In 1971, the State of New York purchased land bordering Lake Minnewaska (lake) for the creation of a state park. The park was under the jurisdiction of the Palisades Interstate Park Commission (PIPC). In 1977, the PIPC purchased a conservation easement over the lake and adjacent land, using funding from the federal Land and Water Conservation Fund. The funding was authorized by the Land and Water Conservation Fund Act of 1965 (Act), 16 U.S.C. §§ 460l-4 to 460l-11. The easement area included the lake, a non-operational golf course, a golf shop, a water-supply system, and wooded land. The easement area exposed scenic vistas and served as a buffer zone between the park and developed areas, but was not accessible to the public. The easement prohibited the landowner from building or developing new facilities within the easement area. The Marriott Corporation (Marriott) acquired an option to purchase the land in 1980. In 1981, the PIPC resolved to amend the easement to allow Marriott to make certain changes, including a golf-course expansion and increased water usage from the lake. A memo issued by the U.S. Department of the Interior (Interior Department) (defendant) in 1978 defined “conversion” to include the conveyance of property interests for non-public outdoor uses. In 1981, the National Park Service (NPS) (defendant) notified the PIPC that federal authorization was not required for the amendment, because the amendment did not constitute a conversion of land to “other than public outdoor recreation uses” under the Act. The Friends of Shawangunks, Inc. and others (plaintiffs) sued the NPS and the Interior Department in district court. The district court held that the amendment was not a conversion, because the easement area was not accessible to the public and was therefore not intended for public, outdoor recreational use under the Act. The plaintiffs appealed.