Seasonal and Inter-Annual Variability in Hyporheic Water Quality Revealed by Continuous Monitoring in a Salmon Spawning Stream

Over a 3.5 year period, levels of dissolved oxygen (DO) saturation were continuously monitored in surface waters and at depths of 150 and 300 mm in the hyporheic zone of a riffle in a montane stream where Atlantic salmon spawn. Throughout this period, DO in surface waters remained close to 100% saturation, but exhibited daily variations related to CO2 cycling driven by diurnal patterns of respiration and photosynthesis. However, in the hyporheic zone, variations were much more dynamic over storm event, seasonal and inter-annual timescales. At 300 mm, DO saturation was generally close to 100% during summer low flows, though levels occasionally fell during warm periods which appeared to be related to diffusion gradients caused by benthic respiration. Such DO decreases at low flows were much more common and marked at 150 mm. During wetter conditions, DO saturation at 300 mm fell to zero for prolonged periods; this is consistent with increased fluxes of groundwater discharging through the hyporheic zone. During the wettest periods this also affects DO saturation at 150 mm. However, during hydrological events, hyporheic water quality is 're-set' as head reversals cause streamwater ingress which results in transient periods of re-oxygenation, which end during the hydrograph recession. This is consistent with stream-ward hydraulic gradients being re-established in riparian ground water as the stream stage falls. The connectivity between groundwater and streamwater through the hyporheic zone is driven by climatic conditions and is reflected in marked inter-annual variability in water quality characteristics. In some cases, this variability may have implications for the ecology of the hyporheic environment-including the survival of salmon eggs-particularly if oxygen levels are affected. Copyright (C) 2009 John Wiley & Sons, Ltd.