Unravelling environmental drivers governing rapid warming and cooling in rivers spanning the conterminous United States

Abrupt river water temperature (Tw) transitions are a key driver of freshwater ecosystem health as this governs how quickly and readily biota can adapt to shifting thermal conditions. However, despite rapid Tw increases (‘surges’) or decreases (‘plummets’) occurring prevalently across various river typologies globally, they remain poorly explored. Most research to date has isolated hydropower, snowmelt and summer storm controls on surges and plummets in individual catchments, but multiple environmental drivers governing such rapid Tw changes across broad geographic domains have been seldom explored. To address this gap, we leveraged high resolution Tw data from 77 locations spanning the conterminous United States, whereby 5159 surges and 4020 plummets (∆±1 °C in a 15-minute period) were identified between 2008-2022. Subsequently, we quantified the effects of natural and anthropogenic environmental controls on surge and plummet tallies. For this, hydropower activity was identified a key driver nationally, as evidenced by its influence at the two monitoring stations exhibiting by far the highest number of surges and plummets (n = 950-1608). Catchment-wide urban cover and snowmelt influences were more consistently associated with surges and plummets, respectively. The same environmental drivers were also tested against different event-based Tw metrics derived for each surge / plummet. For this, climate exerted significant effects on Tw magnitudes and averages (i.e., minimum, mean and maximum Tw), while catchment influences like hydropower and geology yielded stronger influences on Tw variations occurring within each event (e.g., maximum rate of change, the number of Tw increases and decreases). This paper provides a better understanding of hydroclimatic and river catchment conditions governing surges and plummets. Such evidence could help inform management interventions by targeting river environments most sensitive to rapidly fluctuating Tw regimes, which could become more volatile with a changing climate.