Are caused by the action and occur at the same time and place. Example: Removal of vegetation from a project site to allow construction of a facility. Sometimes referred to as a primary effect.
Are reasonably foreseeable consequences of the action but are later in time or further removed in distance from the direct effects. Effects that change land use patterns, population density or growth rate. Example: Increased air pollution from additional vehicles is a foreseeable indirect effect of relocating residents to newly created neighborhood. Downstream sedimentation resulting from project site erosion might also be considered an indirect effect. Sometimes referred to as secondary effects.
Direct and indirect effects of project actions that are greater in significance than just the sum of the direct and indirect effects, when viewed in the context of the total effects of other past, present and reasonable foreseeable future actions.
Cumulative effects are not a wholly different effect from direct or indirect effects of an action. Cumulative effects are merely a way of placing seemingly isolated or insignificant direct and indirect effects in context with respect to overall impacts, both over time and in an area larger than that evaluated for direct and indirect effects. An example is that of a watershed that has lost nearly all of its historical wetlands. Without looking at the total setting the elimination of one acre of wetland in this watershed may be dismissed as insignificant. But, if this is one of the very few remaining naturally occurring wetland in this severely disturbed watershed, then cumulatively this impact may be great ("the straw that broke the camels back.")
Cumulative effects from the construction and/or operation of a proposed flood control or a drainage facility may be very important because the incremental lowering of wetland, habitat, and water quality caused by the proposed project in an already degraded watercourse or drainage basin may have cumulatively significant impacts when looked at along with the conditions of the watershed or the channel up or downstream of the proposed action. Depending on the present condition of the watercourse, the erosion and sediment produced during construction may or may not have potentially significant cumulative impact on an already degraded watercourse.
Some resources may be affected by several cumulative actions (causing cumulative effects) either later in time or removed from the causative action in distance (indirect effects). In other words, the resource may be affected both indirectly and cumulatively.
The concept of cause and effect relationships is easier to understand in terms of the cause as the action and the effect as the impact.