TABLE OF CONTENTS
The Potter Valley Dams
California's three biggest watersheds are the Sacramento, Klamath-Trinity, and Eel. Most rivers in the state are burdened with dams, tunnels, diversions, and canals. Notable among the detrimental effects on anadromous fisheries are California's 1,400 dams that choke off the spawning grounds for the next generation of salmonids. In the upper Eel River, the Potter Valley Project's Cape Horn and Scott dams deny access to hundreds of miles of prime spawning grounds.
Scott Dam, completed in 1922, creates Lake Pillsbury. Built with private funds by Snow Mountain Water & Power Co., it was purchased by PG&E in 1925. Larger than neighboring Cape Horn Dam, Scott Dam is 130 feet high, earth-filled, and covered with plates of concrete. Scott Dam does not provide flood control but rather water storage for the Potter Valley hydroelectric plant.
Scott Dam holds back a diminishing amount of water, because silt has taken up almost a quarter of Lake Pillsbury's capacity. Moreover, as acknowledged in a 1994 U.S. Forest Service study, Scott Dam blocks access to spawning tributaries. This dam has no fish ladderĺ─÷ and there also is no known instance where such a large fish ladder has ever worked. There is a fish ladder at Cape Horn Dam a.k.a. Van Arsdale allowing access to 12 river miles between dams.
Cape Horn Dam and the Potter Valley Project's diversionary tunnel (through a mountain to the Russian River watershed on the other side) were completed in 1908 on the main stem of the Eel River in order to provide hydroelectricity to Ukiah. The 50-foot dam forms Van Arsdale Reservoir, with a surface area of 106 acre-feet; this reservoir is shallow and now filled with sediment but acts as a bay for the diversionary tunnel. Also built by Snow Mountain Water & Power, this project includes a tunnel diverting an average of 180,000 acre-feet (af) of water every year and often up to 240,000 af at the rate of 340 cubic feet per second (cfs), while releasing only 5 cfs to the Eel, until very recently when releases were increased to 25 cfs.
Cape Horn Dam initially was built without a fish ladder and cut off migration for the largest of the Eel River salmonids. Fish migrating here travel more than 800 hundred river miles and climb 4,500 feet. A ladder was built decades later, and a (marginally effective) screen added to keep fish from being sucked down the diversion tunnel. Still, about 50% of fish swimming near the tunnel are lost, but this is an improvement from past practices of sucking all smolts down the tunnel and grinding them up in the power turbines for fish-fertilized water used by Potter Valley farmers. (The valley is noted for its fish-emulsion-green color.)
Linked to the sedimentation in the reservoirs is the alarming fact that rising levels of mercury in Lake Pillsbury have been detected in fish tissue. Bacteria from all the debris that has settled in the bottom of the lake and begun disintegrating creates methyl-mercury (as well as methane gas that contributes to global warming). Another possible source of the contamination is the mercury found naturally in the area's rock outcropping; the rise and fall of lake water increases erosion and may have escalated concentrations of mercury. The Lake County Department of Health Services has posted warnings in the area that children under the age of 6 and pregnant women should not eat any fish from the lake, and all others "should only eat fish caught in Pillsbury occasionally. Regular consumption would be unsafe due to the mercury levels."