Fortification Rock

Fortification Rock, once a landmark hill on the Colorado River before it was inundated, now known as Rock Island,[1] southernmost and tallest of the Boulder Islands[2] in Lake Mead, Clark County, Nevada.

Joseph C. Ives, which shows Fortification Rock, Ives Camp #59 and the course of the Colorado River from its mouth on the Gulf of California to Las Vegas Wash and the location of its features and other expedition camps along the way.

[3] Fortification Rock, now an island, was before the building of Boulder Dam and the filling of Lake Mead, the tallest peak of a small range of hills, a landmark on the west bank of the Colorado River just above the top entrance to the Black Canyon of the Colorado.

It was named by 1st Lieutenant Joseph Christmas Ives, March 11, 1858, during his 1857-1858 expedition to find the upper limit of navigation for the Colorado.

It marked what he considered that highest point for steamboat navigation, during the June to October season of high water.