How do river levels rise in feet so quickly when we only get a couple inches of rain?
In: Physics
When rain falls, it heads downhill and continues moving downhill until there’s no downhill left (i.e., it hits the lowest part of its journey) or it gets absorbed into the ground (or evaporates). When the ground is saturated (like after a heavy rain or just a lot of rain over a short period of time), rainwater isn’t absorbed into the soil and will just continue to flow downhill. Streams and creeks are typically found in small pockets of downhill flow. These fill quickly and they flow downhill as well, joining other creeks and streams, forming small rivers and then larger rivers. After a hard rain, all that water rushing into all those creeks, streams, small rivers, etc. keeps flowing downward to the larger rivers. In any sizable river system there are thousands of these smaller feeder creeks, streams and rivers, all swollen with rainwater, all flowing to the larger river. All that water causes the terminal river to rise rapidly.
Another way to think about it is to think of the size of a river’s drainage basin (the area that feeds into the river). Some are huge, thousands of times larger than the river’s actual area. All the rain that falls in a river’s watershed eventually joins the river unless it is absorbed into the ground or evaporates. If one inch of rain falls over an area of one acre, that’s about 27,000 gallons that now has to flow somewhere. Take the Susquehana River as an example – 464 miles long with a drainage basin of 17,600,000 acres. An inch of rainfall throughout the drainage basin adds nearly 480 billion gallons of water, much of which will make its way to the Susquehana sooner or later. When the ground is saturated, it gets there much sooner, causing the river to rise quickly.
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