42-9 Resolving Apparent Losing–Stream Behavior with Diel Air–Water Temperature Signals

Abstract:

Surface water–groundwater interactions in low-gradient streams draining glaciated terrain remain poorly constrained, particularly where low-permeability diamicton limits groundwater flux while evapotranspiration strongly influences watershed water balance. Previous investigations of a third-order stream in central Illinois documented persistent upward hydraulic gradients from the surficial aquifer to the stream, indicating groundwater discharge. However, streamflow decreases along the study reach, suggesting apparent losing-stream behavior despite gaining hydraulic conditions. Resolving this contradiction is critical for characterizing groundwater flow systems and associated solute transport.

This study uses diel air–water temperature signals to independently evaluate groundwater influence on the stream. Continuous temperature and hydraulic head data were collected at 15-minute intervals from the stream and streambed wells at upstream and downstream locations. Temperature records were reduced to daily maximum air and water temperatures and analyzed using paired air–water signal metrics, including amplitude ratios, to quantify thermal coupling and evaluate spatial and temporal variations in groundwater influence.

Results indicate that groundwater influences stream temperature year-round, although the magnitude of influence varies between locations and through time. Groundwater control on stream temperature increased from late May through early August and decreased during storm events, reflecting shifts in streamflow generation mechanisms. The thermal analysis confirms groundwater upwelling throughout the study reach. Consequently, the observed downstream reduction in discharge is more consistent with evapotranspiration-driven water loss than with seepage from the stream to the aquifer. These findings demonstrate the value of integrating thermal methods with conventional hydraulic measurements to resolve apparent inconsistencies in stream classification and improve understanding of groundwater–surface water exchange in low-gradient glacial landscapes.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.