Black Canyon 2
When you visit the Black Canyon of the Gunnison, you'll be viewing it from the south rim. The north side is much less accessible. But that just makes for better, less cluttered scenes looking across. I couldn't find a distance for the most narrow spot rim to rim, but it has to be a lot less than the depth, which drops your stomach about 2700 feet if you lean out a little. Wouldn't want to be the galloping cowboy or stampeeding critter that discovers the line of rock here is that deep (for the younger among us, that's on old western movie cliche).