River ecosystem health down under: Assessing ecological condition in riverine groundwater zones in Australia

Below many rivers and extending laterally beneath their banks lies a zone o f saturated sediments-the hyporheic zone. This region is the site of dynami c exchanges of water and materials between the groundwater below, lateral a lluvial aquifers, and the river flowing above. In this centrally located ec osystem component, the actions of the microbial biofilms coating the sedime nts are considered to act like a biological filter, enhancing water quality during the exchange process. However, the hyporheic zones in many rivers a re threatened by siltation, toxicants and increasing acidity, physical extr action for gravel, or altered groundwater inputs through pumping. Current p rotocols to assess river health do not explicitly consider that of the hypo rheic zone, despite its central role in many rivers. This oversight is due largely to the relative infancy of the discipline of hyporheic research, li mited communication to river managers by ecologists, and technical difficul ties in sampling the hyporheic zone. Ecosystem health assessments in rivers with potentially important hyporheic zones could include measurements of t he extent of hydrological exchange between the river and the hyporheic zone at a range of scales, the rates of some key interstitial chemical processe s, and perhaps the biodiversity of hyporheic invertebrates. In Australia, t he potential of some of these measurements is being investigated, especiall y in relation to assessing the health of "groundwater-dependent ecosystems. "