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Planning:
Assessing Impacts

You may have heard the saying that goes: A butterfly in Japan flutters its wings, and a tidal wave hits the coast of California. This is cumulative impact. The immediate effects of one small change in land use may be small, as is the fluttering of wings. But look at the effects over time, and look at the effects of many small changes on the scale of a county or a region, and you could be looking at the tidal wave.

Cumulative impact is the total of all the effects of a land use change on a given area (such as a county). Cumulative impact can be broken down into:

Direct effects, which are evident immediately after the change is made;
Indirect effects, which occur later or farther away from where the change was made; and
Additive effects, which are added to similar effects of similar projects. Impacts of a single project may seem insignificant, but when they are added to the effects of other projects, the total can be huge.

Cumulative effects are very difficult to measure because they can affect so many different things. The Council on Environmental Quality (CEQ) lists the following potential cumulative impacts:

long-distance transport of air pollutants, resulting in ecosystem acidification or eutrophication
air pollultion, which degrades air quality of a whole region
release of greenhouse gases, which may cause climate changes
discharges of sediment and of thermal and toxic pollutants, which can disperse throughout large water bodies such as rivers and lakes
reduction or contamination of groundwater supplies
changes in hydrological regimes of rivers and estuaries
long-term containment of hazardous wastes
mobilization of persistent or bioaccumulated substances through the food chain
decreases in the quantity and quality of soils
loss of natural habitats or historic character through residential, commercial, and industrial development
habitat fragmentation from infrastructure construction or changes in land use
habitat degradation from grazing, timber harvesting, and other consumptive uses
disruption of migrating fish and wildlife populations
loss of biological diversity

This list doesn't even include possible cumulative social, economic, and transportation effects, such as impacts on jobs, government spending, schools, highways, and roads.

How can cumulative impacts be considered in land use planning? Some ideas can be borrowed from the experiences of federal agencies, which are required to consider potential cumulative impacts for projects that must have an Environmental Impact Statement (EIS) or an Environmental Assessment (EA). Federal law requires that cumulative impacts be examined in such statements.

The basic steps set out by the CEQ are:

Scoping
Identify the significant cumulative effects issues associated with the proposed action and define the assessment goals.
Establish the geographic scope for the analysis.
Establish the time frame for the analysis.
Identify other actions affecting the resources, ecosystems, and human communities of concern.

Describing the affected environment
Characterize the resources, ecosystems, and human communities identified in scoping in terms of their response to change and capacity to withstand stresses.
Characterize the stresses affecting these resources, ecosystems, and human communities and their relation to regulatory thresholds.
Define a baseline condition for the resources, ecosystems, and human communities.

Determining the environmental consequences
Identify the important cause-and-effect relationships between human activities and resources, ecosystems, and human communities.
Determine the magnitude and significance of cumulative effects.
Modify or add alternatives to avoid, minimize, or mitigate significant cumulative effects.
Monitor the cumulative effects of the selected alternative and adapt management.

Techniques to estimate cumulative impacts of land use changes include consultation with experts, evaluation of similar projects elsewhere, public hearings, and computer tools that use maps, statistical analysis, or dynamic modeling. Our hope it to offer mapping and decision support tools on the ARROW website that can be used to evaluate the impacts of land use changes in the ARROW counties. The maps are now available under Tools and the decision support tool will be available in mid-2005.