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Biological Effects of the Cape Horn Dam on Salmonids in the Upper Mainstem Eel River
By Spencer Stiff
By now I hope that most of you reading this article are familiar with the diversion of Eel River water to the Russian River. I am not going describe the details of this diversion, only the biological effects it has on salmon and steelhead trout downstream of the Cape Horn Dam. I focus primarily on the section referred to as the upper mainstem Eel River, from Cape Horn Dam to Dos Rios. This section has the highest water temperatures in the Eel River. If you would like further information on the diversion or locations referred to in this article, please see the FOER website.
One of the biggest biological problems caused by the diversion is the elevated stream temperatures during the summer months. These temperatures often exceed lethal thresholds for juvenile steelhead trout that rear in the upper mainstem during the summer. In a recent survey by the U.S. Environmental Protection Agency (EPA) for Total Maximum Daily Loads for Temperature and Sediment (TMDL), maximum daily stream temperatures in the upper mainstem Eel between Tomki Creek and Outlet Creek were 24-28 degrees Centigrade. Numerous temperature studies on juvenile steelhead have shown that temperatures from 20 to 24 are stressful and affect growth and health of the fish. Water temps from 25 to 26°C have been proven lethal to juvenile steelhead.
You may be asking yourself how any steelhead can survive in this part of the river through the summer. They use what’s called “thermal refugia”--this occurs where a tributary or spring enters the river, providing a small pocket of cooler water. This type of habitat is limited, however, leading to greater competition for food among juvenile steelhead and an overall decrease in the available rearing habitat, resulting in slower growth rates and lower overall survival rates. Juvenile steelhead must rear in freshwater for one to two years before entering the ocean. The larger they can grow before migrating downstream, the higher their odds for survival upon ocean entry.
Water temperatures between Lake Pillsbury and Cape Horn Dam range from 18 to 20°C during the summer months. This section of river directly above the diversion has been identified in many studies to have optimum rearing conditions for juvenile steelhead trout. The California Department of Fish and Game had plans to try and keep juvenile steelhead in this upper reach because they were aware of how poor the water conditions are below the diversion at Cape Horn.
Article 39 of the FERC relicensing agreement with PG&E states that “The plan shall further provide for the monitoring of the temperature regime downstream of Scott Dam.” However, temperature monitoring by PG&E’s consulting firm was only conducted until early July of each year. In the NOAA Fisheries biological opinion regarding the continued operation of the Potter Valley Diversion, they refer to the ending of the monitoring in July as an “incomplete data set for the whole summer rearing period.” This appears to be skewed science with a predetermined outcome that met the needs of the diversion project.
The current cubic-feet-per-second (cfs) gauge below Cape Horn Dam that measures the amount of flow going to the Eel River offers no public access to the temperature readings. If you look up almost any other stream gauge in California on the USGS or Department of Water Resources websites you can view the current water temperature at that gauging station. Water temperatures below the Van Arsdale reservoir (formed by Cape Horn Dam) are left out, however. These statistics for the upper section of the river need to monitored more intensely to ensure that the current flow schedule isn’t so limited that water temperatures below the diversion exceed the upper levels for steelhead survival.
Another major problem caused by the diversion is the effect that the controlled flows have on returning adult chinook salmon in the fall. These controlled flows do not always coincide with natural rainfall events that are essential to attract adult salmon upstream from the estuary or lower river. This is due partly to the fact that flows are adjusted based on a calculated relationship between Tomki Creek and natural flows that would have occurred in the upper Eel River if the dams were not present. The headwaters of the Eel above the dams receives an average of 70 inches of rain per year. Tomki Creek enters the river several miles downstream of the Cape Horn Dam and receives around 45 inches of rain per year on average. (Tomki Creek is located on the leeward side of Willits Valley and may be in the rain shadow, decreasing duration and intensity of rainstorms.) Based upon these hydrological differences, NOAA Fisheries considers “Tomki Creek gauging to be a poor surrogate for predicting the magnitude and timing of Upper Eel River natural unimpaired flow.”
By failing to adequately mimic the natural flows in the basin, the dam’s operators can delay the upstream migration of adult chinook salmon. This delay can cause increased mortality and predation, along with unintended spawning in the lower reaches of the river where eggs are more susceptible to loss from scour and siltation, giving them a lower survival rate than eggs deposited further upstream. A study of upstream migration rates reported that fall chinook salmon adults traveled 10.2 km/day from the mouth to Dos Rios and 4.4 km/day from Dos Rios to Cape Horn Dam. This shows that the low flow in the upper river slows the returning salmon migration rate by half. Several locations in this upper section have been identified as migration barriers during low flow because of the limited depth in the riffles for adult passage. This needs to be monitored more closely in the future to ensure that adult salmon can continue the upstream migration under the current flow management.
These are just a couple of issues causing the declining runs of salmon and steelhead in the Eel River. I will leave the non-native pikeminnow and faulty fish ladder/screens for another article.