The Bureau of Land Management (BLM) managed 12.1 million acres of the 25-million-acre California Desert Conservation Area (conservation area). Dove Springs Canyon (canyon), which comprised 5,000 acres of the conservation area, constituted 0.025 percent of the BLM-administered lands. Unrestricted off-road vehicle (ORV) use, which was authorized in 3,000 acres of the canyon, caused severe environmental harm. The Sierra Club (Sierra) (plaintiff) petitioned the U.S. secretary of the interior (secretary) (defendant) to close the canyon to ORV use. The secretary failed to do so. Sierra sued the secretary, claiming that the failure to prohibit ORV use in the canyon violated Executive Order No. 11644, as amended by Executive Order No. 11989 (EOs); 43 C.F.R. § 8341.2(a); and §§ 1732(b), 1781(b), and 1781(d) of the Federal Land Policy and Management Act (FLPMA), 43 U.S.C. §§ 1701 et seq. Section 8341.2(a) required the closure of areas that suffered from the “considerable adverse effects” of ORVs. The district court granted summary judgment for the secretary, and Sierra appealed. The secretary argued that ORV impacts in the canyon were not considerable, because “considerable adverse effects” encompassed the entire conservation area, rather than individual parcels within the canyon. The secretary also argued that a broad interpretation of “considerable adverse effects” was consistent with § 1781(a)(4) of the FLPMA, which allowed ORV use “where appropriate.” Sierra argued that (1) a Council on Environmental Quality (CEQ) memo finding that the EOs required the canyon’s closure was entitled to deference, and (2) the secretary’s interpretation of the closure standard was unreasonable, because the FLPMA’s multiple-use mandate and unnecessary-and-undue-degradation standard required the maintenance of the canyon’s environmental quality.