How do river levels rise in feet so quickly when we only get a couple inches of rain?


How do river levels rise in feet so quickly when we only get a couple inches of rain?

In: Physics

When the rain collects around the river, the ground water swells and run off from other places not directly in the river tend to be pulled towards the river. This makes rivers rise quickly — the saturated water bed, and multiple smaller amounts of water that get pulled into the river from smaller tributaries.

Imagine a glass so chock full of water that simple extra drop of water will make it flood to the table.

You add that drop. Does water spill to the table only that extra, unneeded drop? No. Entire top layer will flow off. Or multiple drops of water worth, at least.

Replace glass with a lake, underground reservoir, multiple pools and puddles – rain drops everywhere – and a single drop by inches of rainfall – and you get a chain effect.

Also, water dampens and softens the soil and moves rocks. If a puddle is being held by piece soil or rock and it’s suddenly removed by rain, like the cork in the bathtub, entire puddle will drain.

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.

Water flows from everywhere, downhill, into rivers and lakes. So the river is not just going up from 2” of rain that falls right on the river. It’s getting the water from surround areas, as well as from upstream. And upstream, the river is gathering water from surrounding areas *there*. This is why downstream areas can get the brunt of the flooding while upstream areas see a small rise in levels but nothing damaging.