Mormon Lake, Natural lake in Coconino County, Arizona, US

Mormon Lake is a shallow, intermittent lake in Pleasant Valley, Coconino County, about 35 km southeast of Flagstaff. When full it covers roughly 12 square miles, making it Arizona's largest natural lake, with an average depth of only 10 ft. In dry periods it shrinks to a remnant marsh. It has no surface outlet; Walnut Creek rises near the north shore.

Mormon settlers who located their main communities along the Little Colorado River set up cottage industries in Pleasant Valley: a sawmill in 1876, a dairy in 1878 and a tannery in 1879. All were abandoned when the Little Colorado colonies were disbanded. In 1978 forest archaeologists recorded the mill site, little more than a foundation and piles of rocks, about 5 miles southeast of the lake.

The lake's name honors Mormon settlers who came to northern Arizona in the 1870s and set up a sawmill (1876), a dairy (1878) and a tannery (1879) in Pleasant Valley. Five private USFS recreation tracts—Rockledge, Pilgrim Playground, Dairy Springs, Montezuma Lodge and Double Springs—hold over 80 summer homes on National Forest parcels, not county subdivisions.

Visitors can stay at two campgrounds that offer basic facilities within the surrounding national forest. It helps to check conditions beforehand since weather and seasonal changes can affect how the water and roads look throughout the year.

The lake has no surface outlet, yet Walnut Creek rises just north of its shore and feeds Upper Lake Mary; both are counted in the Canyon Diablo watershed. At full stage the shallow basin covers about 12 square miles, the largest natural lake in Arizona.