The impacts of domestic grazers on the grasslands of the Palouse and Camas Prairies was transitory because much of the areas were rapidly converted to agriculture. However, the canyonlands of the Snake and Clearwater rivers and their tributaries with their much shallower soils, steep topography, and hotter, drier climate, were largely unsuitable for crop production and were consequently used for a much longer period by grazing domestic animals (Tisdale 1986). There, intense grazing and other disturbances have resulted in irreversible changes with the native grasses largely replaced by annual grasses of the genus Bromus and noxious weeds, particularly from the genus Centaurea. The highly competitive plants of both of these genera evolved under similar climatic regimes in Eurasia and were introduced to the U.S. in the late 19th century.