Effects of Dam-Induced Daily River Stage Fluctuations and Sedimentary Architecture of a Large Gravel Bar on Groundwater Flow Paths

The effects of dam-controlled river stage fluctuations on groundwater-surface water connection between persistent gravel bars and rivers are poorly understood. Hydraulic gradients in the hyporheic zone change as river stage fluctuates, thus affecting the magnitude and direction of water flow within riverbanks; but limited research has been conducted on river islands with regular and marked changes in stage. This research investigates the hydrogeology and sedimentology of Hornsby Bend Island (HBI), a large gravel bar with established riparian forest, located on the Colorado River southeast of Austin, Texas, which is subjected to regular storage and release of water. Questions driving this project are: (1) How do regulated stage fluctuations affect transient flow patterns in HBI? (2) What is the sedimentary architecture of the island and how does this control surface water-groundwater exchange? And (3) Is the hydraulic conductivity distribution in the island affecting the surface water-groundwater exchanges flow paths in the bar? Groundwater dynamics and sedimentary architecture of this large gravel bar were monitored by head distributions, groundwater flow characterization, mapping sedimentary units, and hydraulic conductivity measurements of the sediment. Piezometer nests were installed and instrumented with logging pressure-transducers to monitor the evolution of head distributions and groundwater flow paths through several cycles of dam release and storage, and correlated to river discharge from gauging stations upstream of the site and downstream of the dam. Sediments were collected from piezometer locations and analyzed for grain size. Preliminary results show that changes in head within the island follow changes in stage associated with dam release. More permeable sediment have larger changes in head than less permeable sediment, and have the greatest response to stage fluctuations. Groundwater temperature within the island also responds to stage fluctuations.