We can. One presumes it is less efficient than just removing the dam. It is more work versus less work - fish ladders and the like.
Not only that, but a lot of these dams are not even in use any more, so doing additional work doesn't make much sense. Remove the dam, remove the problem, instead of duct tape solutions.
Not only that, but a lot of these dams are not even in use any more, so doing additional work doesn't make much sense. Remove the dam, remove the problem, instead of duct tape solutions.