Camassia quamash or C. leichtlinii (larger blooms, same range).
Erica wrote: The native people protected their camas fields from the enemy, as the camas bulbs were a valuable food. David Douglas did partake of this Indian delicacy; he said that it tasted like baked pears, and he was "nearly blown out of the longhouse by the strength of the wind therefrom!" The fields were weeded of the similar looking, but white-flowered lily (Zygadenus venenosus) which was poisonous.
2018 update: Camas Prairie Cultural Ecosystems Incubator