22-3 Surface Water/Groundwater Exchange in a Multichannel River Floodplain in the White Mountains of New Hampshire

Session: Advances in characterizing groundwater, surface water, and their interactions (Posters)

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Abstract:

The connection between groundwater and surface water through hyporheic exchange can moderate decreases in surface water flow and recharge riparian groundwater stores during low flow periods. The recently restored cobble-bedded Zealand River (44.2461°N, 71.4968°W) in Bethlehem, New Hampshire, has multiple anastomosing channels that may augment groundwater exchange, although the extent of this gain is unknown. We installed surface and groundwater pressure transducers, allowing us to collect continuous records of water surface elevation in the river channels and floodplain from June to September 2025, which we correlated with measured stream discharge to estimate total flow volume at points along these channels throughout the period. Over the length of this reach, total flow volume decreased in the eastern channel and increased in the western channel. These discharge measurements, along with aerial thermal imagery, dilution gauging, temperature records, and conductivity observations, indicate substantial surface water/groundwater exchange. Field observations were used to develop connected groundwater and surface water flow models, which provide a reach-scale estimate of flow patterns and exchange rates within the floodplain of the lower Zealand River. Hydraulic gradients were generally stable in both magnitude and direction over the study period, and water levels remained relatively constant. This suggests that, despite the onset of intense regional drought, surface water provided continual recharge of groundwater, and groundwater supplies were robust enough to continually discharge to surface water, sustaining riparian water availability. As hydrologic extremes continue to increase in frequency, magnitude, and duration, surface water and groundwater exchange becomes an even more important component of water level stabilization, and also contributes to river management and restoration due to its impact on aquatic habitat, temperature, and water quality.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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