A crumbling sewage system at Yellowstone National Park is dumping wastewater into Yellowstone Lake, and traffic and air pollution have beset Grand Canyon National Park, according to the National Parks and Conservation Association. The group released a list of the 10 most endangered national parks yesterday, highlighting an array of problems in the park system. The maintenance backlog alone in the parks runs to about $4 billion. Sens. Harry Reid (D-Nev.) and Bob Graham (D-Fla.) have introduced a bill that would address threats to parks and provide $500 million a year for park improvements.