"Morning glory" type spillway.

A spillway is built to keep water from overtopping a dam. If the water gets to high, it goes down the spillway, usually built to the side of the dam, rather than over the dam, which could damage the dam.

A morning glory spillway is a different kind of spillway, usually built in places where you cannot build a side spillway, like when a dam is built between two rocky hills.

Instead, the morning glory is big fat tube built high enough that water usually isn't high enough to go over its walls. But its walls are at just the right height that water will start going over them, and down the tube, when the water level behind the dam is getting dangerously high, high enough that it could go over the main dam if it got much higher.

The water flows down the tube beyond the dam in a controlled way. It's a way of letting excess water past the dam in a safer way than, you know, just rushing over the dam and maybe destroying it and killing everyone. There may be some flooding but not deadly flooding.

At least I think that's the deal. See this guy for the details.