immediately before it was unified with the state grant in 1906. The Yosemite Firefall, in which the embers from a bonfire were pushed off a cliff near Glacier Point to create a spectacular effect, was ended in 1968 because it was deemed to be inconsistent with park values. The firefall was occasionally performed in the 1870s and became a nightly tradition with the founding of Camp Curry. Since the late 1960s Broader tensions in American society surfaced in Yosemite when a large number of youths gathered in the park over the summer of 1970, triggering a riot on July 4 after rangers tried to evict visitors from camping illegally in Stoneman Meadow. Rioters attacked the rangers with rocks, and pulled mounted rangers from their horses. The National Guard was brought in to restore order. The Yosemite Park and Curry Company was bought by Music Corporation of America (MCA) in 1973. In 1988, concessionaires brought in $500 million ($ as of ), and paid the federal government $12.5 million ($ as of ) for the franchise. Delaware North Companies became the primary concessionaire for Yosemite in 1992. The agreement it signed with the National Park Service increased yearly park revenue from concessionaires to $20 million ($ as of ). The current concessionaire is Aramark. In 1999, four women were killed by Cary Stayner just outside the park. That same year a large rockslide originating at the east side of Glacier Point ended near the Happy Isles of the Merced River, creating a debris field larger than several football fields. Tourism dropped a little after those incidents, but soon returned to its previous level. Human impact Plans for reducing human impact on the park were released by the Park Service in 1980. The General Management Plan specified a reduction of 17 percent in overnight accommodations, a 68 percent reduction in staff housing and removal of golf courses and tennis courts by 1990, yet there were still 1,300 buildings in Yosemite Valley and of the valley floor were covered by parking lots in the late 1990s. The goals were not met, but flooding in January 1997 destroyed park infrastructure in Yosemite Valley. The Yosemite Valley Plan was later established to implement the General Management Plan and over 250 other actions. Forests and meadows The Awahnechee and other aboriginal groups changed the environment of the Yosemite area. Parts of valley floors were intentionally burned