Observed temporal dynamics and thresholds of vertical hydraulic gradients in the bed of a mountain stream during seasonal streamflow recession, Fraser Experimental Forest, Colorado.

It has generally been held that low-flow conditions are most amenable to characterizing hyporheic exchange in mountain streams. However, there are few continual characterizations of exchange or exchange potential in streams, which could support this notion. Metal drivepoints and water level dataloggers (sampling at a 10 min interval) were deployed in steps to determine the changes in vertical hydraulic gradient (VHG) over the 25 cm of streambed depth throughout the summer, 2006 in lower Fool Creek, Fraser Experimental Forest, Colorado. During this period (late June to September), a nearby meteorological station recorded more than 15 individual rain events, ranging from minor (<2.5 mm) to moderate (2.5-10 mm) storms, with the largest being 18.6 mm and 25.4 mm. We found that VHG varied in response to 1) diurnal fluctuation of stream discharge, 2) abrupt changes in discharge from storms, and 3) seasonal streamflow recession. In one particular step, VHG was always negative (indicating downwelling potential), and response to minor storms (<2.5 mm) was minor, increasing ~0.05 cm/cm (i.e., decreased downwelling potential), enough to interrupt diurnal fluctuations. In response to larger storms, VHG in this step increased abruptly, by as much as 0.55 cm/cm, following the response of stream discharge. At the seasonal time scale, VHG in this step generally decreased throughout discharge recession, until mid-July when a more stable condition was established. These results 1) support the notion that hyporheic exchange is best characterized under low flow conditions, and 2) show that groundwater-surface water exchange in response to storms is rapid, affecting the magnitude and spatial distribution of hyporheic exchange fluxes.