How Trees & Soil Prevent Stormwater Pollution

In the early 1900s, Joyce Kilmer wrote in a now famous poem about trees, “Poems are made by fools like me, but only God can make a tree.” California faces a bit of a tree crisis now as more than 100 million trees in its forests have died in recent years. What’s the cause of this massive tree mortality? Scientists from the U.S. Forest Service say it’s likely from back-to-back years of severe drought. Now with all the rain after this long period of drought, some trees are at risk to toppling over, including the ancient Pioneer Cabin Tree, in Calaveras Big Trees State Park.


Trees are very important to our environment. Aside from providing oxygen we all need to breathe, and storing massive amounts of carbon in roots, branches and leaves, trees also help mitigate flooding, purify water and stabilize soils. You may not think “stormwater management” when you see a tree, but in fact trees are some of our best tools for reducing the impacts of heavy rain events and limiting water pollution in our streets and waterways.


When rain falls on impervious surfaces, like buildings, roads and parking lots, it becomes stormwater runoff, which can be a serious water pollution problem when it mixes with whatever is on those surfaces and flows directly into nearby streams, the Bay or Ocean. All kinds of harmful pollutants can end up downstream:


  • Oil, grease and automotive coolants
  • Soaps from car washing
  • Garden and lawn fertilizers and pesticides
  • Bacteria from pet waste 
  • Chemicals from accidental spills or leaky storage containers


When rainwater falls on trees instead of impervious surfaces, it slows down, trickles off leaves and branches and soaks into soils and roots. In addition to reducing flooding by capturing rainwater and slowing down the rate of stormwater, trees and their roots can also clean-up polluted runoff before it gets into our local waters.


Depending on the size and species of a tree, as much as 100 gallons of water can be captured. Experts say this can mean reducing stormwater runoff by two to seven percent[1]. Additionally, urban areas designed with natural landscapes and trees that filter water can cut back on costly capital expenditures that might otherwise be needed to expand the existing infrastructure to manage stormwater.


Through drought and floods, Californians have come to understand how important water is for our state. It is time we acknowledge the role trees and soil play in balancing our environment and water supply.


[1] E. Gregory McPherson, James R. Simpson, Paula J. Peper, Aaron M.N. Crowell, and Qingfu Xiao, Northern California Coast Community Tree Guide: Benefits, Costs, and Strategic Planting (April 2010): 28. Accessed March 15, 2017.