The Water Knife by Paolo Bacigalupi

In a future hammered by climate change and drought, mountain snows have turned to rain, and rain evaporates before it hits the ground. In a fragmenting United States, the cities of Phoenix and Las Vegas skirmish for a dwindling share of the Colorado River. But it is the Las Vegas water knives - assassins, terrorists and spies - who are legendary for protecting Las Vegas' water supplies, and for ensuring Phoenix's ruin.

When rumours of a game-changing water source surface, Las Vegas dispatches elite water knife Angel Velasquez to Phoenix to investigate. There, he discovers hardened journalist Lucy Monroe, who holds the secret to the water source Angel seeks. But Angel isn't the only one hunting for water, Lucy is no pushover, and the death of a despised water knife is a small price to pay in return for the life-giving flow of a river.