Question: What is the name of the broad expanse of flat land in the western United States and western Canada  where the species of plant called the Polygonum sawatchense of the buckwheat family be found?
Sentence (from "Polygonum sawatchense"): It in the western United States and western Canada, from the Pacific Coast to the western Great Plains, so from British Columbia south to California and east as far as New Mexico, Colorado, Saskatchewan, and the Dakotas.
Relevant evidence: No