How do the weather experts know that a river is so many feet above flood stage when it is spread out?

Question

Weather experts may say that the Mississippi is 20 feet above flood stage. How can they know this when the river is dispersed?

Thanks in advance.

Answer 1

They have equipment either near the riverbank or actually in the river that measures the depth at a fixed point.  So let's say the river depth is 10ft, and flood stage is at 15ft...they know its 20 feet above flood stage when the depth at the fixed station is 35 feet.

Answer 2

Most towns have dikes built along the banks. At normal flow, the river level will be many feet lower than the top of them. At flood level, the river has risen just to the top. At 20 feet over, water will flood over them and continue to flow into the towns until the level spreads itself out.