From within the yosemite vally, take the Southside drive to the trail head of the Muir Trail and follow up river about 30 minutes
Spring - early summer is definately the best time to go to Yosemite if you are looking for water falls and river pictures. This is the time that the rivers are flowing full and fast due to the winter melt.
Picture of the Merced river with Half dome in the distance. Picture was taken from the moddle of the very cold river.