The Corps strives to achieve a goal of no net loss of aquatic resource functions and services. The purpose of compensatory mitigation is to develop long term self-sustaining aquatic resources that offset adverse effects and are not dependent on human intervention after the establishment period. The methods of compensatory mitigation include:
• Establishment: manipulation of the physical, chemical or biological characteristics present to develop an aquatic resource that did not previously exist at an upland site. Establishment results in a net gain in aquatic resource area and functions.
• Re-establishment: the manipulation of the physical, chemical, or biological characteristics of a site with the goal of returning natural/historic functions to a former aquatic resource. Re-establishment results in rebuilding a former aquatic resource and results in a gain in aquatic resource area and functions.
• Enhancement: the manipulation of the physical, chemical, or biological characteristics of an aquatic resource to heighten, intensify, or improve a specific aquatic resource function(s). Enhancement results in the gain of selected aquatic resource function(s), but may also lead to a decline in other aquatic resource function(s). Enhancement does not result in a gain in aquatic resource area.
• Rehabilitation: the manipulation of the physical, chemical, or biological characteristics of a site with the goal of repairing natural/historic functions to a degraded aquatic resource. Rehabilitation results in a gain in aquatic resource function, but does not result in a gain in aquatic resource area.
• Preservation: the removal of a threat to, or preventing the decline of, aquatic resources by an action in or near those aquatic resources. This term includes activities commonly associated with the protection and maintenance of aquatic resources through the implementation of appropriate legal and physical mechanisms. Preservation does not result in a gain of aquatic resource area or functions.
The preferred type of compensatory mitigation is re-establishment, and sometimes establishment, as these result in a net gain in both aquatic resource area and function(s). Compensatory mitigation that results in a net gain of aquatic resource function but not area (enhancement and rehabilitation) may also be a viable option provided functional gain would be sufficient to off-set functional loss. Only in rare instances will the Corps accept preservation as an acceptable form of compensatory mitigation.
Compensatory mitigation may be accomplished through:
• Purchase of appropriate credits at a Corps approved mitigation bank
• Payment into a Corps approved in-lieu fee fund, or
• Permittee responsible on-site/off-site; establishment, re-establishment, enhancement, rehabilitation, and/or preservation.
Generally, the use of Corps approved mitigation banks, to fulfill compensatory mitigation requirements, is preferred. However, in many cases the payment into an in-lieu fee fund and/or permittee responsible compensatory mitigation may provide greater benefits to the aquatic environment and/or watershed as a whole. The appropriate method, type, and amount of compensatory mitigation required to off-set unavoidable adverse effects to the aquatic environment is determined by the Corps, and is made on a project-by-project basis.