The jury is still out on the benefits for fish that might stem from the relatively low profile removal of Hemlock Dam from southwest Washington’s Trout Creek.
Planned dam removals on the Olympic Peninsula’s Elwha and Glines Canyon dams on the Elwha River and Condit Dam on Washington’s White Salmon River are designed to provide upriver passage for salmon, steelhead and other fishes where passage has long been blocked.
On the other hand, the Hemlock Dam removal in July 2009 was intended to smooth passage wrinkles and restore river system habitat conditions that are more steelhead friendly. Trout Creek drains into the Wind River which empties into the Columbia.
“The Wind River supports the largest wild and endemic summer steelhead population in the lower Columbia River, and Trout Creek is among the most important spawning and rearing tributaries within this watershed,” according to a research abstract prepared by Washington Department of Fish and Wildlife researcher Dan Rawding. He summed up Trout Creek post-dam monitoring results during last month’s American Fisheries Society annual meeting in Seattle.
The Wind River populations are key components of the Lower Columbia River steelhead "distinct population segment" that was listed under the Endangered Species Act as threatened on March 19, 1998 by NOAA's Fisheries Service.
Preliminary estimates for adult steelhead returning to Trout Creek above the dam site are 57 and 137 for spawn year 2010 and 2011, respectively. The 57-fish total is near the top of the range of annual returns to the creek over the past 15 years.
But those returns don’t tell the tale. This year was a record year for Wind River steelhead spawners overall, according to Bengt Coffin, project manager for the Mt. Adams Ranger District, USDA Forest Service.
During those two spawn years, adult steelhead in Trout Creek made up a similar percentage (about 8 percent) of the overall Wind River steelhead population as before removal, according to Rawding. A sign of success for the restoration program would be if that Trout Creek percentage starts to climb
“It is too early to evaluate steelhead responses to dam removal and more information will be available in future years,” Rawding said.
The reality is that it will take at least two steelhead generations (about five years), or most likely more, before the population response to the dam removal and habitat work becomes clearer.
“From a statistical perspective we could detect if there was a change” in spawner abundance as a result of the work, Rawding said. “But it’s only been two years.”
Next spring the first ocean-ready smolts will emerge from the spawning that took place after dam removal. Addult steelhead that returned to the creek late in 2009 spawned in the spring of 2010. The most common age for outmigrating steelhead there is 2.
Monitoring of that outmigration will show how well those first post-dam spawners did.
And the year after that smolts that head to the ocean will mostly be the product of that huge 2011 spawning year.
“I think that was a real good thing for Trout Creek,” Coffin said of the large steelhead seeding received as a result of the record run.
When the spawners from those 2012 and 2013 outmigrations, and succeeding outmigrations, return, then the statistical picture should begin to take shape.
The main objectives of the dam removal were to assure passage and restore riverine habitat functions.
The dam, which was located at river mile 1.8, had been blamed for impeding upstream and downstream movement of steelhead, increasing water temperatures in lower Trout Creek, and obstructing downstream movement of valuable sediments. A concrete fish ladder built in 1936 is one of the first concrete fish ladders built in the Pacific Northwest, pre-dating the ladder on Bonneville Dam. Opinions vary regarding how effectively the ladder passed fish.
An underwater camera was employed for a year before the dam demolition to investigate.
“If they got in the ladder they didn’t seem to have any trouble getting up the ladder,” Coffin said. But a box trap located at the top of the ladder may have been daunting.
“They definitely didn’t like going in the box trap,” delaying or in some cases turning around, Coffin said. He said there was also some question about the fishes’ ability to find the ladder entrance.
Researcher Patrick Connolly of the U.S. Geological Survey agreed that the ladder-trap structure may have been a bit of an impediment, but not a considerable one.
“We have some data” from PIT tag readings of steelhead re-detected below the dam, he said. “We have a few but it was a small number.”
The biggest benefits to the Trout Creek steelhead may come from the changed habitat.
The dam served as a “sediment plug.” That buildup behind the dam was cleared during the dam removal process.
“It was certainly not a spawning area (the reservoir area) because of the fine sediment,” Connolly said. A free-flowing river is expected to help keep spawning gravels cleaner.
“A lot of work was done to make it a natural river course,” Connolly said. The reservoir also served to heat up the water in summer, sometimes to lethal levels.
The reconstruction of the half-mile of stream channel involved building pools, gravel bars, riffles and stream banks , including the placement of large woody debris, which result in sheltering pools for fish.. The channel reconstruction included use of river rock and whole trees that have been thinned out of overly dense forest stands in the watershed.
In the summer of 2009 the reservoir area was revegetated with about 15,000 trees and shrubs of various native species. That regrowth is for the most doing well, with some alders already shooting up as high as 7 feet, Coffin said. The vegetation is expected to stabilize the river bank and eventually provide shade that helps keep Trout Creek cooler in summer.
Hemlock Dam was constructed to generate electricity for the U. S. Forest Service Wind River District, Wind River Nursery and a Civilian Conservation Corps camp. It was later re-tooled to provide irrigation water for the nursery.
The total length of the dam was 183 feet and the spillway length is 112 feet. The height of the dam from the streambed to the crest of the spillway is 26 feet.
With the closure of the nursery in the 1990s and the Forest Service outpost early this decade, the irrigation and power generation functions were was no longer needed.
The removal project was led by the Forest Service, which manages the land where the dam was located. It also was in charge of the river channel restructuring and revegetation of the former reservoir site, and has long been involved in upstream habitat restoration.
The biological monitoring is led by WDFW and the U.S. Geological Survey, and is supported by funding from the Bonneville Power Administration. Other stakeholders in the project include the Yakama Nation, American Rivers, Washington's Salmon Recovery Funding Board, the U.S. Fish and Wildlife Service, NOAA Fisheries, Ecotrust and the Mid-Columbia Fisheries Enhancement Group.
Just prior to dam removal, an in-stream PIT-tag reading system was installed to allow the identification of passing tagged fish. In addition, juvenile outmigrant monitoring via screw traps will occur near the dam site on Trout Creek, and at three other locations within the Wind River watershed during the same period. That monitoring effort has led to the development of Before-After, and Before-After-Control-Impact, study designs to evaluate various steelhead responses to dam removal.