In January 1865 several dozen members of The Church of Jesus Christ of Latter-day Saints established a settlement called St. Thomas at the confluence of the Muddy and Virgin Rivers in Clark County, Nevada. Its namesake was Thomas Smith, one of the settlers. By 1866, 45 families were building homes and planting crops in the Muddy River Valley.
In 1869, when explorer John Wesley Powell floated out of the Grand Canyon and left the Colorado River, he was met by several locals. His journal records: “Bishop Leithhead of St. Thomas and two or three other Mormons are coming down with a wagon, bringing us supplies.” Powell and his brother went to St. Thomas with the bishop and on to Salt Lake City (see "The Exploration of the Colorado River and its Canyons," by J.W. Powell, page 287).
Formal surveying later revealed that St. Thomas was actually in Nevada, not Utah or Arizona. Tax collectors then demanded back taxes for several years in gold or silver. Not believing that the taxes would end up in the proper hands, the residents of St. Thomas voted to leave after receiving permission from President Brigham Young.
St. Thomas was later repopulated by other settlers. Decades later, the residents of St. Thomas learned that their town would soon be covered by the waters of Lake Mead soon to be created by Boulder (later Hoover) Dam. It was eventually covered by 60-70 feet of water. In recent years, the lake level has lowered sufficiently to allow the ruins of St. Thomas to reappear. Many of the foundations of homes and businesses are now visible.
(Much of the information above comes from interpretive panels at the site of St. Thomas.)