California’s drought and a bark beetle epidemic have caused the largest die-off of Sierra Nevada forests in modern history, raising fears that trees could come crashing down on people or fuel deadly wildfires that could wipe out mountain communities.

Aerial images show vast forests that have turned a rust-color. The epidemic has killed an estimated 40 million trees since 2010 in the central and southern Sierra, and it’s spreading north. Officials who are cutting down and stacking the most dangerous trees in piles across six counties, however, say they are stumped by how to get rid of them all.

First, the good news: This winter, much of the Sierra had a near-average snowpack. Now, the bad news: It has melted early.

Word of the vanishing Sierra snowpack, which usually helps replenish reservoir levels later in the summer, arrives amid uncertainty over how California’s dams will be managed in coming months to protect endangered fish. It also comes at a critical juncture for urban water officials across the state. Wednesday is their deadline to submit updated drought conservation plans that lay out projections of how much water will be available to customers over the next three years.