Under Section 106 of the National Historic Preservation Act and its implementing regulations 36 CFR Part 800, federal agencies, or their delegates, must assess the potential effects of their undertakings on historic properties. When a federal undertaking could affect an eligible historic landscape, a finding of effect must be prepared. It should be based on an understanding of the resource’s values, the range of essential physical features, and its contributing and noncontributing elements.

Possible effect findings are No Effect, No Adverse Effect, and Adverse Effect, all describing the proposed undertaking’s potential effect on the qualities that make the historic landscape eligible for the National Register of Historic Places. The finding of effect should assess the project’s effects on the landscape as a whole, and also on any individually eligible properties within it.

• A finding of No Effect means that a proposed project will not affect the qualities that make the historic landscape eligible for the National Register. Affecting only noncontributing elements will generally be found to constitute no effect on the landscape as a whole.
• No Adverse Effect means that the project could have an effect on the qualities that make the landscape eligible, but the effect will not be adverse; i.e., the undertaking will not diminish the landscape’s integrity. Project effects that would otherwise be adverse can be found to be not adverse when they meet one of the listed exceptions to the Criteria of Adverse Effect (36 CFR 800.9[c]).
• Adverse Effect includes but is not limited to physical destruction, damage, or alteration of the landscape; isolation from or alteration of the setting; introduction of intrusive elements; neglect leading to deterioration or destruction; and transfer, sale, or lease of the property.

For landscapes, the percentage of the whole property which is subject to effect and the importance of the elements being affected can be assessed to help determine the level of potential effects. Specify clearly whether contributing or noncontributing elements will be affected. Note the scale of the landscape, the prominence of the affected elements, the magnitude of the proposed action, and any change which will be apparent following project implementation. Changes involving only noncontributing elements are likely to have no effect, although the possibility of indirect effects such as visual intrusions on other elements must be considered. Minor takings of open land also have limited potential to create a discernible effect on large landscapes. Generally, large landscapes may have a greater ability than small properties to absorb change, but the possibility of effect through even minor changes must be considered For example, a project’s taking of multiple small roadside features might have a cumulative effect on the historic landscape’s significant character-defining qualities .

A project affecting a landscape may be proposed as being necessary for safety reasons or in order to continue the property’s historic use, but such arguments must be carefully examined. Safety or continuing use are not automatic justifications for undertaking projects that may have environmental consequences. The potential effects must be taken into account and weighed against the project’s benefits. Continuing the historic use of a property may even destroy it, such as modern mining which obliterates all traces of earlier mining activity, or construction of a new freeway on the route of an older road. Adversely affecting a property in order to continue its historic use may, on occasion, justify undertaking a project when the project is in the best public interest. In that case, the finding must clearly explain the effects on historic properties, how those effects have been taken into account, and why the project would be in the best public interest despite those effects.

SHPO concurrence in a finding of No Effect completes the process when the undertaking will not affect any historic properties. Findings of either No Adverse Effect or Adverse Effect require both SHPO and ACHP concurrence and subsequent fulfillment of any agreed-upon conditions. If the project has been found to have an Adverse Effect, proposed mitigation treatments will be included in the Finding of Effect and draft agreement document.