Hyporheic zone

The hyporheic zone is the region of sediment and porous space beneath and alongside a stream bed, where there is mixing of shallow groundwater and surface water. The flow dynamics and behavior in this zone (termed hyporheic flow or underflow) is recognized to be important for surface water/groundwater interactions, as well as fish spawning, among other processes. As an innovative urban water management practice, the hyporheic zone can be designed by engineers and actively managed for improvements in both water quality and riparian habitat. The hyporheic zone is the region of sediment and porous space beneath and alongside a stream bed, where there is mixing of shallow groundwater and surface water. The flow dynamics and behavior in this zone (termed hyporheic flow or underflow) is recognized to be important for surface water/groundwater interactions, as well as fish spawning, among other processes. As an innovative urban water management practice, the hyporheic zone can be designed by engineers and actively managed for improvements in both water quality and riparian habitat. The assemblage of organisms which inhabits this zone are called hyporheos. The term hyporheic was originally coined by Traian Orghidan in 1959 by combining two Greek words: hypo (below) and rheos (flow). The hyporheic zone is the area of rapid exchange, where water is moved into and out of the stream bed and carries dissolved gas and solutes, contaminants, microorganisms and particles with it . Depending on the underlying geology and topography, the hyporheic zone can be only several centimeters deep, or extend up to 10s of meters laterally or deep. The conceptual framework of the hyporheic zone as both a mixing and storage zone are integral to the study of hydrology. The first key concept related to the hyporheic zone is that of residence time; water in the channel moves at a much faster rate compared to the hyporheic zone, so this flow of slower water effectively increases the water residence time within the stream channel. Water residence times influence nutrient and carbon processing rates. Longer residence times promote dissolved solute retention, which can be later released back into the channel, delaying or attenuating the signals produced by the stream channel . The other key concept is that of hyporheic exchange , or the speed at which water enters or leaves the subsurface zone. Stream water enters the hyporheic zone temporarily, but eventually the stream water reenters the surface channel or contributes to groundwater storage. The rate of hyporheic exchange is influenced by streambed structure, with shorter water flow paths created by streambed roughness . Longer flowpaths are induced by geomorphic features, such as stream meander patterns, pool-riffle sequences, large woody debris dams, and other features.