FLUID FLOW AND HEAT TRANSPORT WITHIN THE HYPORHEIC AND RIPARIAN ZONES OF A REGULATED RIVER: COLORADO RIVER, AUSTIN, TX

A series of dams regulate the river stage and discharge of the lower Colorado River in Austin, Texas with scheduled daily releases. The releases from the Tom Miller Dam, 23 km upstream from the study site at Hornsby Bend, cause the stage to fluctuate by more than 1.5 m at the site. At Hornsby Bend, the river transitions from a regionally gaining river to a river that both gains and loses on a daily basis due to the stage fluctuations. The altered surface water – groundwater exchanges were characterized throu­gh detailed monitoring of temperature and head in the river, the streambed, and the adjacent bank along a transect perpendicular to the river on two separate occasions for three days. This lateral transect was composed of nineteen vertical profiles spaced one meter apart and eleven piezometers. Each profile was monitored with thermistors at depths of 10, 20, 40 and 80 cm in the streambed. When the Colorado River is losing to the aquifer below, heat from the flooded warmer river penetrates to a depth of approximately 50 cm. When it is gaining, upwelling groundwater limits the penetration of the river’s temperature signal to less than 20 cm.

The temperature, water table elevation, and pressure data consistently showed that river-groundwater exchange, regardless of whether the river is gaining or losing, is largest close to the bank and decreases with distance from the bank. This decrease in vertical groundwater exchange to or from the river with distance from the bank is consistent with theory. Further field studies supplemented by coupled fluid flow and heat transport simulations are needed to fully understand the impacts of dam regulation on the hydrologic, thermal, and ecologic dynamics of rivers and their hyporheic and riparian zones.