As little as six inches of moving water can cause unstable footing. The faster the water is flowing, the shallower the depth required to sweep people off their feet. In deeper water, less current is required to make one unstable due to increased buoyancy. Water is very dense and moving water can easily push you down.
The depth of water is hard to determine because of the high turbidity (muddiness) of flood waters. Use a walking stick to investigate the area in front of you.
