Hyporheic temperature dynamics and heat exchange near channel-spanning logs

The flow of river water around large woody debris (LWD) creates pressure gradients along the riverbed that drive river-groundwater mixing, or hyporheic exchange, and heat transport within the hyporheic zone. We quantify hyporheic fluid and heat exchange induced by current interaction with channel-spanning logs using two approaches: laboratory flume experiments and numerical simulations that link turbulent open-channel fluid flow, porous fluid flow, and heat transport. Flume and numerical experiments show that logs produce a characteristic diel temperature pattern within sediment that shifts with log blockage ratio (a fraction of the channel depth blocked by the log), channel Froude number, and sediment permeability. Upstream of a log, downwelling water transports the river's diel thermal signal deep into the sediment. Downstream, upwelling water from shallow flow paths has a thermal signal similar to the surface water, while upwelling water from deep flow paths forms a wedge of buffered (low-amplitude) temperatures. Since most hyporheic water emerges from shallow flow paths, upwelling water has limited potential to buffer surface water temperature. Because of historic channel clearing practices, modern rivers have unnaturally low densities of LWD. A key implication is that LWD removal has contributed to thermal homogenization and potential degradation of hyporheic habitats. LWD reintroduction is a promising strategy to improve vertical connectivity in rivers and increase thermal patchiness within the hyporheic zone. However, hyporheic exchange near LWD may not impact diel surface water temperatures at the reach scale.