Removable spillway weir

We humans owe a debt of gratitude to fish—after all, it’s a little bump that developed in their spines that is believed to be the precursor to the human brain. All the more reason to be concerned about preserving the salmon. In my childhood memoir, I recount the plight of these noble fish as they try to carry out their life’s mission, travelling up and down the Columbia across the dams (p. 93):

The young fish heading out to the ocean end up in a pipe called a “penstock,” filled with highly pressurized, fast-moving water, which hurtles them into a turbine rotating at 120rpm. Although it sounds like they ought to be chopped to bits, actually around ninety percent of the youngsters survive. Remember, though, that they have to make their way through not just one, but more than half-a-dozen dams before reaching the ocean. No more than fifty to sixty percent manage that.

Some dams also have a “juvenile fish bypass system” which is basically some screens in front of the turbines which deflect the fish into pipes which either dump them downstream, or in some case, into a holding pool from which they are carried downstream in a truck. The number of fish successfully deflected is called the “fish guidance efficiency,” and it ranges from thirty to eighty percent depending on the season and type of fish. Another concept is simply to spill water around the times of the migrations, with the fish getting an exhilarating ride over the dam.

Unfortunately, my description of “spilling water” over the dams was incorrect. In fact, the approach is to let the fish cross the dam under the spillways, through a high-pressure pipe whose entrance is sixty feet below the surface of the dam. This requires the salmon to make a harrowing dive to find the entrace, before being shot through the tunnel into the pool below the dam—where some are so disoriented they perish on the spot.

Now a new approach has been developed: the removable spillway weir, which requires the juveniles to dive just fifteen feet. They pass through under lower “veclocities” and “pressures”. The new weir, costing a cool $20 million, is said to improve survivability by several percentage points—which may not seem like much, until you consider that the salmon are navigating a half dozen dams or more, so the percentages add up. It weighs over 2 million pounds, and is 115 feet tall and 83 feet wide. The “removable” part of the name indicates that the entire device can be dropped to the bottom in flood situations. This device also supposedly “wastes” only 1/4 as much water per fish as existing approaches. It’s been installed on a trial basis at a dam on the Snake River.

But sorry, folks, you are still killing salmon. You’ve got to tear down the dams and let the salmon come and go as they wish.

One Response to “Removable spillway weir”

  1. winsailor Says:

    Why can’t the salmon be directed to go back down the fish ladders?

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