A Thermal Mosaic for the Willamette River

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The U.S. Army Corps of Engineers operates 12 dams in the Willamette River Basin. The dams alter natural temperature and hydrologic regimes. Unnatural water temperatures can negatively impact all life stages of salmonid fish species. Water temperature in the mainstem Willamette River regularly exceeds the standard of 18.0 °C (64.4 °F) designated for salmon and trout rearing and migration for mid-May to mid-October upstream of Newberg as well as the 20.0 °C (68.0 °F) criterion designated for salmon and steelhead as a migration corridor downstream of Newberg. Tributaries and backwaters can be used by fish as cold-water refuges. To understand the extent to which temperature regimes of these off-channel conditions can be defined, it is necessary to determine how those off-channel conditions compare to conditions that occur in the main channel. This study will develop a critical piece of a “Thermal Mosaic” for the Willamette River that quantifies the water temperature variations in the mainstem Willamette River during spring and summer under varying streamflows.

Temperature graphs
Figure 1. Water temperatures in the Willamette River frequently exceed temperature standards intended to protect life stages of salmon and steelhead. (Public domain.)

During April-May of 2016, water managers (U.S. Army Corps of Engineers [USACE], Oregon Water Resources Department), fisheries management agencies (National Marine Fisheries Services, Oregon Department of Fish and Wildlife), and scientific researchers (U.S. Geological Survey, Oregon State University) met to discuss and review the current scientific understanding of how flow management affects native and resident fish in the Willamette River, especially threatened spring-run Chinook salmon (Oncorhynchus tshawytschya) and winter-run steelhead (O. mykiss). This workshop identified water temperature in the mainstem Willamette River as an inhibiting factor for multiple life stages of these salmonids. Figure 1 illustrates the regular exceedance of  Oregon Department of Environmental Quality temperature standards (red line) in the Willamette River.

Map of Willamette Basin
Figure 2.  Map showing the Willamette River Basin and stream network in western Oregon, along with the river reaches covered by the TMDL models (dark blue). (Public domain.)

To better understand temperature conditions in the Willamette River and its major tributaries, and to develop a temperature total maximum daily load (TMDL) for those rivers, a set of 2-dimensional hydrodynamic and temperature models (CE-QUAL-W2) of the mainstem Willamette River and its largest tributaries was constructed. The extent of the modeled reaches are shown in figure 2 by the darker blue lines on the stream network. After their development for the temperature TMDL, these models were verified and used in several studies to obtain a better understanding of the temporal and spatial temperature effects of the USACE-operated Willamette Project dams. The models are available and well documented, were originally constructed for the 2001–02 period, but have been applied under a range of measured and hypothetical flow and temperature conditions.

In response to extremely low flows and warm air temperatures during 2015, USACE asked USGS to assist in monitoring the temperature and water quality of off-channel areas associated with the middle and upper mainstem Willamette River (upstream of Newberg, Oregon). These data have helped to quantify the abundance of cold-water habitat in sloughs, side-channels, and seeps under varying flows during the summers of 2015 and 2016. However, 2015 monitoring data have not yet been analyzed within the context of the spatial and temporal variations in mainstem water temperatures throughout the middle and upper mainstem Willamette River. To understand the extent to which off-channel conditions can be defined and the areas used as cold-water refuge areas by fish, it is necessary to assess how those off-channel conditions compare to conditions that occur in the main channel. This study will develop a critical piece of the “Thermal Mosaic” for the Willamette River that quantifies the water temperature conditions and variations in the mainstem Willamette River during spring and summer under varying streamflow.