Construction Projects To Improve Fish Survival Are Winding Down At Corps Of Engineers Dams
- May 18, 2020
- John Harrison
The U.S. Army Corps of Engineers is nearing the end of a series of construction projects intended to make passage easier and survival higher for juvenile and adult salmon at dams on the lower Columbia and Snake rivers, and is beginning to transition to ongoing operations and maintenance for passage and survival structures.
The 2008 Biological Opinion on hydropower dam operations, issued by NOAA Fisheries to protect Endangered Species Act-listed salmon and steelhead, included specific direction to the Corps to improve passage survival at its dams, particularly for juvenile fish migrating downriver to the ocean.
“That really set in motion the Corps taking specific actions at all mainstem Columbia and lower Snake dams to provide improved fish passage,” Tim Dykstra, senior fish program manager for the Corps’ Northwestern Division, told the Council at its May meeting. “As part of the overhaul; we focused on big things – surface passage at all the dams, spillway weirs, a corner collector at the Bonneville Second Powerhouse. We also installed a lot of changes and overhauls to juvenile bypass systems, such as repositioning outfall pipes to ensure [juvenile] fish are placed back into the river at locations that provide high survival.”
The projects are funded by Congressional appropriations under the Corps’ Columbia River Fish Mitigation Program. A portion of the cost is repaid by the Bonneville Power Administration, as part of its obligation to mitigate the impacts of hydropower dams on fish and wildlife in the Columbia River Basin. Hydropower dams operated by the Corps generate electricity that is sold by Bonneville.
Dykstra, joined by NOAA Fisheries research biologist Gordon Axle, reported on several examples of recently completed projects that have measurably improved fish survival.
At Lower Granite Dam and Little Goose dams, the Corps installed devices that pull cool water from far below the surface and mix it with warmer water from near the surface to moderate its temperature in the adult fish ladders, which encourages fish to enter and cross the dams. Before the device was installed, the river below the dam would warm in the summer sun, and fish would wait for cooler water before crossing. Cool water releases from Dworshak Reservoir upriver helped cool the water flowing through the ladders, but maybe too much – the sudden shock of cold water also caused delayed migration. Similar temperature changes occurred at Little Goose. These small decreases were enough to improve fish passage, Dykstra said.
Also at Lower Granite, which is the first Snake River dam juvenile salmon and steelhead from Idaho encounter on their way to the ocean, the outfall of the discharge pipe for the fish bypass system was extended to near the middle of the river. This helps fish avoid near-shore predators by dropping them into swifter, deeper water, increasing their chances of survival.
Another construction project at Lower Granite, completed in January of this year, is proving that juvenile fish with transponder (PIT) tags can be detected passing over a spillway, an important accomplishment and something Dykstra said, “we are very excited about.” Tagged fish that go through the juvenile bypass system can be recorded, but until now fish that went through the rapid current of a spillway could not. An array of detectors was built into a spillway at the dam. When activated, the array began detecting passing fish with great accuracy. “We have to see how it will perform over the long term; it’s still experimental, but we are very pleased with its performance,” Dykstra said.
Axle of NOAA said the experimental system has improved the detection of tagged fish at Lower Granite by 492 percent. In addition to tagged juvenile fish, the system also detects steelhead kelts, which are adult fish that have spawned and are returning to the ocean, a characteristic unique to steelhead, and also upstream-migrating adults that cross the dam but then are swept back downriver over the dam. He said the improved detection is providing critical new information for fish passage research.
At Little Goose, a device was installed that will greatly speed the operation of a movable spillway weir so that the dam can respond more quickly to changing river conditions when juvenile fish are migrating.
At another Snake River dam, Ice Harbor, a new turbine runner, installed in May 2019, is designed to reduce the risk of injuries to juvenile fish from striking the runner blades, and also reduce water pressure in the turbine pit, where rapid decompression can harm or kill fish. Tests with tiny balloons to represent juvenile fish so far have indicated 98 percent survival 48 hours after turbine passage and a reduction in turbine-strike injuries by more than half. “The problem of rapid decompression has been largely addressed and greatly reduced with this new turbine design,” Dykstra said, adding, “our goal is to develop a turbine that is safer for juvenile fish; turbines across the system need an overhaul to improve both fish passage survival and hydropower generation.”
More turbine replacements are planned at Ice Harbor, the oldest of the four lower Snake dams, and at McNary and John Day dams downriver on the Columbia, over the next five years or so.
The 2008 Biological Opinion established juvenile fish survival requirements for the Corps dams and, Dykstra said, “we have conducted tests, and by and large we are basically meeting or exceeding those metrics.”
He called the recent construction projects “the tail end of implementing the 2008 BiOp,” adding, “I think a lot of the low-hanging fruit has been plucked in terms of overhauling fish passage.”
The next step is focusing on river operations to improve fish passage survival, such as implementing the 2018 flexible spill agreement, he said. Under that agreement, operations are changed to spill water over dams to aid juvenile fish passage at times when demand for hydropower is low, and reduce spill at times when demand for power is higher.
“That’s why what we’ve seen, for example in the flex spill agreement, is an acknowledgement that we’ve done pretty much all we can do structurally at the dams,” he said.