When you go whitewater rafting for the first time, one of the cardinal rules they teach you is NEVER EVER STAND UP IN THE RIVER if the raft flips over or if you get bumped out of it.

Instead, you are taught to float with your legs up off the bottom and use your arms to swim or paddle to safety.

Why is this?

The reason is simple. Even in a calmly flowing river, the force of the flow is strong. If you try to walk on the bottom, you may step in a hole and get stuck... or you may catch your foot on a submerged tree and get stuck.

When you become stuck, the flow of the river may push you under since your foot is now stuck... and you may drown in this predicament.

So yes, it is drowning that causes flooding deaths. These people were swept away quickly and their natural reaction is to try to stand. With so much debris, victims are quickly caught up in it and pushed under by the flow. Many times that is it for them. You've got less than a minute to escape typically, and you will be in an extreme panic which will not help you.