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Criterion 4 Indicator 20
Percent of Stream Kilometers in Forested Catchments in Which Stream Flow and Timing Has Significantly Deviated from the Historic Range of Variation
Changes in stream flow and timing can cause flooding or low flows. Insufficient flows can cause stream temperatures to rise to levels that are lethal or detrimental to some species of fish. Increased peak flows, or more frequent floods or high flows, can move spawning gravels or accelerate erosion. Either increased or decreased flows could indicate a general decline in watershed health.

Can This Indicator Be Quantified
Not at this time.
The U.S. Geological Survey (USGS) collects stream flow data at gauging stations on many rivers in Oregon. However, the relationship between forest practices and stream flow is not well enough established to use the gauging station data as an indicator of forest management’s effects on the variation in stream flow and timing.
Stream flows have been shown to increase in small watersheds when significant percentages of the watersheds are logged (i.e., 15 or 20 percent). Timber harvesting increases stream flow by reducing water losses from interception and evapotranspiration. The effects become smaller over time, but can last 20 to 30 years in western Oregon (Adams, 1994). Stream flow increases are proportionally greatest during the fall when soils in logged areas have more water than soils in forested areas. During floods or high flows, stream flows in logged areas do not change very much from flow levels in unlogged watersheds, since the ground is fully saturated in both cases.
The relationship between timber harvest and increased stream flows has been established for small watersheds. However, in the case of large watersheds, generally only a relatively small portion of the watershed is harvested at any one time, and no changes have been found in stream flow timing due to timber harvest. When peak flow data for large watersheds is examined, the analysis has not found any changes in peak flow magnitudes that can be attributed to timber harvest (Duncan, 1986).
Some studies show that timber harvesting can increase the flow of water during rain-on-snow events. These events occur when heavy rain falls on a snowpack in the mountains. Greater amounts of snow accumulate in clearcuts than in forested areas; then, during the rainstorm, more rain reaches the snow and melts it faster in open areas than in forested areas (Harr, 1992). However, harvesting intensity alone is not a good indicator of the overall effects of forest practices on hydrologic changes, even during rain-on-snow events. It may be possible to improve the prediction of timber harvesting's effects on peak flows by having additional information on elevation, aspect, stand age, and other factors (Connelly, 1992).

Trend information is not available.
There is insufficient information to quantify the range of natural variation in stream flows, for most streams in Oregon. Since streams have a lot of natural variation in their flow, there is inherently some uncertainty in estimating how forest practices are affecting those flows. If a stream has highly variable flow and only a few years of records on its flow, it is very difficult to tell the difference between natural variations and human-caused changes. If stream flow records have been kept for many years on a stream, there is less uncertainty.
This difficulty exists whether the stream’s average flow or flood patterns are being examined. Natural climate cycles have been observed that are much wetter or dryer than average, and last longer than ten years. Because climate cycles and natural variability can make it so difficult to detect human-caused effects and long-term trends, it is essential to have stream flow records for at least 50 years on a particular stream, if real trends are to be detected (Wahl, 1995). This type of data is not available for the vast majority of streams in Oregon.

Data Source and Availability
Stream flow data for 928 sites is available from the USGS web site at: http://waterdata.usgs.gov/nwis-w/OR
Data is available in several different forms from the USGS for 928 USGS data stations in Oregon.

Reliability of Data


Recommended Action for Data Collection
Oregon has many streams, and there is a great deal of natural variation in stream flow. Also, it takes many years of observations to understand stream flow and timing. For all these reasons, it would be very expensive to implement a comprehensive monitoring system. There is also a great deal of disagreement in the scientific community about how to model the effects of forest roads and harvesting. A large-scale assessment by hydrologists would be needed to develop a monitoring program to measure changes in stream flow.

Interception — The capture of precipitation on plant surfaces, which then may either evaporate or fall to the ground.
Evapotranspiration — Water loss to the atmosphere through evaporation of precipitation intercepted by plant and other surfaces and through vaporization of moisture taken up from the soil via roots and plant conducting tissues.

Selected References
Adams, Paul W., and Jeanette Ringer. 1994. The effects of timber harvesting & forest roads on water quantity & quality in the Pacific Northwest: summary & annotated bibliography. Oregon State University, Forest Engineering Department, Corvallis, OR. 147 pp.
Connelly, B. A., and T. W. Cundy. 1992. Cumulative effects of forest management on peak streamflows during rain-on-snow events. In: Interdisciplinary approaches in hydrology and hydrogeology, pp. 470-484. American Institute of Hydrology.
Duncan, S. H. 1986. Peak discharge during thirty years of sustained yield timber management in two fifth-order watersheds in Washington State. Northwest Science 60(4):258-264.
Harr, R. D., and B. A. Coffin. 1992. Influence of timber harvesting on rain-on-snow: a mechanism for cumulative watershed effects. In: Interdisciplinary approaches in hydrology and hydrogeology, pp. 455-469. American Institute of Hydrology.
Wahl, Kenneth L., and Wilbert O. Thomas, Jr., Robert M. Hirsch. 1995. Stream-gauging program of the U.S. Geological Survey. U.S. Geological Survey, Reston, VA. Circular 1123.