18 CFR § 707.9 - Tiering.

In accordance with the CEQ NEPA regulations 40 CFR 1502.4(d) and 1508.28(a), this NEPA rule emphasizes the use of tiering to relate broad and narrow actions. The level of detail in EIS's and EA's prepared by RBC's, WRC interagency committees or WRC coordinating committees will reflect the level of detail in the plans, particularly the comprehensive and policy nature of comprehensive, coordinated, joint plans or elements or revisions thereof. These EIS's are not intended to substitute for individual statements on individual projects as more detailed planning and analysis will be required for major Federal actions proposed in these plans. The “policy” or “overview” EIS should serve as the framework and introduction for a more site-specific project EIS developed by the implementing Federal agency. Environmental impact statements for regional water resource management and Level B plans will generally address the items in the recommended format (40 CFR 1502.10) on the basis of water and related land resources of an entire region or river basin. This is the level of consideration at which the environmental issues and considerations are most relevant to decisionmaking. They may also address groups of interrelated or individual plan elements where these involve significant environmental considerations.