Seasonal and storm dynamics of the hyporheic zone of a 4th-order mountain stream .1. Hydrologic processes

The objective of this study was to quantify fluxes of ground water and advected channel water through the shallow aquifer adjacent to a 4th-order mountain stream. A network of wells was installed from 1989 to 1992. Water-table elevations were measured seasonally and during storms. These data were used to calibrate MODFLOW, a 2-dimensional groundwater flow model. The fluxes of water through the subsurface were estimated from the head distributions predicted by the model for 8 steady state model runs bracketing the observed range in baseflow conditions, and for 1 transient simulation of a large storm. The overall pattern of subsurface flow changed little over the course of the year, even though the relative flux of advected channel water and ground water changed among seasons and during storms. Apparently the longitudinal gradient of the main valley, the location of the stream, and the influence of secondary channels determined the pattern of subsurface flows. Subsurface fluxes through a gravel bar were dominated by advected channel water but fluxes through the floodplain were dominated by ground water. Flow rates were positively correlated to estimated stream discharge during base-flow periods, but decreased slightly during storms because of precipitation inputs to the aquifer. The mean residence time of water stored within the aquifer was approximately 10 d for the gravel bar and 30 d for the floodplain during baseflow periods. Even though precipitation during the simulated storm equaled 12% and 23% of the water stored in the gravel bar and the floodplain, respectively, the mean residence time of water remained long.