the extent to which the hyporheic zone develops adjacent to the stream channel. Ongoing research appears to have resolved these issues by considering the spatial and temporal variability of the hyporheic zone that results from fluctuating stream stage. This work has been conducted at the riffle scale using numerical groundwater flow and transport models (MODFLOW-2000, MT3DMS and MODPATH) to predict the sub-surface transport and fate of nutrients entering the stream bed. This research demonstrates that cyclical fluctuations in stream stage can radically affect nutrient flow paths, increase travel times and enhance the extent to which nutrients pervade the sub-surface. These findings are preliminary but have important implications in the study of hyporheos communities that may survive gradual changes to their living conditions e.g. water chemistry and fluxes, by migrating to more hospitable aquatic habits, but may be unable to respond to changes caused by extreme hydrological events.