For Yosemite Falls, the best light doesn't match up with the biggest flow. As the water volume grows in spring, the sun rises further and further north, and the light doesn't reach the falls until 9:30 to 10:00 a.m. in April and May.
But Bridalveil Fall is better situated for spring light. From late March through about the end of April the sun sets through a gap to the west, and Bridalveil receives beautiful golden light just before sunset. The accompanying photo was made on March 27th, 2007 from Tunnel View. In May and June the light gets cut off earlier, but it's still nice.
Bridalveil Fall can be photographed from across the river along Northside Drive (location 4 in my book, The Photographer's Guide to Yosemite), Valley View (location 5), Tunnel View (location 8), or from the Bridalveil Fall parking lot and trail (location 7). You can see a rainbow right from the parking lot between about 5:00 - 6:00 p.m. throughout the spring. (Unfortunately the book is temporarily out of print; it should be available again sometime this spring.)
The poppies in the Merced River canyon are still going strong. While the earliest-blooming spots - mainly in the burned areas - may start to fade soon, other areas have just started flowering and should last a little longer.