Many lakes have a giant hole in them near the dam. That's how it looks anyway - you have the smooth surface of the water and then there is a big hole in the lake. It seems impossible, but what you are seeing is the top of something called a morning glory spillway, also known as a bell-mouth spillway.
The classic hole-in-the-lake:
Here the water in the lake is so deep that a whirlpool forms around the spillway:
This is what the mouth of a morning glory spillway looks like when there is no water in it:
At the other end of these spillways you find big pipes to let the water out. The first video above is shot at Lake Berryessa. The other end looks like this:
The Glory Hole is a 30′diameter spillway pipe (approx. 700ft. in length) that comes down off Lake Berryessa, near Winters, Calif. It's a massive, perfectly transitioned, ridable full-pipe, that is sought after by just as many, as the infamous Baldy full-pipe down in Southern, Calif. The only difference between the two is that the Glory Hole is a little more challenging to get to. Not only do you have to hike several miles, but you then have to take a raft ride down the river to get to it as well. If your lucky enough and make it that far without taking a spill, the pipe is then all yours to ride.
Spillway pipes are sometimes massive and complex:
Obviously the exit pipe will often be full of water rather than empty. These two videos show you the whole picture. The Owyhee Reservoir has a morning glory spillway that you can see in this satellite photo:
The top of the spillway looks like this:
The bottom looks like this:
The spillway for Yuba Lake, when half-full, has recreational applications:
You might wonder why they use a bell-mouth structure like this. It seems incredibly dangerous to any boat or swimmer that gets near it. The thing to keep in mind is that any spillway is dangerous to boats and swimmers, so they put up fences and buoys to keep people away.
They use spillways like this on dams that have vertical faces. On most earthen dams, the face of the dam is sloped and they can use a normal "water flows over the top of the dam" kind of spillway like this:
Many concrete dams with vertical faces go the bell-mouth route since the over-the-top route would cause wearing or erosion at the bottom of the dam.
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