 He led his party up the Colorado to a point just above the head of Black Canyon of the Colorado and beyond to the vicinity of Fortification Rock.Next day, they went two miles farther to Las Vegas Wash, which Ives thought might be the Virgin River, but had doubts because it seemed too small.The difficulties of the rapids above Fortification Rock convinced Ives that the river at Fortification Rock was the practical head of navigation 550 miles above the mouth of the river