>Surface Water Temperature

Surface Water Temperature

This is a fundamental measure for aquatic organism well-being.
Physical / Chemical
Maintain Water Quality

QT: Indicators

What is it?

Surface water temperature is a constant variable of any waterbody and can be measured and summarized in various ways. Maximum seven day average daily maximum (7DADM) was calculated for each year and for each subwatershed. Each year was represented by the highest seven-day average maximum water temperature within the year. This metric is identical to the Maximum Weekly Maximum Temperature metric in the literature.

Why is it important?

One of the consequences of increased withdrawal of river water for human uses is an increase in water temperature due to lowered volume. Increase of river temperatures from their natural levels has far-reaching effects on local ecology, including alteration of community processes and facilitating invasion by exotic species (Poole & Berman 2001). Restoring natural flow regimes and thus natural temperatures is critical to restoring a healthy natural system.

Native salmonid species are of great ecological, economic, and cultural importance to local communities. They also serve as strong indicators of habitat quality and integrity in river systems, particularly with regard to water temperature, sediment load, and barriers to passage. They are well-studied, including behavioral and physiological responses to temperature extremes. The Central Valley spring-run Chinook salmon is listed as a threatened species under the Endangered Species Act (ESA), giving them a high priority for restoration. The main threats to the remaining populations are loss and degradation of habitat. In particular, rising water temperature combined with upstream dams has greatly diminished available juvenile summer habitat. Within the Feather River Watershed, only two populations persist. One, in the Feather River itself, is completely dependent on the Feather River Fish Hatchery to maintain itself. The other, in the Yuba River, is of unknown status. There are occasionally spawning salmon in the Lower Bear River.

Maximum water temperature is a critical part of habitat quality for salmonids. Temperature affects every aspect of salmonid biology, from feeding and growth rates to migration and spawning, and stress levels and survival (Carter 2005). Rainbow trout, for example, are more severely impacted by temperatures in excess of 20°C than by fishing pressure (Runge & Peterson 2008). Upstream diversion of water for human usage increases downstream temperatures, as the lower remaining volume warms more quickly. Due to upstream barriers such as dams, only less-suitable, high-temperature regions are available for spawning and summer feeding. Anthropogenic temperature increases have been identified as key contributors to salmon decline (USEPA 2003).

Target or Desired Condition

USEPA suggests as a guideline that a river sustaining salmonid populations should not have 7DADM temperatures over 18°C to avoid impairment of salmon health. Similarly, migratory portions of the river should not exceed Maximum Weekly Maximum Temperatures of 20°C and temperatures greater than 22°C will cause broad mortality (USEPA 2003). For core rearing areas in mid-to-upper parts of the river basin, a maximum of 16°C may be appropriate. Experimental studies indicate that spawning temperatures up to 16.5°C do not have deleterious effects on juvenile salmon, but mortality increases markedly after that point (Geist et al. 2006). These temperature guidelines, along with additional information from Brett et al. (1982), were used to convert monthly maximum 7DADM into a 0-100 scale.

A score of 100 is equivalent to the USEPA’s stated protective criteria of 18°C 7DADM for secondary foraging/rearing areas. A score of 0 will be equivalent to 25°C 7DADM, the lethal point for juvenile Chinook salmon. Intermediate scores were scaled using an adaptation of the Brett et al. (1982) growth curve. Only the right side of the curve was used; temperatures below the USEPA protective criterion were still scored as 100. Brett et al. (1982) estimate that natural populations of Chinook feed at roughly 60% of saturation (or R=0.6, the lowest growth curve). Because of daily temperature fluctuation, 7DADM temperatures are equivalent to constant laboratory temperatures roughly 1-2°C colder (USEPA 2003).

What can influence or stress condition?

The major factor which raises water temperature is decreased flow within the river. Low water volume allows the sun to warm the river much faster, and temperatures increase rapidly as the water moves downstream. Prolonged decreased flow (as opposed to seasonal variations) is most often due to human water use; water is retained in reservoirs and diverted to urban centers or for agricultural use, and only a small fraction is released into the original channel. Land-uses can contribute to higher temperatures, including logging, agriculture, and urban development. Increasing temperature due to climate change is another possible factor.

Temporal and spatial resolution

Temperature is monitored throughout the Feather River Watershed, but not consistently over time or space. For example, there are few sites in the very large North Fork Feather and many in the comparatively small South Yuba. In addition, temperatures are collected using a combination of monthly grab samples and continuously monitoring Hobo-temps (thermometers left in the waterways). Continuous monitoring provides the most consistent source of temperature data and indicator calculation, but it is conducted on only a few sites. A critical feature of watershed-wide monitoring would be the establishment of a network of continuous temperature measuring devices that covers all important waterways and times of the year.

How sure are we about our findings? (Things to keep in mind)

The overall condition assessment based on 7DADM tells part of the story, but is best calculated based on complete data-sets. Water temperature is a straightforward parameter to measure, but is complicated to interpret. For example, the North Yuba received a lower condition score than the South Yuba (Table 1), but maximum temperatures in the South Yuba ( Figure 6B) can get higher than in the North Yuba (Figure 6A, Table 4). Similarly, Deer Creek received a score of 100, but has also experienced high maximum temperatures (Table 4). The most significant problem is the lack of long-term continuous temperature data for all sites. Although continuous data exists for many sites, it is limited to only the most recent years. The majority of sites have only single daily or monthly measurements, which makes it impossible to estimate a reliable maximum temperature. Scores based on these “false maxima” are likely to under-represent temperature problems in the watershed. The lack of reliable long-term time series makes trend estimation difficult as well.

The sites included were not edited to provide balanced spatial resolution, and some monitoring stations may not be representative of the subwatershed. For instance, the Lower Yuba score includes temperatures from Dry Creek. Dry Creek has higher temperatures than the Lower Yuba, and excluding its data would give a final score for the Lower Yuba of 41, not 26. Due to these and other factors, subwatersheds varied broadly in the confidence in findings (see Table 4). Subwatersheds with the lowest confidence scores are: Deer Creek, Lower Bear, Lower Feather, and North Fork Feather. These regions typically had fewer sampling sites, or lacked data for true temperature maxima. Highest confidence subwatersheds are: East Branch North Fork Feather, Middle Yuba, South Yuba, and Upper Bear. Differences in quality of data may explain some of the results, such as the high score for the Lower Feather compared to the Lower Yuba.

What did we find out/How are we doing?

The current states of the subwatersheds are shown in Table 1 and Figure 3. Scores ranged from a low of 20 for the Lower Yuba to 100 for Deer Creek and the Lower Feather. Many of the sampling sites were excluded from analysis due to data limitations. Many sites had only one year of data, sometimes represented by a single point. Most problematic was the prevalence of monthly samples at irregular times, which clearly do not represent temperature maxima. However, there were sufficient daily datasets from each subwatershed to perform trend analyses. Due to the aforementioned data limitations, only current state assessments and annual Mann-Kendall analyses were performed for each subwatershed. More details are available in later tables.

Trends Analysis

The majority of subwatersheds had positive temperature trends, but only one was statistically significant. This was the Lower Feather, which had the second highest score for current state. Deer Creek had a significant negative slope, but the limited data available indicate an unreliable trend. Regional Mann-Kendall analysis was conducted on each subwatershed, using the individual sites as separate regions. Only datasets with three or more years were included, to minimize noise and avoid a disproportionate contribution from the many 2008-2009 datasets. As suggested by the results from the individual regions, the overall trend for the entire watershed was positive but not significant (tau-b = 0.011, p = 0.728).