The concrete plugging the tunnels can be removed, and has been before. Here's a project in Oregon where the original diversion tunnel plug was removed, to allo

The concrete plugging the tunnels can be removed, and has been before. Here's a project in Oregon where the original diversion tunnel plug was removed, to allow drawing cold water from the bottom of the lake to feed the river: https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2005/triservice/track1/schlenker.pdf https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2005/tr...