It’s a picture-perfect day in Southern California. The sun is beating down on this Carlsbad beach, where volleyballs hit the sand and surfers paddle out into the waves. Just steps from here, the salty water lapping the shore is being transformed.

This beach neighbors the largest desalination facility in the Western Hemisphere. The Carlsbad Desalination Plant uses a complex web of pipes, tanks and specialized filters to pull salt and impurities out of ocean water, turning it into part of the drinking supply for San Diego County.

Water managers are feeling the crunch of a supply-demand imbalance along the Colorado River. Fresh water reserves are shrinking as climate change squeezes the river that supplies 40 million people and fields of crops across seven states. Some have proposed desalination technology as a way to augment that supply, easing the strain on a river that supplies a growing population from Wyoming to Mexico. Experts say it could be part of the solution, but likely won’t make much of a dent in the region’s water crisis.

At the Carlsbad plant, former seawater poured into a cup from a freshwater spigot. Michelle Peters, technical and compliance manager for plant operator Poseidon Water, held it and took a drink.

“At 10 a.m, the morning surfers were swimming in it off the coast in the ocean here,” she said. “Now it’s high-quality drinking water, ready for consumption.”