An improved design for assessing impacts of watershed practices on small streams

Assessment of impacts of watershed management practices (logging, grazing, etc.)on stream ecosystems is an important problem facing resource managers. Considerableeffort has been put into studies of such effects, yet results have too often been equiv-ocal. Difficulty may arise from the complexity of multiple pathways through whichthese practices may influence stream ecosystems. We suggest that another source ofdifficulty may lie in the design of the various studies. In this paper we review fourapproaches to assessing impacts of watershed practices that have been taken in Oregonand suggest the most efficient design in terms of time, effort, and power to discriminateeffects.
Study designs differ in the way they address a research problem. Each study muststrike a balance between the level of precision and generality that it attains. Further-more, limitations on time, funds, and manpower dictate that one choose the designthat most efficiently allocates effort, and that best suits the study objective.
The approaches can be classified in four categories according to whether theyinvolve study before treatment (before-after) or not (post-treatment) and whether theyinvolve one or two streams (intensive) or many (extensive). Extensive studies are usuallyof short duration. Intensive studies must, by nature, extend over at least several yearsto account for natural variation. The total effort may be about the same; extensivestudies involve single observations at many sites, and intensive studies involve manyobservations at the same site.