Irrigation activities are a major control on water movement and storage
in irrigated river valleys in the Intermountain West, USA. Particularly
in dry years, surface water diversions can deplete streams over the
summer irrigation season, leading to more variable stream temperatures
and increased risk for resident aquatic species. Cooler lateral inflows
derived from irrigation activities can mitigate the impacts of depletion
by buffering main channel stream temperatures. Given the increasing
susceptibility of depleted streams to climate and land use changes,
understanding stream temperature patterns and controls in these systems
is critical. We used intensive field monitoring over three summers and
thermal aerial imagery to characterize stream temperature patterns and
irrigation influences in a 2.5 km reach of a small agricultural stream
in northern Utah. Considering variable hydrology, weather, channel
morphology, diversions, and lateral inflows we found stream temperatures
to be relatively insensitive to flow depletion or lateral inflows in a
wet year but very sensitive in drier years. Irrigation-related lateral
inflows reduced longitudinal warming and diel variability during drier
years and at times prevented temperatures from reaching stressful or
lethal limits. Reaches with substantial lateral inflow contributions
also had a greater areal proportion of low temperatures and spatial
temperature diversity. These trends were enhanced by differences in
channel morphology, with greater spatial and temporal variability in
multi-thread than single-thread reaches. Study results highlight
critical flow and weather conditions driving increased temperature
variability that will likely become more extreme with additional climate
change related reductions in baseflow. Regardless of the cause, this
study highlights that decreased instream flows increase the importance
of identifying, quantifying, and maintaining lateral inflows to maintain
instream temperatures and preservation of these inflows should be
considered in future water management decisions.