When heavy rains cause flooding, the water moves downstream under the pull of gravity. For a given flow rate, if the water moved very fast, the depth of the moving water could be shallow. For the same...

When heavy rains cause flooding, the water moves downstream under the pull of gravity. For a given flow rate, if the water moved very fast, the depth of the moving water could be shallow. For the same flow rate if the water moved less rapidly, the moving water would be very deep. What physics determines which of these two states nature chooses?