HYPORHEIC EXCHANGE AS A REGULATOR OF PERIPHYTON DYNAMICS

Hyporheic exchange in streams and small rivers has been an area of active research over the past few decades, in large part due to the importance of exchange effects on water quality. In contrast, the exchange of water and solutes between medium size rivers and their adjacent aquifers is less well studied, in part due to difficulties in adequate instrumentation. We are investigating mixing and exchange processes and the effects on periphyton dynamics within a 207-m long and 72-m wide channel bar in the Truckee River of northern Nevada. At the study site, the Truckee River has a mean flow rate of approximately 800 cfs (23 cms).

A preliminary assessment of the perimeter of the channel bar for anomalies in temperature, specific conductivity (SC), pH and dissolved oxygen (DO) has been completed. Groundwater discharge may be indicated in some locations by changes in river temperature, pH, and DO. Several locations displayed lower surface water temperatures than the mean. Similar locations were identified in a long term monitoring study by relatively constant and lower temperatures over time compared to surface water. Hydraulic head gradients have revealed dual groundwater mounds (Δh ~1.0 and ~1.2 m) at the upstream and the downstream portion of the channel bar, respectively. A mid-bar depression is present with an adjacent groundwater ridge (Δh ~0.6 m) connecting the two groundwater mounds. Manifestation of groundwater upwelling to the surface is being assessed through monitoring of periphyton distributions.