Following a very heavy rainstorm that dumps 15 cm of rain over a day or two, the Russian River just north of us in CA can have a peak flow of as much as ~70,000 cubic feet/second for about 48 hours.
Calculate the area of land (in square km) over which 15 cm of rain would have to have fallen to produce the volume of water flowing down the river at that rate for 48 hours.
Assume that the ground is sufficiently saturated that all rainfall is runoff and the air is so humid that no evapotranspiration occurs during the period from the storm to the end of the peak flow.
Is your answer reasonable? Explain.