The google maps link here shows pretty clearly, along with the terrain mode, that the Merced River in Yosemite National Park is only a few feet in height from taking an alternate way to the north of the falls. If you look at the intersection of the gully and the river there is very little leeway to prevent the river from modifying its course given a large flood, and, indeed, if you look at the pictures that people have uploaded, there are stone walls built by the park service to at least prevent the existing trail from washing away.
My core question is: did the river ever actually take this more obvious course than falling several hundred feet down a waterfall? Perhaps blocked by a rockfall at some point? The only other explanation I can think of is if the alternative course is man made by some combination of prospectors and park rangers, but it seems far too wide and deep to make it worth it versus just making a staircase.