Historical (1900s-1940s) and present-day (2003 – 2023) spring Chinook salmon (Oncorhynchus tshawytscha) spawning distribution in the Willamette River Basin, Oregon

Spawning distribution maps for Upper Willamette River (UWR) spring Chinook salmon were created for the historical period (approximately 1910-19 40), and for the present-day period (2003-2023). These maps were created as a tool to visualize generalized, reach-scale patterns of spawning distribution before and after the construction of high-head dams and other large-scale anthropogenic river alterations in the Willamette River Basin. The area of interest was the Willamette River and its major salmon spawning tributaries draining the Cascade Range. The historical spawning distribution dataset was derived from qualitative and descriptive information collected from habitat surveys in the early 20th century (Mattson, 1948; Myers and others, 2006; Willis and others, 1960). The historical spawning maps depict spawning distributions prior to construction of many Willamette Basin high-head dams (which spanned 1911-1969), however many other alterations to fisheries populations and habitats had occurred by the 1940s (LaVigne and others, 2008), hence the historical data does not represent conditions prior to Euro-American settlement. The present-day spawning map was derived from qualitative spawning data from 2003 – 2005 and quantitative spawning data from 2006-2023 (Schroeder and others, 2008; Sharpe and others, 2017; Lindsey, Morasch, and Paulsen 2019, 2020; CFS, 2023; EAS, 2024). The present-day spawning maps depict generalized distributions and quantities of spawning for the period 2006-2023, approximately 50 years after completion of dam construction. However, year-to-year conditions can vary substantially based on environmental conditions, management actions and other factors.