As a paddler you will be very familiar with flow patterns that exist in the normal river situation. As the river floods, water flows in places that only rarely see water, where rocks are sharp, not worn smooth, where trees grow, banks don't follow a natural water worn shape, and the power and energy of the water increases faster than most people appreciate. Physics tells us that twice the speed of the current means four times the force it can generate if something gets in its way. Different channel shapes have totally different characteristics as the flow increases. Pool drop rivers turn into continuous swift water with river wide hydraulics at narrows.
Braided rivers spread out bank to bank and the big rivers get deceptively dangerous.
Flow patterns which are not apparent at normal flows, start to make their presence felt. All water in flowing channels moves in a vortex like a big double corkscrew running downstream. This is just a swirl on the surface normally, but in a flood water mushrooms up from river banks and folds down on the eddy lines with tremendous power. The nett effect is that if you are in the river you will be unable to reach the safety of the banks. Each time the flow slows down you get pulled under as the river's kinetic energy converts into vertical axis rotational flow.