OBSERVATIONS OF HYPORHEIC ZONE PROCESSES DURING SHORT-TERM, DAILY WATER RELEASES ON THE DEERFIELD RIVER IN CHARLEMONT, MASSACHUSETTS

The region located directly beneath the sediment in a stream bed where groundwater and surface water mixing occurs is known as the hyporheic zone. The transient response of the hyporheic zone is not yet clearly understood, however, it is known that critical reactions occurring in these areas shape both ground and surface water chemistry. In this study we use short-term river stage fluctuations characterize the transient nature of the hyporheic zone during strong changes in hydraulic gradients.

River stage changes typically take place over long time scales, as stream head changes naturally occur once or several times per year (for example, during spring snow melt or drought seasons). In contrast to a natural system, the flow at study site, located on the Deerfield River in Charlemont, MA, is regulated by an upstream dam. Dam releases occur daily, raising the river stage 1-2 ft over a 3-hour time period. After this release, the river stage drops down to average flow rates before the next release the following day. This allows the unique opportunity to monitor hyporheic zone response on short time scales that are otherwise difficult to measure.

Shallow groundwater, surface water, and release water were defined independently as they have differing magnitudes of dissolved oxygen, electrical conductivity and temperature. The depth of the hyporheic zone was located by installing piezometers at various depths within the riverbed. Each piezometer was sampled throughout the stage rise and recession. Through independent measurements of hydraulic head, we conclude that the vertical hydraulic gradient reverses directions. Before the release, the surface water is gaining water from the groundwater and, after the release, the stream loses water to the groundwater. Additionally, the hyporheic zone shrinks and swells with the reversal of the hydraulic gradient and concentration gradients when stream water enters the shallow sediment. These observations are important for stream and resource management decisions as well as understanding the dynamic response of these systems to perturbations.