You can turn a culvert into a waterslide with just a few boards and some rope.
As you can see on the video, we tied ropes onto the boards, used the boards to block off the culvert, and then let the streambed fill up with water. When the reservoir was full, we climbed over the boards into the culvert and removed the braces from the boards. The boards were forced into the culvert but the ropes kept them from following us down.
Once the boards are removed the water rushes into the culvert to shoot us all the way through until we splash down into the pool at the other side of the culvert. The ride is about 1/4 mile (400 meters) long.
We rode on foam boogie boards but some people chose to just ride on their butts. (They shredded their clothes and sometimes their skin--NOT recommended.)
Before we did this, we made sure that we knew exactly what was inside of the culvert and in the splash pool at the bottom so as not to hurt ourselves or anyone else.