The effects of river stage fluctuations on the hyporheic and parafluvial ecology of the Hunter River, New South Wales

Abstract

The hyporheic zone is the area of saturated sediments underlying many gravel-bed rivers where channel water actively exchanges with interstitial water. Through a series of biological, physical, and chemical filtration processes, the hyporheic zone influences the water quality of the surface stream. Lateral to the hyporheic zone is the parafluvial zone, the saturated area below gravel bars, which can have a similar filtration role. The ability of the hyporheic and parafluvial zones to act as filters largely depends on surface discharge. Fluctuations in discharge are needed to prevent the clogging of sediment pore-spaces, and to vary the rate at which nutrients and oxygen are transported into the hyporheic zone. Sediment packing, porosity and size, the amount of microbial and invertebrate activity, and stream topographical profile are other factors that control hyporheic filtration. Filtration efficiency is a measure of the rate at which dissolved nutrients and physico-chemical variables of a parcel of water are transformed during a period of interstitial flow. It can be gauged by measuring gradients of nutrients and physico-chemical variables along subsurface flowpaths... This study is the first broad-scale investigation into the hyporheic zone of any large Australian regulated river. It uncovered a rich invertebrate fauna, an active microbial biota, and significantly improves our understanding of how environmental flows benefit the hyporheic zone. In streams with strong connections to the aquifer, such as the Hunter River, hyporheic biological processes can be maintained through environmental flow releases in the surface channel. Controlled manipulations in river stage may be a useful means of improving surface water and groundwater quality through hyporheic and parafluvial filtration.