The quarry site was declared a National Monument in 1915. During the 1930's, a WPA (Works Progress Administration) project expanded the quarry face, but no new fossils were exposed or excavated. The monument boundaries were expanded in 1938 from the original 80-acre tract surrounding the dinosaur quarry in Utah, to its present extent of over 200,000 acres in Utah and Colorado, encompassing the spectacular canyons of the Green and Yampa Rivers. In addition to its dinosaurs, the National Park Service manages and protects a variety of other natural and cultural resources within these expanded boundaries.
The Yampa River is the last major tributary of the entire Upper Colorado River Drainage that has not been dammed, and the river system is home to a number of endangered fish species, including the Colorado squawfish and humpback chub. For visitors to the monument, only the dinosaur quarry itself exceeds whitewater rafting in popularity. the scenery, geology, and the plants and animals that make up the natural environment or ecosystem, are also important resources.