Nooksack Watershed Summer Water Temperature Trends

by Eric Hirst

Over the past several years, I’ve written articles that document declines in summer streamflow throughout the Nooksack River basin. (1) This paper expands these prior analyses with an examination of water temperatures. I focus on summer (July, August, September) because that is when flows are lowest, and salmon and other wildlife need more — and colder — water than is actually flowing.

The U.S. Geological Survey (USGS) and other agencies measure and publish data on daily stream temperatures. I focus on the 7-day average of daily maximum temperatures (7-DAM). (2) While streamflow measurements are available, in some cases, for several decades, water temperature data are much more limited. This article, therefore, deals with the 17-year period from 2008 through 2024. (3) I analyze data from the same five stations studied in my earlier papers: mainstem Nooksack River at Ferndale; North, Middle, and South forks; and Fishtrap Creek.

Figure 1

Fig. 1. Mainstem Nooksack River summer streamflow and 7-DAM water temperature.

Results

Figures 1 – 5 show summer flows and maximum daily temperatures for these sites.

These data show:

  • The numbers vary greatly from year to year and across the five locations, for both flows and temperatures.
  •  Nevertheless, clear trends exist:
      *  Flows are declining over time, at rates of 3 to almost 6 percent per year.
      * Temperatures are increasing, by less than 0.1 to 0.2oC per year.
  • Temperature increases and flow decreases are correlated, with R2 values that range from 0.1 to 0.9. (4/5)
  •  Although correlation does not necessarily imply causation, these results strongly suggest that lower summer streamflows throughout the Nooksack watershed lead to higher stream temperatures. And, higher temperatures lead to lower dissolved-oxygen levels. (6)
Figure 2

Fig. 2. North Fork summer streamflow and 7-DAM water temperature.

Interpretation

Although these temperature data are interesting, alone they tell us nothing about salmon health and survival. Unfortunately, there are no hard and fast rules about upper limits on water temperature for healthy salmon populations. The desirable temperature range differs among salmon types and lifecycle stage (eggs, alevins, fry, smolts, and adults). As temperatures increase, salmon weaken, are more susceptible to disease, less able to reproduce and migrate; and, if the temperatures are too high, they die.

Chinook salmon may experience adverse effects at 20- 22oC, sockeye at 14-16oC, and coho at 18-20oC. (7) The Washington Dept. of Ecology specifies temperature criteria that range from 12 to 20oC. (8) For purposes of this article, I assume that 18oC is the boundary, above which salmon suffer.

Figure 3

Fig. 3. Middle Fork summer streamflow and 7-DAM water temperature.

As shown in Figs. 1-5, maximum daily temperatures, on average, have not yet reached this (somewhat arbitrary) 18oC threshold. Clearly, temperatures are increasing as streamflows are decreasing. And projections of future declines in streamflow suggest that temperatures will continue to increase in the future. (9)

 

Figure 4

Fig. 4. South Fork summer streamflow and 7-DAM water temperature. (Data for 2008 are not available.)

Stream temperatures in the North and Middle forks are several degrees cooler than for the other three locations. But here too, temperatures are increasing with time. The relationships between declining flows and increasing temperatures suggest that the 18oC threshold will soon be passed throughout the watershed.

Figure 5

Fig. 5. Fishtrap Creek summer streamflow and 7-DAM water temperature.

Although each of the five watersheds examined here differ in detail, the trends in streamflow, temperature, and the relationship between the two are similar. One difference is clear: stream temperatures are less sensitive to streamflow changes in Fishtrap Creek than in the other four watersheds.

Summary

The future prospects for salmon in the Nooksack watershed are grim. This stark conclusion is based on streamflow and water temperature at five locations over the past 17 years.

Table 1

Table 1. Summary of Nooksack Watershed Results, 2008 – 2024

Flows are declining at a rate of 3 to 6 percent per year, depending on location (Table 1). And, temperatures are increasing by 0.04 to 0.18oC per year. Several projections suggest that these adverse trends will continue. Modeling the effects of climate change on the three forks shows substantial increases in stream temperature over the coming decades. Results show 7-DAM increases around 2050 of about 2.8, 3.4, and 4.3oC for the South, Middle, and North forks. (10)

These five watersheds differ in many ways: elevation, size, water sources (glaciers, snow, rainfall, groundwater), and out-of-stream water use. Nevertheless, the summer temperature and flow trends are remarkably similar — and all adverse to salmon health. Worse, these trends compound: as flows decline, fish need colder water, the opposite of what is happening.

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Eric Hirst has a Ph.D. in engineering from Stanford University, worked at Oak Ridge National Laboratory for 30 years as a policy analyst on energy efficiency and the structure of the electricity industry. He spent the last eight years of his career as a consultant.

Endnotes

  1. E. Hirst, “Is Time Running Out? Streamflow Trends in the Nooksack River Basin,” Whatcom Watch 32(12), Dec.2023.
  2. A. Richter and S.A. Kolmes, “Maximum Temperature Limits for Chinook, Coho, and Chum Salmon, and Steelhead Trout in the Pacifi c Northwest,” Reviews in Fisheries Science, 13:23– 49, 2005. Results using average or maximum daily temperatures show similar trends to those presented here.
  3. These temperature data contain many missing elements and are sometimes internally inconsistent. Specifically, reported average temperatures are sometimes higher than reported maximum temperatures, an impossibility. The missing-data issue is particularly severe for the Middle Fork; fortunately, temperature data are available from a site managed by the Nooksack Indian Tribe (NIT).
  4.  R2, the coefficient of determination, measures the proportion of variance in a dependent variable that can be explained by an independent variable. R2 can take values between 0 and 1, with higher values indicating that the model explains more variability. For example, an R-squared of 0.60 means that one variable explains 60 percent of the variance in the other variable.
  5.  Other factors affect stream temperatures, especially air temperatures. Riparian vegetation (shading), rainfall, groundwater flows, and out-of-stream human uses of water might also affect stream temperatures.
  6.  See endnote 2.
  7.  N.B. Mayer et al., “Thermal tolerance in Pacific salmon: A systematic review of species, populations, life stages and methodologies,” Fish and Fisheries, 2024.
  8.  Washington Dept. of Ecology, WAC 173-201A-200, Fresh water designated uses and criteria, Feb. 2023.
  9.  See endnote 1.
  10.  Table 6, S. Truitt, Modeling the Effects of Climate Change on Stream Temperature in the Nooksack River Basin, Western Washington Univ., Feb. 2018.

 

 

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