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Fall 1991

The New Alsea Watershed Study:
Hydrologic Recovery and Cumulative Effects Assessment

(Editor's note: This was downloaded from the EPA's new nonpoint source electronic bulletin board - see page 14 for details. This originally appeared in Hydata, July 1990.)

The Alsea Watershed Study has been reactivated by Dr. John D. Stednick, who is on sabbatical with NCASI (National Council for Air and Stream Improvement) and the Forest Engineering Department at Oregon State University. The current water resources research is partially supported by NCASI.

Introduction

The original Alsea Watershed Study was the first long-term watershed study to evaluate the effects of logging on stream physical and biologic properties. Three small tributaries of the Alsea River, Oregon, were studied from 1959 to 1973. Deer Creek - 304 ha (750 acres) was patchcut in three cuts of about 25 ha each, with a vegetation buffer strip left along the main stream channel. Needle Branch - 71 ha (175 acres) was completely clearcut without stream protection. Flynn Creek - 203 ha (500 acres) remained undisturbed as a control area. Roads were constructed in 1965 and logging took place from March through October 1966. Post-logging monitoring continued until the fall of 1973.

Results

Annual water yield was increased over 51 cm by clearcutting and broadcast burning in Needle Branch. Annual water yield increased by lesser amounts at Deer Creek.

Elevated stream temperatures were among the most dramatic effects observed for Needle Branch, where streamside vegetation was removed. Because Needle Branch is such a small watershed, this may account for the temperature increases being among the largest ever observed. By 1973, temperatures had returned to pre-logging values. No temperature changes were observed on the patchcut watershed, where riparian shade remained intact.

Suspended sediment concentrations were variable in time and space. Suspended sediment concentrations significantly increased over expected values during the first winter after slash burning on Needle Branch. Sediment yields declined to near-normal levels four years later. Increases were also observed on the patchcut watershed but the source was attributed to periodic road fill failures which persisted for one year.

These and other findings from the Alsea study were used to help developed state forest practice regulations in Oregon, and much of the nation. For example, most regulations require the use of buffer strips along streams to protect stream banks, to provide shade, and to keep slash out of streams.

The New Alsea Study

Increased water yields resulting from timber harvesting are variable by site, and vegetation type and stocking. Increased annual water yields decrease over time as vegetation reestablishes until the site is fully stocked. For 13 years of post-treatment record, at the H. J. Andrews, the increased annual water yield decreased in a linear fashion. A predictive equation using time after harvest was developed to determine hydrologic recovery. This equation predicts that Needle Branch will return to pre-treatment levels of annual water yield in 1991.The newly reactivated water quality monitoring program and streamflow data collection program will provide data to assess hydrologic recovery and cumulative effects of silvicultural treatments on water resources in the Oregon Coast Range. Identification of potential long-term water quality and quantity changes are needed to define cumulative impacts (changes to the environment caused by the interaction of natural ecosystem processes with the effects of two or more forest practices). Current cumulative impact assessment efforts tend to use only physical characteristics of the stream channel.

Flynn Creek has been designated by the United States Forest Service as a long-term Research Natural Area and will remain undisturbed. Needle Branch has remained undisturbed since the timber harvest and slash burn. Some pre- commercial thinning has been done in the fully stocked second growth Douglas- fir stand. Annual water yield, and other streamflow characteristics, will determine the validity of the hydrologic recovery model. Deer Creek has had two additional harvesting entries, one unit in 1978, and two units in 1987. The multiple entries in Deer Creek will provide the opportunity to assess the ability to predict hydrologic recovery and to identify potential cumulative impacts on water resources.

The New Alsea Watershed Study will be an important addition to the long term hydrologic data base. The study will provide needed input for current cumulative watershed effects models. Various agencies have been contacted to determine additional research opportunities and to identify additional research funding opportunities.

Dr. Stednick may be reached at the Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523.

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