It has been 20 years since cumulative effects were statutorially defined, yet people still ask me what they are. And I don't mean just anyone - these are watershed specialists and researchers who work with cumulative watershed effects (CWEs) almost daily. The confusion is ironic, since "cumulative impact" is one of the few technical terms that has been saddled with a legal definition. According to the Council on Environmental Quality's interpretation of the National Environmental Policy Act, "Cumulative impact" is the impact on the environment which results from the incremental impact of the action when added to other past, present, and reasonably foreseeable future actions regardless of what agency...or person undertakes such other actions. (CEQ Guidelines, 40 CFR 1508.7, issued 23 April 1971)
Succinct, straightforward, and apparently all-encompassing. It is the all-encompassedness that has caused the problem.
The definition's breadth did not sit well with those trying to figure out what to look at to find CWEs. A literal interpretation would include virtually any land-use impact, so efforts were made to restrict it to a particular flavor of impact. The most popular contender was the synergistic effect, where the combined impact of two activities is greater than the sum of the two occurring independently. This had the advantage of circumscribing a particular, narrow set of effects, but had the disadvantage of not being what the legal definition meant. The ultimate arbitrator in this issue is the courts, and to them a legal definition is a legal definition. We simply do not have the luxury of being able to limit the definition to make the problem easier to handle.
But looked at from a different point of view, the definition controversy is a red herring anyway. The formal definition implies breadth because that is what it is intended to imply. It does not refer so much to a particular variety of impact as to the context in which all impacts must be evaluated. Because of widespread air pollution and potential human-induced climatic change, and because of the variety of land-use activities occurring in most watersheds, almost all impacts are influenced by multiple land uses. To fully understand any impact, therefore, all potential influences must be evaluated; the impact must be analyzed as a cumulative effect of multiple influences.
The novelty of the definition thus lies not in the particular type of effect being described, since almost all impacts reflect multiple influences. Instead, it lies in the definition's implications: the message is that watershed impacts must be evaluated in the context of all potential uses influencing them, rather than on the scale of a single activity, project, or plan. This, in turn, implies that analysis of potential effects must consider far broader space and time scales than were previously considered in land-use planning. In the case of timber production, for example, we must consider activities and impacts occurring anyplace in a drainage basin-and in the estuary beyond-in our analyses, and we must consider patterns of impact and recovery over multiple disturbance cycles.
Cumulative effects are thus nothing new; they represent the same types
of environmental changes that concerned the ancient Greeks. What is new
is the mandate to evaluate and understand them.