To kick off National Park Week, which starts today, we"re turning our lens on Vernal Fall in California"s Yosemite National Park. This time of year, the waterfall flows in a torrent, but by late summer it can be reduced to a trickle of small streams slipping over the edge. Most years, Vernal Fall is at full power by May, as the spring thaw in the mountains melts the snowpack, turning the falls into a raging tumble of water spilling to the valley floor where the Merced River flows.