Warming of the Willamette River, 1850–present: the effects of climate change and direct human interventions

Abstract. Using archival research methods, we found and combined data from multiple sources to produce a unique, 140 year record of daily water temperature (T_w) in the lower Willamette River, Oregon (1881–1890, 1941–present). Additional daily weather and river flow records from the 1850s onwards are used to develop and validate a statistical regression model of T_w for 1850–2020. The model simulates the time-lagged response of T_w to air temperature and river flow, and is calibrated for three distinct time periods: the late 19th, mid 20th, and early 21st centuries. Results show that T_w has trended upwards at ~1.1 °C /century since the mid-19th century, with the largest shift in January/February (1.3 °C /century) and the smallest in May/June (~ 0.8 °C /century). The duration that the river exceeds the ecologically important threshold of 20 °C has increased by ~20 days since the 1800s, to ~60 d yr^-1. Moreover, cold water days below 2 °C have virtually disappeared, and the river no longer freezes. Since ~1900, changes are primarily correlated with increases in air temperature (T_w increase of 0.81 ±0.25 °C) but also occur due to increased reservoir capacity, altered land use and river morphology, and other anthropogenic changes (0.34 ±0.12 °C). Managed release of water influences T_w seasonally, with an average reduction of 0.27 °C and 0.56 °C estimated for August and September. System changes have decreased daily variability (σ) by 0.44 °C, increased thermal memory, and reduced interannual variability. These system changes fundamentally alter the response of T_w to climate change, posing additional stressors on fauna.