Development of CE-QUAL-W2 models for the Middle Fork Willamette and South Santiam Rivers, Oregon

Hydrodynamic (CE-QUAL-W2) models of Hills Creek Lake (HCL), Lookout Point Lake (LOP), and Dexter Lake (DEX) on the Middle Fork Willamette River (MFWR), and models of Green Peter Lake and Foster Lake on the South Santiam River systems in western Oregon were updated and recalibrated for a wide range of flow and meteorological conditions. These CE-QUAL-W2 models originally were developed by West Consultants, Inc., for the U.S. Army Corps of Engineers. This study by the U.S. Geological Survey included a reassessment of the models’ calibration in more recent years—2002, 2006, 2008, and 2011—categorized respectively as low, normal, high, and extremely high flow calendar years. These years incorporated current dam-operation practices and more available data than the time period used in the original calibration. Modeled water temperatures downstream of both HCL and LOP-DEX on the MFWR were within an average of 0.68 degree Celsius (°C) of measured values; modeled temperatures downstream of Foster Dam on the South Santiam River were within an average of 0.65°C of measured values. A new CE-QUAL-W2 model was developed and calibrated for the riverine MFWR reach between Hills Creek Dam and the head of LOP, allowing an evaluation of the flow and temperature conditions in the entire MFWR system from HCL to Dexter Dam.

The complex bathymetry and long residence time of HCL, combined with the relatively deep location of the power and regulating outlet structures at Hills Creek Dam, led to a HCL model that was highly sensitive to several outlet and geometric parameters related to dam structures (STR TOP, STR BOT, STR WIDTH). Release temperatures from HCL were important and often persisted downstream as they were incorporated in the MFWR model and the LOP-DEX model (downstream of MFWR). The models tended to underpredict the measured temperature of water releases from Dexter Dam during the late-September-through-December drawdown period in 2002, and again (to a lesser extent) in 2011, but simulations were much more accurate in 2006 and 2008. This episodic model bias may have been a result of hot, dry conditions; lower lake elevations; and earlier drawdown at both HCL and LOP in 2002. These dry conditions in 2002 may have contradicted assumptions inherent in the estimation of certain model inputs, such as unmeasured inflows and water temperatures, which may respond differently during dry years than during normal and wet years.

This report documents the development and calibration of new and revised flow and water-temperature models for riverine and reservoir reaches in the Middle Fork Willamette River and South Santiam River systems. Methods and model parameter values were established for the accurate simulation of flows and temperatures in these systems under current conditions. By extension, these models should be able to accurately simulate flows and temperatures under potential future conditions in which dam operations and dam outlet structures may be changed as part of a strategy to improve habitat, fish passage, and temperature conditions for endangered fish.