Project Summary
Management activities in forested watershed may produce changes that are cumulative; i.e. changes that accrue either offsite or over time. Agencies responsible for long-term planning and management of watershed resources must now consider the potential environmental damage associated with cumulative effects. Unfortunately, sufficient information is lacking to critically evaluate the cumulative effects problem.
A critical component of the problem is whether management activities affect hillslope processes (e.g. erosion, runoff) and adversely affect downstream beneficial uses of water (e.g. fish production and water quality). The problem from a biological perspective is whether alteration of watershed and stream channel processes "significantly" change the following components of the stream system: 1) the morphological characteristics of the stream environment (i.e. physical and biological habitat), 2) the "normal" disturbance regime of the stream environment, and 3) the health, abundance, and production of important stream biota.
At this time biotic changes cannot be predicted from modeled or measured changes in the physical environment of watershed or stream channels. In addition, thresholds for change cannot be estimated given our current knowledge of how these systems operate. We have proposed an empirical approach to the problem. Our goals are to: 1) advance understanding of cumulative effects, and 2) provide tested and scientifically valid methods for predicting and detecting cumulative effects on forest lands as they relate to the stream environment.
Achieving these goals requires that we determine whether cumulative effects exist, and if so, whether some types of basins are more sensitive than others. The most effective approach to this problem is to examine watershed, stream channel, and biotic variables over a range of impact levels and forest management activities. The study includes approximately 50 different 10,000- to 20,000-acre watersheds on 14 different National Forests in the Sierra Nevada and Klamath Mountains of California. We have selected physical variables that are relevant to both research and management as well as enough biotic variables to characterize the biological integrity of the stream community.
All data are collected on a basin-wide scale to allow upstream-downstream evaluation of cumulative effects as well as between-basin comparisons. Data are collected on stream channel characteristics, fish habitat availability, fish abundance and health, and aquatic invertebrate abundance. Inventories are made of landslides, streambank erosion, and large woody debris. Management-related variables for hillslopes are extracted from air photos, resource maps, and planning maps.
From these data, empirical relationships between watershed management (type and intensity) and biotic variables can be used to predict cumulative effects in other watersheds. Much of the information necessary to make such predictions is currently being collected on National Forests or is being required as part of National Forest Planning. Hence, once relationships are established, most forest managers will already have the information to predict the cumulative impact of past, present, and future management activities on aquatic biota. Predicted effects can then be evaluated in context of both ecological significance and the statutes governing land management activities.
Present Situation
At present, we have completed a pilot study (4 watersheds in 1987), data collection and partial analysis on 25 watershed surveyed in 1988, and data collection on 25 additional watersheds surveyed in 1989. Four benchmark watersheds have been resurveyed in each year to provide information on annual variability. One watershed was resurveyed by three different field crews in 1989 to provide information on between-crew variability. Completion reports on this extensive phase of the research are planned for 1990. Research will continue at several sites as we enter the next phase of our research program at the PSW Station in Berkeley.
This study is being conducted in cooperation with Region 5 of the USDA-Forest Service and the California Department of Forestry and Fire Protection.
Lynn Decker may be reached at (415) 486-3456
