HYPORHEIC FLOWS IN A RIPARIAN TERRACE OF A COASTAL TEMPERATE RAINFOREST RIVER

摘要

Research in the last decade has confirmed the existence and importance of the hyporheic (subsurface) zone, a previously unrecognized river subsystem. Hyporheic zones exist where rivers flow through porous gravel that allows high infiltration rates of surface water. These conditions are the norm for most western Washington rivers, suggesting that hyporheic processes are important controls on river ecosystem function in this region. We have been studying the seasonal pattern of subsurface hydrology beneath a floodplain terrace in the Queets River (Olympic National Park, Washington) since summer 1998. Depending upon flowpath location and path, hyporheic water flows beneath a variety of overlying riparian soils whose available carbon and nutrient pools vary widely with the successional stage and age of the overlying vegetation. During periods of low flow, flowpaths occur as matrix flow through the terrace. In contrast, during periods of high flow, flowpaths are focused and water flows along preferential flowpaths. This interaction between subsurface hydrology and riparian soils results in a patchwork mosaic of nutrients that support subsurface secondary production and ultimately control the flux of metabolic endproducts delivered to the surface stream. Preserving terrestrial-aquatic linkages via subsurface flow is important for maintaining surface productivity and other river ecosystem functions.