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He keeps referencing a "plunge pool" half way down the new slipway. Does anyone happen to know what the purpose of the plunge pool is?

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I think it's the natural depression that eroded where the water first comes off the top half of the chute.

I watched everything and I confirm, it’s what was the crack, and it doesn’t exist anymore, it’s filled in. At about the same place the are putting an aeration feature to mix air in the water and make it less destructive to the concrete downstream.

It's simply where the water from the broken spillway plunged down into the hillside.

They talk about two plunge pools (upper and lower), basically two different depressions in the stable bedrock. They do not have a purpose, but are rather just the result of the disaster.

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