07 Sep 2022
07 Sep 2022
Status: this preprint is open for discussion.

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

Stefan A. Talke1, David A. Jay2, and Heida L. Diefenderfer3,4 Stefan A. Talke et al.
  • 1Civil and Environmental Engineering, California Polytechnic State University, San Luis Obispo, California
  • 2Civil and Environmental Engineering, Portland State University, Portland, Oregon
  • 3Coastal Sciences Division, Pacific Northwest National Laboratory, Sequim, Washington
  • 4School of Environmental and Forest Sciences, University of Washington, Seattle, Washington

Abstract. Using archival research methods, we found and combined data from multiple sources to produce a unique, 140 year record of daily water temperature (Tw) 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 Tw for 1850–2020. The model simulates the time-lagged response of Tw 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 Tw 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 (Tw 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 Tw 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 Tw to climate change, posing additional stressors on fauna.

Stefan A. Talke et al.

Status: open (until 02 Nov 2022)

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Stefan A. Talke et al.

Stefan A. Talke et al.


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Short summary
This manuscript uses archival measurements and a statistical model to show that water temperatures in the Willamette River have trended upwards since 1850, with the largest increase occurring in winter and the smallest in late spring. Approximately 30 % of the increase is attributable to system changes, and 70 % to warming air temperature (climate change). The number of warm water days has significantly increased, and near freezing conditions, common his-torically, no longer occur.