HYPORHEIC FLUX AND STREAMBED DYNAMICS IN A POSTGLACIAL SYSTEM

The exchange of water between streams and groundwater systems is governed by the sedimentary environment of the channel bottom and hydraulic gradient, which are themselves a product of watershed setting and climate. The setting of Central Maine’s landscape exhibits postglacial conditions sculpted by the Laurentide ice sheet that retreated from the region 10 to 15 thousand years ago. Sediment transport in this system is more dependent on local geomorphology and is typically low compared to settings south of the last glacial maximum’s extent. Using a combination of field measurements, spatial data analysis, and numerical modeling this research addresses the link between stream setting, bed dynamics, and hyporheic flux in Maine’s postglacial setting.

We instrumented three streams in the Lakes Region of Central Maine to monitor sediment transport and hyporheic exchange from the spring of 2015 through the fall of 2016. Monitoring efforts included sediment tracers, scour chains, and repeat facies mapping and bulk sampling to estimate sediment transport dynamics. Site instrumentation also included multiple piezometers to evaluate the vertical heat profile in channel bed sediments. Heat flux through the channel bed is used as an indirect measure for the vertical flow of water using the principle that the advection of heat by water moving through channel bed sediment controls the subsurface temperature distribution.

Comparison of sediment dynamics between our study reaches indicates conformity to previous research demonstrating the dominate importance of local geomorphic conditions to bed sediment transport in this postglacial environment. Sediment entrainment predictions based on hydraulic conditions and grain size distributions estimate stability during sub-annual flows. Field measurements and qualitative observations indicate that local geomorphology produces much higher than predicted transport at one study site and much lower than predicted at another. Transport data and observations are being coupled with numerical modeling results of hyporheic flux using temperature data from these sites. Ultimately, these data will be analyzed relative to watershed modelling results of flow conditions under varied climate and land cover conditions.