Supposedly you swim downwards, in a downstream direction, when you reach the river bottom.

Note the path of the blue arrows escaping downstream in the diagram.

The catch being that you're submerged, being spun around in the currents and disoriented, and you have a very brief window to claw yourself along the riverbed.

I can't find the source but have seen reports where people have been caught in a weir, drowned, and remained spinning in the currents for a couple of days before they're eventually spat out by chance and the bodies are recovered. They've well earned the name "drowning machines".

I have no intention of going anywhere near one of these dams, if I can help it.