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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5216

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Conclusions

Estimates of vertical flux across the sediment–water interface were made by direct measurement using seepage meters and a heat as tracer method in a 100-m reach of the lower Merced River. Results of the temperature modeling efforts indicate that the Merced River at the study reach is generally a slightly gaining stream with very small head differences between surface water and ground water, and has flow reversals that can occur during high streamflow events. The period of study included a large range of streamflow that affected the streambed characteristics and hydraulic conductivity of the streambed. The high streamflow events associated with storm runoff events and large releases at the upstream dam resulted in an increase in hydraulic conductivity that probably was due to the scouring of fines accumulated during periods of low streamflow. The application of heat as a tracer method resulted in average vertical flux estimates across the sediment–water interface of 0.4–2.2 cm/day for the study period. The range of vertical hydraulic conductivities used in heat as a tracer model (VS2DH) was 1–10 m/day at the study reach.

The use of seepage meters to directly measure vertical flux generally failed in this high-energy system because of slow seepage rates and a mobile streambed that scoured or buried the seepage meters. Slug tests and grain-size analysis were directly compared to characterize the streambed hydraulic conductivity. Lateral hydraulic conductivities determined using slug tests ranged from 40 to 250 m/day at the upstream transect and from 10 to 100 m/day at the downstream transect. The locations of screened intervals likely explains variability in range of estimates. Variability in hydraulic conductivity for the grain-size method ranged from 50 to 110 m/day and 20 to 70 m/day for the upstream and downstream transects, respectively. The relative percent difference between hydraulic conductivity estimates by grain size and slug test for wells screened at 0.5 m below the sediment–water interface was 0–9 percent at the upstream transect and 80–133 percent at the downstream transect.

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