Flows hot and cold: long-term evidence of rapid river water temperature fluctuations across the conterminous United States

River water temperature (T_w) regimes are fundamental to freshwater ecosystem health and socioeconomic activities. Most T_w research has focussed on magnitudes and averages, but rates of thermal change have been drastically understudied. Rapid T_w increases (‘surges’) or decreases (‘plummets’) have been observed across individual catchments and short-term periods, but remain poorly characterized across broader space-time domains. To address this, we collated high-resolution T_w data spanning the conterminous United States (US) between 2008-2023. We demonstrated the national-scale prevalence of surges (n = 6507) and plummets (n = 4787) that were recorded at 88 of the 102 monitoring stations. Both event types spanned freezing (snowmelt-fed systems) to extremely hot (>40 °C—geothermal influences) conditions. Most surges and plummets exhibited constrained durations (<1 h), amplitudes (≈1 °C) and rates of change (≈Δ1 °C/15 min), but some reached 24 h, 18.8 °C, and Δ11.3 °C/15 min, respectively. Successive transitions between rapid T_w warming and cooling occurred predominantly in regulated systems, indicating dam-induced T_w volatility. Surge and plummet characteristics differed between US climate regions. Such events were less widespread and frequent in the Northwest and West, with surges here most often occurring during regional droughts and heatwaves. Plummets recurred most consistently in the Southwest during summer months, and were also most common during notable hot and dry periods. Surges were most prevalent in space and time across the Southeast, and again were most common in summer. Surge and plummet counts were less variable year-to-year in the Northeast US, but significantly decreased across the study period. This research provides a critical step in characterizing rapid T_w changes across broad spatial and temporal scales, thus opening prospects for future research exploring how varying catchment properties and hydroclimatic gradients govern surge and plummet dynamics. Such insights are critical for informing evidence-based management solutions targeting extreme T_w variations and volatility.