Temperature Effects of Point Sources, Riparian Shading, and Dam Operations on the Willamette River

Models

North Santiam R. at Mill City, OR (Photo by I. Wigger, 4-Jun-2002)

The Willamette flow and temperature models were constructed using CE-QUAL-W2, a two-dimensional (longitudinal, vertical) model from the U.S. Army Corps of Engineers (Cole and Wells, 2002). CE-QUAL-W2 is a physically based mechanistic model that simulates gravity- and wind-driven flow through a network of interconnected river channels or reservoir reaches by using channel geometry and slope, bottom friction, wind shear, density effects, and upstream/downstream flow or water-level data. Algorithms to calculate the effect of hydraulic structures such as weirs, pumps, and spillways are included. Horizontal and vertical velocities, flow, and stage are simulated.

Water temperature is modeled in CE-QUAL-W2 by using a detailed expression of the energy budget of the water body. The model includes algorithms to calculate the effects of both topographic and vegetative shading. Using latitude, longitude, time of day, and the water body's orientation, the model determines at each time step the presence or absence of a topographic or vegetative shadow on the water surface, the length of any shadow, and the degree to which that shadow shields the water body from solar radiation. Model inputs include meteorological data, topographic shading angles, tree-top elevations, distance to the vegetation, and solar-reduction factors associated with the riparian canopy that vary by location. This detailed representation of the heat budget and the effects of riparian shading was one of the major reasons that ODEQ chose to use CE-QUAL-W2 for the Willamette temperature TMDL analysis.

In addition to modeling flow and water temperature, CE-QUAL-W2 can simulate many water-quality constituents. CE-QUAL-W2 has open source code, good documentation, and a large user community. In addition, it has a long history of successful application to a wide range of lake, reservoir, estuary, and river systems (Cole and Wells, 2002). USGS users have found that CE-QUAL-W2 is capable of simulating water temperature with a mean absolute error of 0.5 to 1.0^oC (Bales and others, 2001; Green, 2001; Rounds and Wood, 2001; Sullivan and Rounds, 2005 and 2006).

The Willamette modeling suite is composed of nine submodels. These models can be linked together by passing the output of any upstream models to the input of downstream models. Such connections can be made using filters and scripts so that the linkages are automatic and transparent. The nine submodels include:

• Lower Willamette River, with connections to the Columbia River
   
• Middle Willamette River, from RM 26.5 (Willamette Falls) to RM 85.5 upstream of Salem
   
• Upper Willamette River, as far upstream as the confluence of the Coast and Middle Forks
   
• Clackamas River, the lower 26 miles downstream of River Mill Dam
   
• Santiam and North Santiam Rivers, downstream of Big Cliff Dam
   
• South Santiam River, downstream of Foster Dam
   
• Long Tom River, downstream of Fern Ridge Dam
   
• McKenzie River as far upstream as its confluence with the South Fork McKenzie River, plus the South Fork downstream of Cougar Dam
   
• Coast and Middle Forks Willamette River, downstream of Cottage Grove Dam on the Coast Fork and Dexter Dam on the Middle Fork, including the Row River downstream of Dorena Dam and Fall Creek downstream of Fall Creek Dam.

In general, these models include the entire main-stem Willamette River and most of its major tributaries as far upstream as the first major dam on each tributary (map). Version 3.12 of CE-QUAL-W2 was used to build all submodels. The Santiam and North Santiam River model was constructed by USGS (Sullivan and Rounds, 2004). The South Santiam River model was constructed by ODEQ with assistance from Dr. Scott Wells' research team at Portland State University (PSU). The rest of the models were constructed by the PSU modeling team (Annear and others, 2004a and 2004b; Berger and others, 2004).

All of the models were calibrated to measured temperatures at many locations for June 1 to October 31, 2001, and April 1 to October 31, 2002. The summer of 2001 was a drought period, with low flows at or near post-dam 7Q₁₀ low-flow levels in many of the modeled rivers. The 7Q₁₀ is the lowest 7-day average streamflow that would be expected to occur once in 10 years. Hydrologic conditions in 2002, in contrast, were more typical. The models' water-temperature predictions were in good agreement with measured data; mean absolute errors generally were less than 1.0^oC (Berger and others, 2004; Sullivan and Rounds, 2004).

The models used in this investigation are available online from the downloads page of this web site.