How Does Forest Harvesting Affect Water Quality?

Because forests grow in high rainfall areas, most streams originate in or pass through forestland, especially west of the Cascades. Clean water is critical to our quality of life, and because in the West so much of our useable water passes through forests, discussions about water supplies and quality invariably touch on forest management practices.

In general, forests have beneficial effects on water quality. Forest soils are very porous and most precipitation that falls on a forest infiltrates rather than flows across the surface, which can cause erosion. Forest soils are also an effective filter, removing organic and inorganic chemicals from precipitation.

The closer streams are to the forest, the clearer they generally are, which is why so many municipal reservoirs are located in forests. Erosion is a significant threat to water quality, and any disturbance that causes erosion, including forestry activities, has come under scrutiny.

Forest roads are a potential source of sediment from erosion, especially during major storms. Most research has focused on older forest roads built in the 60's and 70's, but considerably less erosion has been reported from newer roads built to meet current Forest Practices Act design standards.

Harvesting to the edge of streams can also lead to erosion as well as increased stream temperatures. However, Forest Practices Act rules adopted in 1987 require specific buffer widths along fish-bearing and other streams, mandating retention of a minimum number of conifers within buffers.

These rules were strengthened in 1994 to increase conifer retention requirements with the objective of creating mature forest conditions along streams and improving fish habitat.

Chemical contamination fore forestry operations is another potential concern, especially in regard to drinking water. Forest Practices Act rules are designed to prevent chemical contamination of streams, and monitoring has shown them to be effective.

SOURCE: Oregon Forest Resources Institute, Forest Fact Book.