The original fish passage structure at Cape Horn Dam, which impounds Van Arsdale Reservoir, was inadequate. The Cape Horn Dam Fish Ladder was modified in 1962, and again in 1987, to facilitate fish passage. When Scott Dam was built, it blocked about 36 miles of the Mainstem Eel River from returning Chinook salmon and 58 miles of steelhead spawning habitat (VTN 1982.). This is not the thousands of miles of spawning tributaries stated by FOER. Scott Dam also changed the flows and water temperature in the Eel River above Cape Horn Dam. However, often ignored in statements by the Friends of the Eel River, is the fact that there are still 12 Mainstem river miles (and miles of tributaries) of spawning habitat between Cape Horn and Scott Dam. Another fact left out of the Friends of the Eel River's "story" is that the Potter Valley Project blocks less than 10% of the Eel River's total watershed. The Friends of the Eel River insinuate, over and over, that the Potter Valley Project has cut the entire mighty Eel River off to migratory runs of salmon and steelhead.
7.3% of the Eel River watershed was cut off from migrating salmon and steelhead 88 years ago. 35.7 miles of Chinook main river and tributary spawning habitat and 58.4 miles of steelhead river and tributary spawning habitat was lost when Scott Dam was completed (VTN 1982). Extrapolation of historical data and estimates of the number of spawning salmon per river mile, show that up to 10.4% of the Eel River Chinook salmon population was lost when Scott Dam was built (VTN 1982). It is more difficult to estimate how much of the steelhead population was lost due to hatchery releases, different age classes and juvenile steelhead enhanced growth rates in the river between Scott Dam and Cape Horn Dam.
We absolutely know that spawning and rearing habitat for Chinook salmon and steelhead was lost when Scott Dam was built. However, it is doubtful that Coho salmon successfully spawned in significant numbers above Tomki Creek, which is below Van Arsdale Reservoir and 15 river miles below Scott Dam (VTN 1982).
After the lengthy Environmental Impact Study and the Section 7 Consultation process was completed in 2004, resulting in the FERC Final Order, the flows below the Potter Valley Project have been adjusted to more closely mimic natural flows and to make the Potter Valley Project as invisible as possible. Note, we did not say that Potter Valley Project is invisible to the Eel River! However, the Potter Valley Project has never been so carefully tuned to react to natural conditions as it is today. Could it be better? Probably - but not in the way one might think. Before the Final Order was put into place, and during the time NMFS was preparing their Biological Opinion, there was concern from the California Department of Fish and Game (see CDFG letter to FERC dated December, 2002) that the summer flows below Cape Horn Dam were going to be too high in the summer and would provide more habitat for, and enhance the population of, predatory pikeminnow. Of particular concern was the endangered population of Coho salmon in Outlet Creek. (Outlet Creek is the farthest that Coho salmon consistently travel, and successfully spawn today, in the Eel River. It is known that Coho salmon did, in the past, successfully spawn as far up river as Tomki Creek.) It remains to be seen if CDFG was correct.