THE FAMILIAR AS A FRONTIER: PERSISTENT TRANSIENT STREAM-GROUNDWATER INTERACTIONS

Processes occurring along the interface between streams and groundwater are key components of stream, hyporheic zone, riparian zone, and aquifer physical and biogeochemical functioning. Most previous studies of stream-groundwater interaction (SGWI) investigated steady-state or quasi-steady state regimes while few analyzed the effects of seasonal perturbation but with synoptic static measurements. In this talk, we present cases where regular and short-term river stage fluctuations profoundly impact SGWI, as well as a case where it does not. At the first study site on the Colorado River, 10 miles downstream of Austin TX, river stage regularly fluctuates by more than a meter within a 24-hour cycle due to hydroelectric dam releases. This causes the river, which receives regional baseflow, to transition between strongly gaining and strongly losing within the same day. We monitored water table elevation, temperature (T), and electrical conductivity (EC) in the riparian zone and in an island. We profiled head, T, and EC vertically along a lateral transect of the streambed during several cycles. The results show rapid pumping of water in and out of the island, the sub-channel hyporheic zone, and the riparian zone. A cubic meter of water goes in and out of the riparian zone per square meter of bank per day. At one location, the vertical upwards fluxes along the streambed range from 5 m/d to -4.5 m/d over one dam-release cycle. About 5,000 cubic meters of groundwater move in and out of the island during the same cycle. This rapid movement of water is consistent with pronounced changes in T and EC in these zones. At this site, natural SGWIs and any related ecosystem services have been drastically altered. On the other hand, observations from a dense network of piezometer clusters within a pointbar of an experimental meandering river indicate uniformity of hyporheic flowpaths across a flood cycle.

The physical processes we investigated exert first-order control on ecological and biogeochemical processes. The coupled mechanical and biogeochemical effects of persistent periodic SGWI should be a focus of future research as more than half of major rivers are dammed and all experience natural processes (e.g., ET and snowmelt) that may induce regular fluctuations in stage and discharge.