Where does the rain go? Understanding river basins and watersheds

Have you ever wondered where all the rain goes? It goes to the same place as snowmelt when temperatures thaw out in the spring.

It’s called a river basin, or a watershed, and whether you know it, all the land we stand on is part of a river basin. 

Some water will seep into the shallow ground, moving through the soil and helping keep grass, trees and plants healthy. It can even travel deeper down, recharging groundwater aquifers.

Once the ground becomes saturated, a river basin collects runoff and excess water and drains it into a larger body of water.

Depending on where in the basin, it can move into small bodies of water like creeks, streams and rivers, and eventually to outflow points into larger bodies of water like a reservoir, bay or the ocean.

In the U.S., most rain runoff eventually finds its way into the Gulf of Mexico, Atlantic and Pacific Ocean, or the Great Lakes. 

This map below from Grasshopper Geography shows all the permanent and temporary streams and rivers of the Lower 48 in the U.S., divided into catchment areas. 

The Mississippi/Atchafalaya River Basin (highlighted in pink) is one of the largest in the world. It includes parts or all of 31 U.S. states and two Canadian provinces, collecting precipitation for 41% of the contiguous United States.

The Mississippi River’s water level closely correlates to precipitation totals across the basin. Persistent, heavy rainfall or excessive snowmelt can cause high water levels and put sections of the river into flood stage, threatening communities along the river.

Conversely, severe drought across the Mississippi River basin can lead to low water levels, which can disrupt the shipping industry and expose some oddities from the bottom of the river.

Snowmelt

When snow melts, the water goes to the same place that rain would go. Generally, 10 to 12 inches of snowfall produces 1 inch of liquid water.

In the western U.S. mountains, environmental and utility experts work to conserve that water and replenish lakes and reservoirs that shrink or dry up during times of drought. In the Central U.S., most of the snowmelt ends up in the Mississippi River.

The annual snowpack in mountain ranges can help forecast potential flooding concerns when temperatures warm up. If there’s a large snowpack, rapid snowmelt can cause flooding.

Below, you can see a video from Dec. 2023, after heavy rainfall and snowmelt from warm temperatures combined and led to significant flooding and high waters in Fairfax, Vt. 

It’s more common during the spring, but also possible if temperatures climb well above normal during winter.

Snowmelt can be just as important as rain for areas when it comes to recharging groundwater and supplying freshwater for people and animals. Less snowfall means less snowmelt and less water that could be resourceful for us. 

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