DYNAMIC CHANNEL BAR EXCHANGE WITH AN EMPHASIS ON GROUNDWATER MOUNDING

Large-scale fluvial obstructions such as channel bars produce spatially and temporally variant gradients in hydraulic head and influence flowpath delineation, porewater fluxes, and temperature distribution. Hydrodynamic processes within channel bars, specifically groundwater and surface water exchange, are not well understood due to the difficulty of interpreting dynamic gradients and subsurface heterogeneities.

A network of piezometers was installed within and around a channel bar located within the Truckee River, a dense 6th order river network, primarily in northwest Nevada. Extensive instrumentation was deployed which included temperature loggers, pressure transducers, and stage recorders. Several methods were simultaneously utilized to quantify water and heat fluxes and to interpret the hydrodynamic processes through the channel bar. Water level measurements over time provided insight into the temporal scale and the spatial relationships of exchange.

Measurements of hydraulic head over the past year indicated a temporally constant groundwater mound (0.6 m above stage), followed by a groundwater depression (0.6 m below stage) along the length of the channel bar. The dimensions of the potentiometric surface appear to be constant irrespective of river stage and do not conform to the topography of the channel bar. Hydraulic conductivity estimates of the bar from several slug tests (Bouwer and Rice method) were less than 20 m d-1, with calculated average linear pore velocities between 0.1 to 3.7 m d-1. Estimates of pore-scale Reynolds number values, within the channel bar areas, were between 10-5 and 10-1. Near the channel bar and river boundary, the vertical head gradient can vary up to 0.1 over a diel period.

These results demonstrate that the potentiometric surface gradients do not change, however the complete ground water surface rises and falls with river stage. Further, low to moderate hydraulic conductivity, velocity, and Reynolds numbers suggest most exchange occurs near the groundwater and surface water interface. The fact that the groundwater mound persists throughout the year suggests that its potentiometric surface may not be controlled by infiltration and/or evaporation, but rather geologic structure.