The Colorado River is nearing collapse. It’s Trump’s problem now.

The Colorado River currently supports 40 million people and $1.4 trillion in annual economic activity in seven U.S. states and Mexico — but it was never intended to be stretched so thin.

A century-old legal framework promises those users more water than there is to go around. The river’s flow has shrunk by about 20 percent over the last century as climate change has made the West more arid. As water has vanished, states have clashed over how to divide up what remains. The core dispute...