Urban Forests Improve Water Quality

Waterways and lakes in and near urban areas can be polluted by soil erosion and water runoff that may contain fertilizers and pesticides from landscapes, oil, and sewage. Trees and vegetation can help reduce water quality problems in communities by decreasing stormwater runoff and soil erosion. Trees also absorb some of the nutrients in the soil that would otherwise be washed away. Communities can have cleaner water by managing existing natural vegetation, planting additional trees, and reducing the use of pesticides and fertilizers.

Stormwater runoff

In many communities, the rate and volume of stormwater runoff have increased beyond the capacity of existing stormwater drainage systems. This is caused by continued development of hard, impermeable surfaces such as roads and parking lots that cannot absorb water, thus changing natural drainage patterns. Additionally, the increased runoff rate causes stream channelization, resulting in a degradation of water quality downstream.

The canopy of trees provides interception of rainfall, which can slow the rate and reduce the total volume of stormwater runoff. Planting trees in riparian buffers will help stabilize steam banks and improve water quality. Some of this water remains in the crown and returns to the atmosphere through evaporation. Other rainwater will eventually fall to the ground (through-fall), where it will infiltrate into the soil or continue as runoff .

Many urban forestry activities, such as creating open spaces, saving trees on construction sites, and planting trees after construction, can help reduce the amount of stormwater runoff that enters storm sewers and streams.

Raw sewage spillover

During heavy rainstorms, problems occur when water floods into the sanitary sewage system. If the sewage system is old and leaky or if the treatment facility cannot handle all the stormwater runoff, raw sewage spills over into natural waterways. This can cause a dangerous increase of bacteria in the water. Communities with this problem may be charged large fines, suffer lawsuits from downstream users of the waterways, have to make costly improvements to the sanitary sewer system, or stop further development until water-treatment facilities are improved. Trees, vegetation, and wetlands can help prevent this problem by interrupting and absorbing stormwater runoff.

Soil erosion

Long-term loss of riparian vegetation has been linked to both stream bank erosion and channel widening (Oliver and Hinckley 1987; Shields et al. 1994). Trees, more than any other vegetation, can limit soil erosion by helping control stormwater flow. Fibrous root systems hold soil in place so that it is not washed away by rain or flowing water (Harris et al. 2003; Wynn and Mostaghimi 2006). Tree roots help stabilize stream banks, protecting stream habitat and preventing accelerated sediment movement downstream.


Photo Credit: Raina Sheridan

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Harris MR, Lamb D and Erskine PD (2003) An investigation into the possible inhibitory effects of white cypress pine (Callitris glaucophylla) litter on the germination and growth of associated ground cover species. Australian Journal of Botany, 18(51): 93-102

Oliver, C. D., and T. M. Hinckley. 1987. Species, Stand Structures, and Silvicultural Manipulation Patterns for the Streamside Zone. p. 259-276. In Streamside Management: Forestry and Fishery Interactions. University of Washington. College of Forest Resources. Contribution No. 57.

Shields, Jr., F. D., S. S. Knight, and C. M. Cooper. 1994. Effects of channel incision on base flow stream habitats and fishes. Environmental Management. 18(1):43-57.

Wynn, T., and S. Mostaghimi. 2006.  Effects of riparian vegetation on stream bank subaerial processes in southwestern Virginia, USA.  Earth Surface Processes and Landforms  31:399-413.

By: Ed Macie, Regional Urban Forester, USFS Southern Region