Colorado boasts over a hundred cattle feedlots capable of holding over a thousand animals, according to the state's Department of Public Health and Environment. Many of those operations are close to waterways. Like in other states, it's up to cattlemen and regulators to keep manure out of streams and groundwater. Environmentalists worry that will get trickier as historic rain events become more common.Water levels are up this winter at Lake Mead, a gauge for the Colorado River's ability to supply 30 million people with water, thanks partly to a surprising hero: lettuce.

Farmers' switching to lettuce, which uses less water because it's cultivated only part of the year, from alfalfa, a thirsty year-round crop, helped push the lake to 1,087.6 feet (331.5 meters) above sea level as of Jan. 31. That's more than 1 foot higher than a year ago and above the benchmark of 1,075 feet, at which point regional water restrictions kick in.

The improvement, which brought a sigh of relief to a dry region, is mostly due to a record snowpack in the Sierra Nevada mountains that ended California's drought by early 2017. But some credit goes to farmers, the biggest users of the region's water, who in some places have been doing exactly what climate experts say they should be doing-switching crops for conservation reasons.

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