It is likely the most enduring legend of Lost Imagineering: Thunder Mesa and its signature attraction, the Western River Expedition. Press materials from 1969 provided this description:
"THUNDER MESA....A major attraction in the Frontierland area will be this spectacular panorama, where the old west will live again through a series of exciting adventures. Designed to resemble a 'table-top mountain," typical of those on southwestern deserts, it will include a pueblo-style village and other attractions, including the -Western River Expedition," a frontier fantasy on the grand scale of 'Pirates of the Caribbean' in California's Disneyland."
The very best account of the history and development of Thunder Mesa can be found at Widen Your World. Take a look!