>>509654915 (OP)The existing riverwater wants to flow at its normal pace
The floodwater flows quickly. When it hits the water flowing slowly, it doesn't necessarily push it forward, instead it flows over top of it. The result is that the current is very, very strong near the surface, but slower underneath. This creates a pressure differential that pulls downward, plunging you under. The floodwater becomes a wall that prevents you from surfacing. You can pulled under and trapped there until you drown. The forces involved are well beyond what any human could overcome, no matter how strong of a swimmer they are.