Many of these structures have concrete channels downstream of the weir which means that the stopper/keeper/recirculating water is equally strong across the width of the river. It can also be extremely hard to swim with any sort of accuracy due to being constantly recirculated and not-very buoyant due to the entrained air.

There are other subtleties in 3D as well; some weirs form a downstream-pointing V when viewed from above the dam - these are generally considered safer as the currents will (generally) move you to the middle of the river and downstream where the water is more likely to be escapable - see [1].

Others can form an upstream-pointing V which has the opposite effect where the currents will move you towards the middle of the river but upstream back towards the stopper/keeper/hydraulic.

[1] https://assets.atlasobscura.com/media/W1siZiIsInVwbG9hZHMvcG...