BOZEMAN, Mont. — IN Yosemite National Park, officials need roughly $19 million to upgrade an aging sewer system to prevent spills like the one that leaked thousands of gallons of raw sewage into the Merced River 15 years ago. In Grand Canyon National Park, more than $100 million is needed to repair the water system and $44 million to fix the trail network for the park’s four million visitors a year.

Throughout the national park system, an enormous backlog of deferred maintenance is eroding the visitor experience and threatening the very resources that the National Park Service was created to protect. Earlier this year, the park service announced that the cost of deferred maintenance had reached $11.5 billion.

Included in the backlog: $5.6 billion for park roads, $1.8 billion for buildings, nearly $473 million for trails, $255 million for wastewater systems and $62 million for campgrounds.

Unfortunately, the park service is not alone. At last estimate, the maintenance backlog for its parent agency, the Interior Department, which also includes the Fish and Wildlife Service and Bureau of Land Management, was put at between $13.2 billion and $19.3 billion.