Hyporheic zone

All water saturated sediments, located beneath and alongside a stream, containing a certain amount of surface water. If the stream flows over impermeable bedrock, it will not develop a hyporheic zone. The hyporheic zone may exclusively consist of surface water (case of streams perched above the alluvial groundwater) or characterized by a mixture of surface water and groundwater (when there are exchanges with groundwater). These different configurations are found along the same stream, basin headwaters have reduced or absent hyporheic zones unlike the alluvial plain. The hyporheic zone plays an important role in the self-purification of streams.