Black Canyon on the Colorado River, from the 1871 Wheeler survey, courtesy USGS. This is the area eventually chosen some years later for what was called Boulder or Hoover Dam:
Black Canyon of the Colorado River, looking below from Camp 7 in Nevada. Photo by T.H. O’Sullivan.U.S. Geographical Surveys West of the One Hundredth Meridian (Wheeler Survey, 1871 Expedition).