The National Environmental Policy Act (NEPA) was the first major environmental law in the United States and is often called the "Magna Carta" of Federal environmental laws. NEPA requires Federal Agencies to assess the environmental effects of their proposed actions prior to making decisions. To implement NEPA's policies, Congress prescribed a procedure, commonly referred to as "the NEPA process" or "the environmental impact assessment process."

Mitigation in an important mechanism Federal agencies can use to minimize the potential adverse environmental impacts associated with the actions of the Federal agency. Agencies can use mitigation to reduce environmental impacts in several ways, including (1) avoiding an impact by not taking action or parts of an action; (2) minimizing an impact by limiting the degree or magnitude of the action and its implementation; (3) rectifying an impact by repairing, rehabilitating, or restoring the affected environment; (4) reducing or eliminating an impact over time, through preservation and maintenance operations during the life of the action; and (5) compensating for an impact by replacing or providing substitute resources or environments.

When unavoidable habitat losses do occur, the Corps will offset such losses through compensatory mitigation to replace the lost habitat. Compensatory mitigation is an important part of project construction and could include habitat restoration or enhancement projects or credit purchases. Generally and to the extent possible, the mitigation projects will be implemented in the same river basin where the project impacts occur.