Hydraulic and thermal effects of in-stream structure-induced hyporheic exchange across a range of hydraulic conductivities

In‐stream structure‐induced hyporheic exchange and associated thermal dynamics affect stream ecosystems. Their importance is controlled by spatial variability of sediment hydraulic conductivity (K). We calibrated a computational fluid dynamics (CFD) model of surface and groundwater hydraulics near a channel‐spanning weir (represents log dams, boulder weirs) to field data and varied K from 10−7 to 10−2 m/s (silt to gravel). Surface water stopped cresting the weir for K > 10−3 m/s. Non‐Darcy hyporheic flow was also prevalent for K > 10−3 m/s, and velocity errors using non‐CFD models ranged up to 32.2%. We also modeled weir‐induced heat transport during summer. As K increased from 10−7 to 10−3 m/s, weir‐induced hyporheic heat advection steadily increased. Cooling and buffering along hyporheic flow paths decreased with increasing K, particularly above K = 10−5 and 10−4 m/s, respectively. Vertical heat conduction between surface water and groundwater near the weir decreased with increasing K, particularly for K > 10−5 m/s. Conduction between hyporheic flow paths and adjacent groundwater helped cool hyporheic flow. Downstream surface water cooling by hyporheic advection increased steadily with K as increases in hyporheic flow overwhelmed decreases in cooling along hyporheic flow paths. Yet such effects were small (0.016°C) even at K = 10−3 m/s. The largest thermal effect of weir‐induced exchange was therefore spatial expansion of subsurface diel variability (particularly for K > 10−5 m/s) which affects benthic habitat and chemical reactions. The specific values of K where such trend shifts occur is likely variable among streams based on flow conditions, but we expect the presence of such trend shifts to be widespread. Key Points Hyporheic exchange is strongly controlled by hydraulic conductivity (K) Weir‐induced hyporheic water and heat exchange vary most at high K Largest thermal effect of exchange is increased subsurface heterogeneity