Stream-Temperature Assessment

Key Elements

• A total of 112.4, 6.3, and 55.7 mi of McKenzie River basin mainstem and tributary stream reaches are listed as thermally impaired for salmonid and bull trout spawning and rearing.
• Streamflow below Cougar and Blue River Dams has been or currently is cooler in the summer and warmer in the fall since regulation.
• USGS stream-temperature data has been collected at 14 active and now inactive USGS streamflow-gaging stations since as early as 1951 in the McKenzie River basin.
• Blue River and South Fork McKenzie River stream temperatures have a slight effect on stream temperatures at McKenzie River near Vida during September and October.

Stream temperature in the McKenzie River basin is major concern with regard to the habitat of fish and other aquatic species. For many aquatic plants and animals, optimal temperatures are critical for different phases of their life cycle. Temperature also has a direct relation to water-quality parameters, such as dissolved oxygen concentrations, biochemical oxygen demand rates, algae production, and contaminant toxicity.

Stream-Temperature Criteria

In the McKenzie River basin, anthropogenic factors that affect stream temperatures include the removal of riparian shade buffers, channel geomorphology alterations, canal streamflow diversions, and reservoir streamflow releases (Oregon Department of Environmental Quality, 2006). The ODEQ surveyed streams and rivers using Federal Clean Water Act standards to determine which reaches were thermally impaired and did not meet State temperature standards for salmonid rearing, spawning, and cold water refuges. Stream reaches in violation of the standards were placed on the Federal Clean Water Act section 303(d) list (Oregon Department of Environmental Quality, 2009), and a Total Maximum Daily Load (TMDL) plan must be developed. McKenzie River basin stream reaches listed by the ODEQ and the temperature criteria they exceeded are shown in table 16. In total there were 112.4, 6.3, and 55.7 mi of stream reaches listed as thermally impaired for salmonid and bull trout spawning and rearing. As part of the McKenzie River TMDL plan, monthly target streamflow release temperatures were developed for the Cougar and Blue River dams (table 17). Summer release targets are in the optimum/preferred range for all life stages of the Chinook salmon. Spring and fall targets are in the optimum/preferred range for Chinook incubation and juvenile rearing (Gregory and others, 2007a) (table 18).

Effect of Reservoirs

A flood control reservoir can significantly affect downstream water temperature. Similar to lakes and other natural water bodies, a thermocline typically will develop in a reservoir during the spring and summer as the upper layers are warmed by solar radiation. Water releases from the top of thermally stratified reservoirs will be warmer than reservoir inflows, whereas releases from the reservoir bottom will be colder. In autumn, when the upper layer temperatures have cooled, thermal stratification is eliminated as vertical mixing occurs. At this time, the reservoir typically is drawn down to increase the winter flood storage capacity, and the downstream water temperatures will reflect reservoir temperatures. Streamflow releases from Cougar Dam, prior to the recent construction of the water temperature control (WTC) tower in 2006, and Blue River Dam came from a lower level of the reservoir pool. As a result, summer streamflows were unnaturally cooler and fall temperatures were unnaturally warmer.

Altered streamflows caused by reservoir regulation can affect the downstream temperature regime in other ways as well. When winter and spring flooding extremes are reduced and summer streamflows are increased, natural channel processes are affected. For example, Gregory and others (2007a) found in the Willamette River basin that alcoves on floodplains and gravel bars produce pockets of water cooler than that in the well-mixed portions of the stream. A decrease in these alcoves caused by streamflow regime alteration could potentially reduce the availability of “temperature refuges” that temperature-sensitive aquatic species utilize during periods of heat stress.

Stream-Temperature Data

The USGS has been collecting stream-temperature data in the McKenzie River basin since 1951 at 14 active and now inactive USGS streamflow-gaging stations on the mainstem and tributaries (table 19). Records for these sites include daily minimum, maximum, and mean stream temperatures. Although the records for some of these sites contain some data gaps, standard USGS data collection and publication protocols were always used. Stream temperatures in the channel cross sections were periodically checked and verified with the monitoring instrumentation (U.S. Geological Survey, 2009).

Results and Discussion

The Carmen-Smith–Trail Bridge Dam complex in Reach 1 has not affected the natural seasonal pattern of stream temperatures (Stillwater Sciences, 2006a). However, water temperatures in the upper basin stream reaches are listed as thermally impaired in violation of the bull trout 12°C temperature criterion (Oregon Department of Environmental Quality, 2006). Mean daily maximum stream temperatures for the McKenzie River below Trail Bridge Dam (14158850) and McKenzie River at McKenzie Bridge (14159000) stations (both located downstream of the Carmen-Smith–Trail Bridge Dam complex) have a stream temperature seasonal distribution similar to that at Horse Creek near McKenzie Bridge (14159100) (which is unaffected by the dam complex) (fig. 20). However, the annual maximum stream temperature for McKenzie River below Trail Bridge Dam (14158850) occurs slightly later in the summer than at the two other stations. (The graphs were created by computing the mean of all daily maximum temperatures for each calendar day from a record of a site.)

On the South Fork McKenzie River (Reach 3) natural temperature patterns (warm summer and cool winter) are apparent in data collected at the streamflow-gaging station upstream of the Cougar Reservoir (14159200) and pre-dam period data collected at the streamflow-gaging station downstream of the reservoir (14159500) (fig. 21). However, the post-dam period (1961–2001) had unnaturally cool temperatures for most of the summer and unnaturally warm temperatures in the fall. Since construction of the WTC tower in 2006, the seasonal distribution of temperatures has returned to a more natural pattern.

For the Blue River (Reach 5), the effect of the dam on stream temperatures is readily apparent when compared to the natural conditions of the pre-dam period (fig. 22). In mid-July stream temperatures during the post-dam period are almost 10°C cooler than the pre-dam stream temperatures. In early October, the temperatures during the post-dam period are more than 4°C warmer than pre-dam temperatures. At the time this report was written there were no plans to construct a WTC tower for the Blue River Dam. However, the downstream thermal effect of Blue River is limited because the mean annual streamflow of Blue River (14162200) is only approximately 10 percent of the mean annual streamflow at the McKenzie River near Vida (14162500) streamflow-gaging station.

Overall, stream temperatures increase from upstream to downstream throughout the length of the McKenzie River (fig. 23). With the exception of Blue River, all the locations have their maximum temperatures in July or early August. Blue River and South Fork McKenzie River stream temperatures appear to have a slight effect at the McKenzie River near Vida (14162500) during September and October. Although the Coburg monitoring station (Reach 12) is downstream of the Walterville station (Reach 10), the Walterville stream temperatures are higher than the Coburg stream temperatures. Because the Walterville site is located on a reach of the river below which streamflow is diverted by the Walterville canal, the higher stream temperatures could be a result of year-round streamflows that are 1,000 to 2,000 ft³/s less than reaches not affected by canal diversions in the lower McKenzie River basin.

First posted February 8, 2010