Environmental Flow Studies for Middle Fork Willamette, McKenzie, and Santiam River Basins

Science Center Objects

Environmental flows are defined as "streamflow needed to sustain ecosystems while continuing to meet human needs."

Background

The Sustainable Rivers Project is a partnership of The Nature Conservancy and the U.S. Army Corps of Engineers formed in 2002 and aimed at developing, implementing, and refining environmental flow requirements downstream of dams. Environmental flows can be defined as the streamflow needed to sustain ecosystems while continuing to meet human needs. Developing environmental flow requirements typically involves a collective of stakeholders working together to identify and prioritize streamflow objectives. Since the creation of the partnership, Sustainable Rivers Project projects have been implemented on a number of rivers around the Nation. Within the Willamette River basin, environmental flow guidelines were developed in Phase 1 studies for the Coast and Middle Fork Willamette River basins. USGS involvement with the Sustainable Rivers Project began with environmental flow studies for the McKenzie and Santiam River basins, described below.

 

McKenzie River Basin

Since the early 1900s, streamflow in the McKenzie River basin has been altered through the construction of dams and canals. Two major flood control dams, a hydropower dam complex, and two hydropower canals significantly affect streamflows in the river. The dams also affect stream temperature. Flow releases from one of the flood control dams are cooler in summer and warmer in fall in comparison to unregulated flow conditions, before the dam was constructed.

The McKenzie River supports several populations of salmonids, including spring Chinook salmon, and winter and summer steelhead, as well as native rainbow, cutthroat, and bull trout. In 2006, the Oregon Department of Environmental Quality listed a total of 112.4, 6.3, and 55.7 miles of the McKenzie River basin mainstem and tributary stream reaches as thermally impaired for salmonid rearing, salmonid spawning, and bull trout habitat, respectively.The intial phase of the USGS study component provides McKenzie River basin stakeholders with a compilation of relevant streamflow, geomorphic, and ecological data and analyses necessary for them to prescribe the rate, frequency, duration, and timing of flow releases from McKenzie River basin dams for various downstream locations.

 

Santiam River Basin

The U.S. Army Corps of Engineers (USACE) operates four dams in the Santiam River Basin, which are used primarily for flood control, hydropower production, recreation, and water-quality improvement. Detroit and Big Cliff Dams were constructed in 1953 on the North Santiam River. Green Peter and Foster Dams were completed in 1967 on the South Santiam River. The impacts of the structures have included a decrease in the frequency and magnitude of floods and an increase in low flows. For three North Santiam River reaches, the median of annual 1-day maximum streamflows decreased 42–50 percent because of streamflow regulation. Likewise, for three reaches in the South Santiam River basin, the median of annual 1-day maximum streamflows decreased 39–52 percent because of regulation. Using streamflow data and the results from the analysis of the data, water managers can identify the rate, frequency, duration, and timing of flow releases from Santiam River basin dams needed at downstream locations to achieve specific ecological and geomorphic objectives.

McKenzie River Basin

Dams have altered downstream channel morphology and ecologic communities by reducing the magnitude and frequency of floods and diminishing sediment transport by trapping bed material. Bank stabilization, highway construction, and reductions of in-channel wood have contributed to the loss of riparian habitat. A comparison of aerial photography taken in 1939 and 2005 showed substantial decreases in secondary channels, gravel bars, and channel sinuosity, particularly along the lower alluvial reaches of the McKenzie River. In addition, bed armoring and incision may have contributed to habitat degradation. Peak streamflow reduction has led to vegetation colonization and stabilization of formerly active bar surfaces. The large flood control dams on Blue River and South Fork McKenzie River likely have had the greatest effect on downstream habitats because these sediment- and flood-rich tributaries historically contributed a disproportionate volume of bed material, wood, and peak flows in comparison with the spring-fed tributaries of the upper McKenzie River Basin.

 

Santiam River Basin

In contrast to their effect on high flows, the dams in the Santiam River Basin have increased low flows. The median of annual 7-day minimum flows in six of the seven study reaches increased under regulated streamflow conditions between 60 and 334 percent. On a seasonal basis, median monthly streamflows decreased from February to May and increased from September to January in all the reaches. However, the magnitude of these impacts usually decreased farther downstream from dams because of cumulative inflow from unregulated tributaries and groundwater entering the North, South, and main-stem Santiam Rivers below the dams.