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Type N Experimental Buffer Treatment Study

  • What: This study assesses how timber harvest activities, such as using different vegetative riparian buffer treatments in relatively small basins, affect a suite of input processes (heat, litter, sediment, and wood) and how changes in those processes affect downstream fish-bearing waters. The study will also link changes in stream conditions and input processes to changes in abundance of amphibians.

  • How:  By comparing individual buffer treatments to each of their pre-treatment (reference) conditions, this study seeks to assess the degree to which forestry practices may impact public resources.

  • Scope: This study focuses on non fish-bearing (Type N) basins that are located in the geographic range of the targeted amphibian populations. The sites were of harvest rotation age, and less than 120 acres in size to allow for harvest to be completed in one operation.

  • Duration: All basins will be studied for a minimum of two years pre-harvest, one year during harvest, and two years post-harvest.

  • Investigators: The agencies involved in data collection include the Washington State Department of Ecology, the Washington State Department of Fish and Wildlife, and the Weyerhaeuser Company.


Type N describes perennial and seasonal non fish-bearing streams under Washington State’s current stream-typing system. Although physical barriers, higher gradients, and seasonal low flows make Type N streams inaccessible to fish, exports from these relatively small basins are important to downstream fish-bearing waters. In contrast to larger, fish-bearing streams, Type N are more influenced by hill-slope processes. Land use practices in Type N basins streams may have consequences for fish populations downstream, and for stream biota and water quality in the Type N stream itself.

The Forests and Fish Law established forest practice rules that would contribute to the recovery of protected salmonids and other species as well as the enhancement of water quality in streams (FFR 1999, ESHB 2091). Under the rules, Type N streams receive a 50-foot buffer along 50 percent of the perennial stream length, a 50-foot buffer around perennial sensitive areas, and a 30-foot equipment limitation zone along the entire perennial and seasonal stream length. The effectiveness of the buffer prescriptions in protecting aquatic and riparian species and in maintaining water quality, however, has not been tested. The purpose of the Type N Experimental Buffer Treatment Study (Type N study) is to assess the influence of different buffer prescriptions on riparian inputs, stream-associated amphibians, water quality, exports to downstream fish-bearing waters, and fish.

Objective and Purpose

At the time of negotiations determining harvest regulations, almost no published studies existed on the efficacy of buffers for headwater streams or on clear guidance addressing buffer design (Hayes et al. 2005). This study should allow clear conclusions to be drawn in evaluating how logging practices impact resource targets such as amphibians.

Comparison of individual treatments to reference treatments (unharvested basins with comparable type and age of stands) will help determine whether observed changes are attributable to environmental variation or harvest treatment effect.

The critical questions for the Department of Ecology are What are the effects of different buffer treatments on:

  1. Stream temperature within the Type N stream and in downstream fish-bearing waters.
  2. Stream flow, suspended sediment, and nutrient export from Type N streams.
  3. Organic matter input to and export from Type N streams.
  4. Macroinvertebrate export from Type N streams.


Ehinger, W. and S. Estrella. 2007. Quality Assurance Project Plan: Type N Experimental Buffer Treatment Study: Addressing Buffer Effectiveness on Riparian Inputs, Water Quality, and Exports to Fish-Bearing Waters in Basaltic Lithologies. Washington State Department of Ecology, Environmental Assessment Program, Lacey, WA. 28 pp. + appendices. Publication No. 07-03-103.

Hayes, M. P., W. J. Ehinger, R. E. Bilby, J. G. MacCracken, R. Palmquist, T. Quinn, D. Schuett-Hames, and A. Storfer. 2005. Study Plan for the Type N Experimental Buffer Treatment Study: Addressing Buffer Effectiveness on Stream-Associated Amphibians, Riparian Inputs and Water Quality, and Exports to and Fish in Downstream (Type F) Waters in Basaltic Lithologies of the Coastal Areas and the South Cascades of Washington State. Report submitted for the State of Washington Forest Practices Board Adaptive Management Program. 68 pp. [15 July].

Lewis, J. 1996. Turbidity-Controlled Suspended Sediment Sampling for Runoff-Event Load Estimation. Water Resources Research 32: 2299-2310