Heat Transport variability Across the Streambed of a Large, Regulated River Subject to Hydropeaking

Dams affect over half of the Earth’s large river systems. Storage of water and its release for peak demand (hydropeaking) changes the thermal regime of the river and impacts the surface and groundwater interactions downstream of a dam. Temperature is an important ecological variable influencing fish, invertebrates, microbial communities and nutrient processing, and can also be used as a tracer of groundwater flow in sediment. Despite this importance, little is known about how dams affect temperatures downstream across the river corridor, particularly temperatures in the subsurface. This study investigates the impact on thermal regime and surface and groundwater interactions hydropeaking has on a 4th order dam-regulated river on several spatial scales. Two transects of thermistors recorded temperature gradients in the riverbed over the course of several flood pulses at 5-minute intervals. One transect was across the channel spanning the 68 m from bank-to-bank and the other was along the bank. The cross-channel transect additionally had piezometers with instruments collecting temperature, pressure and electrical conductivity to corroborate temperature measurement interpretations. The findings were that near-bank and in-channel temperature profiles respond differently to hydropeaking. Hydropeaking reverses head gradients daily near the bank and cause the river to fluctuate between gaining and losing water on hour timescales while in the channel, gradient reversals do not occur. Near the bank, stage increases causes warmer
surface water to penetrate into the subsurface and during the receding limb, cooler groundwater upwells as the river returns to base flow conditions. Temperature ranges near the bank in the subsurface exceed those observed in the stream. Flux and gradient reversals localized at the bank explain temperature distributions in the streambed sediments. Temperature differences and ranges near the bank in the subsurface cannot be explained by conduction alone and advective heat transport through groundwater flow provides a mechanism that explains the temperature distributions through time. Evidence from pressure and temperature sensors moving downstream along the bank verify this effect is not only localized to the cross channel transect. These measurements serve as observational evidence for the impact loading from rapid stage change has on subsurface sediments preventing the reversal of pressure gradients in the channel while causing them near the bank. This impact is analogous to tidal fluctuations and has been show in modeling in both marine and freshwater environments.