Posted on July 20, 2021
PORTLAND, Ore. — Humans. What other sentient being designs a tool requiring hands and feet and expects animals without limbs to use it? Alas, the answer is humans.
Humans created a ladder for fish, which is quite effective in certain situations – but isn’t a blanket solution to every fish passage problem.
And while most humans would agree that ladders can be useful for climbing short distances, perhaps 20-50 feet– another tool – like an elevator or truck – may be a better option to climb hundreds of feet. Otherwise, there would need to be more infrastructure to support that ladder, or perhaps it would need to be a staircase at that point.
This is similar for fish when moving them up and downstream.
The U.S. Army Corps of Engineers has several tools to move fish upstream of its 40-400-foot-tall dams. For shorter dams, where water elevation in the reservoir is relatively stable or minor fluctuations in elevation, the Corps generally uses fish ladders. At taller dams, where the water elevation can fluctuate by hundreds of feet, the Corps employs the “trap and haul” method.
Sometimes, even the smartest sentient beings – humans – need reminding that a ladder may not be the only solution to a particular problem. Even Abraham Maslow recognized this when he said, “I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail.” However, humans do have other options, especially when engineers need to consider other complications.
Pool levels in reservoirs vary greatly in the Willamette Valley Project because of its flood risk management responsibility – meaning a reservoir level could be up to 170 feet lower in the winter than the summer and the water can have significant fluctuations in one day. This is one factor that make fish ladders at these high-head dams impractical according to Marie Phillips, Hydraulic and Coastal Design section chief.
“A traditional fish ladder provides a rise of approximately one foot for every 10 feet of length” said Phillips. “For a dam that is several hundred feet in height, which many of the Willamette Valley projects are, the length of ladder needed would correspondingly be several thousand feet in length,” she said.
Building such a structure isn’t impossible; however, it would be massive – and unneeded since trap and haul works. In fact, small fish ladders already exist at the base of most of these dams that lead up to adult fish collection facilities. Corps biologists then collect and sort fish that migrate up the ladders. Fish, like salmon that need to migrate upstream are then placed in tanks and then a truck, which drives around the dam (or dams) and releases fish upstream. Biologists return fish that don’t need to migrate back to the river below the dams.
However, even if the Corps decided to build a fish ladder at these types of dams, there are other problems: after fish climb to the top of the dam via a fish ladder, there would be difficulties getting them back down 100 or more feet to the reservoir below at any given time of year.
“We don’t have openings through the dam to let them [salmon] out at specific intervals over that range,” explained Phillips. Even if we added control sections to the ladder to accommodate fluctuations, we would need outlets approximately every 15 feet.”
That could equate to more than 10 holes in the dam.
The next problem: water temperatures. The shallower water in the fish ladder could get too warm from sun exposure, which would also harm fish. Additionally, an unresolved issue at many of these dams is juvenile downstream passage – not upstream passage. Fish ladders wouldn’t help that situation, either.
“Juvenile fish move downstream differently than adults move upstream,” said Phillips. “Traditional fish ladders are designed for upstream passage with small rise and resting areas and this is not the preferred passage method for juvenile fish.”
The Corps has been looking into the best way to solve this problem in the Willamette Valley for several years. Some proposals at Detroit and Cougar dams involve floating barges to capture fish for transport downstream, similar to the successful trap and haul method to move fish upstream.