Yellowstone National Park is celebrating its 150th anniversary as it faces its biggest challenge in decades. Floodwaters that tore through the park this week destroyed potentially hundreds of bridges, washed out miles of roads and drove out more than 10,000 visitors. The scope of the damage is still being tallied by Yellowstone officials, but based on other national park disasters, it could take years and cost upwards of $1 billion to rebuild in an environmentally sensitive landscape. Park officials hope to reopen the southern half of the park next week but the northern half likely won’t reopen this year.