Healthy Forests for Healthy Water

small runoff waterThough seemingly unrelated, the health of a forest directly affects the health of the surrounding waterways. While the trees depend on the local water supply for survival, they perform valuable filtering services on this water, which helps to keep the local supply clean. One of the most valuable ways trees help to protect the local water bodies is by reducing the amount of runoff water flowing across the landscape.

Reining in Runoff

While not a type of pollution in the strictest sense, excess runoff water is a serious problem for local watersheds. Excess runoff water can overwhelm creeks and streams, causing widespread flooding. Additionally, it accelerates the rate of erosion, rapidly undermines stream banks, undercuts roads and can cause significant destruction to the local infrastructure.

The sediments collected in the runoff water eventually make their way into streams, making the water cloudy, which can harm many aquatic organisms. In urban areas, runoff water collects numerous pollutants in the form of oils, fuels and debris, which ultimately flow into the rivers, ponds and swamps of the region.

Mechanisms of Mitigation

Trees – and more importantly forests – reduce runoff water and the amount of sediment reaching local streams and creeks in a variety of ways.

  • They reduce the amount of groundwater in the area through the process of transpiration. This allows the soil to absorb more water during the rain, thus reducing the amount of runoff water reaching streams
  • Trees collect water on their surfaces, and release it slowly to the ground, which helps to reduce the amount of runoff water in an area.
  • The growth pattern of tree roots and the activities of the microorganism near their roots encourage increased rates of water infiltration into the soil, which further reduces runoff water.
  • The trees’ roots help retain soil and prevent erosion, which protects yards, roadways and – most importantly – stream banks from collapsing.
  • The canopies of trees protect the soil from the pounding of raindrops, which reduces the amount of sediment that reaches local waterways.

Complicated Considerations

One common way that trees are used to reduce the amount of runoff water flowing across parking lots, driveways and other small paved areas is by planting a small group of trees near the low point of the surface. This can help you reduce the runoff from your property and increase the property’s value at the same time. The trick for those living in the Los Angeles area is to select species that are native to the state – and thus equipped to deal with periodic droughts – yet still able to cope with the periodically saturated soil that they will be exposed to. California laurels (Umbellularia californica), western hemlocks (Tsuga heterophylla) and California buckeye (Aesculus californica) are excellent choices, but if you prefer non-native species, consider black gums (Nyssa sylvatica) or bald cypresses (Taxodium distichum).

Healthy Water through History

Humans have understood that trees affect water quality for hundreds of years. According to, residents of Rio de Janeiro began advocating for the protection of the area’s coastal rainforest to prevent declining water quality in 1658. Later, after a series of droughts reduced the available water in the mid-1800s, reforestation efforts were begun in the region of Tijuca, along the Atlantic Coast. (Messenger, 2010) Today, the Tijuca National Forest is one of the largest urban forests in the world, covering approximately 12 square miles, and helping to provide clean drinking water for Rio’s nearly 12 million residents.


Messenger, S. (2010, August 22). World’s Largest Urban Forest Was Planted by Hand. Retrieved from