A COMPARISON OF METHODS TO DETERMINE THE RATE OF HYPORHEIC EXCHANGE FLOW

We are investing the influence of hyporheic exchange flow on stream temperature. An obstacle, however, is that the flow rate in the hyporheic zone in small headwater streams is difficult to measure. Here, we compare three approaches to quantify hyporheic exchange rates for a 200-meter reach including calculations based on hydraulic gradients in the streambed, one-dimensional injection tracers, and vertical temperature profiles. The study reach, Dry Creek, a small (1-5cfs) riffle-step-pool mountain stream flowing through the granitic Idaho Batholith of the Boise Front. Dry Creek is a small, high-energy, perennial mountain stream with a highly permeable bed. The vertical hydraulic gradient between the water in the hyporheic zone (30cm below the stream) and the stream is used to determine flow into riffle sections and returning to a downstream pool. A chloride injection experiment at several locations is used to assess transient storage in pools and the hyporheic zone. Time delay of diurnal variation of vertical temperature below the stream was used to determine advection, with the assumption that the heat transfer by conduction is relatively small. All three methods have been used in similar environments and stream to characterize exchange flow between the stream and the saturated sediment beneath the stream.