Beautiful flowers, poisonous plant. This is Veratrum californicum or False Hellebore. Here are some details from Wikipedia: Veratrum californicum.
Veratrum californicum (California corn lily, white or California false hellebore) is a poisonous plant native to mountain meadows at 3500 to 11,000 ft elevation in southwestern North America, the Sierra Nevada and Rocky Mountains, and as far north as Alaska and as far south as Durango. It grows 1 to 2 meters tall, with an erect, unbranched, heavily leafy stem resembling a cornstalk. It prefers quite moist soil, and can cover large areas in dense stands near streams or in wet meadows. Many inch-wide flowers cluster along the often-branched top of the stout stem; they have 6 white tepals, a green center, 6 stamens, and a 3-branched pistil (see image below). The buds are tight green spheres. The heavily veined, bright green leaves can be more than a foot long.
Veratrum californicum displays mast seeding; populations bloom and seed little in most years, but in occasional years bloom and seed heavily in synchrony.