Heat transfer theory suggests that floodplain soils in dryland riverine ecosystems can be cooled by hyporheic flows generated during spring floods. I compared soil temperature cycles and associated hydrologic factors on a free‐flowing river to those on a river where flows and surface water temperatures are now regulated. Spring surface water temperatures were comparable on the 2 rivers, as was apparent diffusivity of the soil under mature Populus fremontii in a year when severe drought produced similar soil moisture regimes. Over 9 years of monitoring, mean annual maximum soil temperature was higher on the regulated river than on the free‐flowing river (10 cm depth: 33 vs. 23 °C; 40 cm depth: 30 vs. 20°C, respectively), and sinusoidal models of the annual temperature cycle at each depth indicated higher means and greater amplitudes on the regulated river. The annual maximum soil temperature was inversely related to peak flood discharge on the free‐flowing river but not on the regulated river. Temporal shifts in the lag between diel cycles at 40 and 10 cm depths—an index of soil thermal diffusivity—suggested that the capillary fringe is strongly involved in heat exchange. An increase in the lag during some water table declines suggested that shallow soils may undergo flood‐induced evaporative cooling. Hyporheic recharge can be an ecologically important determinant of growing‐season soil temperatures at plant rooting depth in dryland river floodplains. Reductions in spring flood magnitude due to river regulation, water abstraction, or climate change can increase these temperatures and thereby alter ecosystem structure and functioning.
