Lake Mary may refer to one of two reservoirs in northern Arizona, southeast of Flagstaff. The name may also be used to refer to the two lakes as a whole. The pair of lakes impound the intermittent Walnut Creek upstream from Walnut Canyon. Recreational facilities at both lakes are maintained under the authority of the Coconino National Forest.
Upper Lake Mary, the further upstream of the two lakes, is formed by a small earthen dam on Walnut Creek. The lake is long and narrow, with a maximum length of about 5 miles (8 km), and a maximum width of about 2,000 feet (610 meters). During dry seasons the lake will narrow and shorten. The lake is named after Mary Riordan, a daughter of the wealthy lumber barons who built the lake in 1905 for a water supply for Flagstaff.
Upper Lake Mary is also stocked with several fish species for angling, including crappie, sunfish, channel catfish, northern pike and walleye. Largemouth bass, yellow bass and yellow perch are sometimes reported to be found in the lake, as well.