Abstract Title: Shallow vs Deep Groundwater Discharge Influences the Thermal Stability of Streams: A Continental-Scale Analysis

Abstract Submitted to: HYDROLOGY

Abstract Text:

Groundwater discharge to streams sustains sensitive aquatic ecosystems throughout stream
networks, as these zones are essential to maintaining thermal stability and water quality.
However, the depth of the contributing groundwater flow path can affect the vulnerability of
these ecosystems to landscape and climate modifications. Here we use paired multi-year air-
water temperature signals to categorize 1786 varied size stream sites across the continental
United States. These categories parse streams with strong shallow or deeper groundwater
connectivity from those with reduced groundwater connectivity and sites with thermal regimes
likely influenced by major upstream dam operation. Nationally, 40% of stream sites were
identified as having clear groundwater influence, of which approximately 50% are dominated by
shallow groundwater. Of those shallow groundwater sites with a longer-term temperature
records (14-30 years) we observed a long-term warming at a similar proportion to sites with
reduced groundwater contributions. Sites with deep groundwater signature tended to have long
term temperature records that were stable or cooling. When compared to watershed
characteristics streams with thermal regimes closely coupled to air temperature were associated
with low watershed slope and high watershed impervious coverage, indicating groundwater
disconnection in human-modified landscapes. These results demonstrate that the vulnerability
of groundwater-influenced streams to warming depends on the relative flow path depth of the
source groundwater requiring unique management strategies.