There's a dramatic difference between Vernal Fall in the spring and in the autumn. The waterfalls of Yosemite Valley usually peak sometime in May, and if you're a waterfallphile, visit then and you'll feel fully rewarded. Autumn is more for slabs-o'-rockphiles. There will be more water on the Mist Trail falls than on Yosemite or Bridalveil Falls, though, at any time of year, since the Mist Trail is served by the Merced River rather than the mere creeks that feed the other two falls. When the water flow looks like this on Vernal Fall, you can be confident that Yosemite Falls has dried up altogether.