Yosemite Falls--and other waterfalls in Yosemite--reach their peak flow near the end of May, but usually remain high well into June. By August, waterfalls have little water or (in the case of Yosemite Falls) may be dry.

Summer
(June through September)
See lots of the park...and other visitors

Areas to visit: All areas of the park are usually accessible by car by late May or early June, although services along the Tioga Road often open a bit later in June.

Climate: Warm to hot, with occasional rain (usually as afternoon thundershowers, especially at the higher elevations).
Yosemite Valley & Wawona (4,000 ft / 1,200 m): 87°F (31°C) / 51°F (10°C)

Rivers & Waterfalls: Most of the water flowing in Yosemite comes from snowmelt in the high country, so runoff decreases during the dry summer. Peak runoff typically occurs in May or June, with some waterfalls (including Yosemite Falls) often only a trickle or completely dry by August. Other waterfalls, including Vernal, Nevada, and Bridalveil, run all year, however their flow can be very low by late summer.

Wildflowers:
Yosemite Valley & Wawona: Most blooming occurs in June, with redbud, Sierra onion, lupine, Mariposa lily, pentstemon, and flowering dogwood beginning in May.
Tuolumne Meadows: The season begins in late summer for subalpine flowers. Beginning around July, look for little elephant's heads, gentian, pentstemon, yarrow, and shooting stars.