In 1987, supported by funding from the R-5 Range and Watershed Management Staff and the help of many people too numerous to mention, I conducted a survey of watersheds thought to display Cumulative Watershed Effects (CWE). My intent of was to determine differences in channel condition between watersheds deemed as suffering from cumulative watershed effects, and those deemed as being in acceptable condition by watersheds specialists working on various National Forests. A second goal was to compare channel condition data with Equivalent Roaded Area (ERA) estimates for each watershed. Aquatic invertebrates, channel stability (Pfancuch) and channel geometry measurements where used as indicators of channel condition in twenty one watersheds on six National Forests in California. Eight of the watersheds were from the coastal mountains of the Shasta-Trinity and Mendocino National Forests, the remaining thirteen were Sierran watersheds.

Aquatic macroinvertebrates have long been used as indicators of water quality and stream health, and to assess the impacts of a variety of land management impacts, including mining, logging and road construction, grazing and wildfire. Shannon Diversity was used to display information from the invertebrate samples.

In brief, watersheds identified as suffering from adverse cumulative effects by watershed specialists were in poorer condition (lower invertebrate diversity and lower channel stability) than a paired "control" watershed in twelve of thirteen comparisons (some control watersheds were used in more than one comparison). Both aquatic invertebrate community diversity (Shannon Diversity) and channel stability (Pfankuch) were significantly lower when the thirteen treatment streams were compared to the eleven control streams. The lowest invertebrate diversities were found in those watersheds that displayed active mass wasting of the inner gorge. As would be expected, channel stabilities were also lowest in this group. Watersheds burned by wildfire also displayed low invertebrate diversity and channel stability. There was no discernible difference in channel geometry (cross sectional area, cross sectional area/watershed size or any other tested characteristics) between any of the watershed groups.

Though treated watershed ERA was higher than control watershed ERA in every comparison, a correlation between ERA and other indicators of watershed condition was not found. The presence of significant mass wasting appeared to overwhelm any upslope influences. On those watersheds without mass wasting, the degree of disturbance to sensitive lands (particularly streamside management zones) had the strongest (negative) correlation to both benthic diversity and channel stability.

The results indicate poor correlation between ERA and the watershed condition factors measured. Rather than a criticism of the ERA methodology for evaluating cumulative effects, I think the results should be used to highlight the limitations of an ERA based methodology, and flexibility to adapt the method to watersheds with particular characteristics. The ERA methodology is not meant to serve as a predictive model of watershed condition, sediment delivery or any other watershed output or parameter. Rather, it is a system that can be used to account for watershed disturbance, and provide a sound basis for determining (in the absence of or in conjunction with other data) when risk of adverse effects from any proposed activity is high enough to warrant a more detailed and in depth evaluation. The results of this study indicate that by weighting disturbance on sensitive land types more heavily (perhaps geometrically so) than disturbance on stable lands, confidence in the ERA value might be increased.

Perhaps the most interesting aspect of the study is what the results suggest about recovery of channels from disturbance. Several of the streams which displayed active mass wasting had been disturbed over twenty years prior to this investigation. Likewise, some watersheds had been the site of wildfires that burned ten to twenty years prior to the sampling. With both types of disturbance, there appeared to be little recovery of the aquatic community. Evidently, there are types of disturbances which are so severe in the short term (a wildfire for example) or so persistent over time (continued mass wasting) that the "natural" ability of a channel and its biota to recover is disrupted. Unfortunately, responses of stream systems (especially the biological component) from both natural and man caused disturbance are poorly documented. A summary of the available literature prepared by Gerald Niemi and others of the University of Minnesota Natural Resources Research Institute in 1989 revealed that stream communities recover very quickly (a matter of days or weeks) from "pulse" or one time disturbances, such as poisonings or spills. Recovery from "chronic" or long lasting disturbances, such as channel alterations, is much slower, maybe as long as years. In natural systems, disturbances which caused long term changes to channel habitats had the longest recovery periods. If there is a need to differentiate CWE from "other" watershed disturbances, then perhaps the disruption of natural recovery processes is worth investigating further.

Allowing subjective definition of watersheds with CWE was interesting in that it revealed the range of disturbances, conditions and dominant processes on forested watersheds, but it caused serious problems in evaluating the data. Though all the "treatment" watersheds selected for study had been categorized as suffering from CWE, it became apparent that the natural processes at work (mass wasting, wildfire) made real distinctions among the treatment group. It is arguable whether the watersheds studied represented cumulative effects, or rather simply a range in degree of disturbance. The inference is simple. Instead of trying to classify and segregate effects, the need is to try to consider each watershed and activity as unique, and make the best effort to evaluate processes and effects of concern, whether one might term them cumulative or not. All the while remembering how complex watersheds are.

Ken Roby may be reached at (916) 284-7126