CARSON CITY, Nev. — Record-breaking heat and historic drought in the U.S. West are doing little to discourage cities from planning to welcome millions of new residents in the decades ahead.

From Phoenix to Boise, officials are preparing for a future both with more people and less water, seeking to balance growth and conservation. Development is constrained by the fact that 46 percent of the 11-state Western region is federal land, managed by agencies like the U.S. Forest Service and Bureau of Land Management that are tasked with maintaining it for future generations.

That’s led officials in states like Nevada and Utah to lobby the federal government to approve land transfers to allow developers to build homes and businesses on what had been public land. Supporters in the two states have won over environmentalists in the past with provisions that allocate proceeds to conservation projects, preserve other federal lands and prevent road construction, logging or energy exploration.

A small group of opponents is arguing that routinely approving these kinds of “swaps” to facilitate growth isn’t sustainable, particularly in areas that rely on a shrinking water supply. For the seven states that depend on the Colorado River — Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming — a regional drought is so severe that less water is flowing to Lake Mead and Lake Powell, the two man-made reservoirs where river water is stored.

If the level of Lake Mead keeps dropping through the summer as projected, the federal government will likely issue its first-ever official shortage declaration, which will prompt cuts in the share of water Arizona and Nevada receive.

The predicament is playing out in the Las Vegas, Nev., area, where environmental groups, local officials and homebuilders united behind a proposal from U.S. Sen. Catherine Cortez Masto that was heard in the Senate recently.

The Nevada Democrat is pushing what she calls the largest conservation bill in state history to designate more than 3,125 square miles of land for additional protections — roughly the size of Delaware and Rhode Island combined — and 48 square miles for commercial and residential development, which is about the size of San Francisco.

Some conservationists support the proposal because it would add federal land to the Red Rock Canyon National Conservation Area for recreation and reclassify some undeveloped parts of Clark County, where Las Vegas is located, and the Desert National Wildlife Refuge as Bureau of Land Management “wilderness areas,” which carry stronger protections than national parks.

Jocelyn Torres, field director for the Conservation Lands Foundation, said at the Senate hearing Wednesday the protections would restore lands to more efficiently capture carbon, which would help mitigate rising temperatures.

The effort mirrors land management pushes made over the past decade in Washington and Emery counties in Utah to designate wilderness and sell other parcels to developers to meet growth projections. The U.S. Census Bureau reported that St. George, in Washington County, was the nation’s fifth-fastest growing metro area last year.

In both regions, affordable housing is among officials’ top concerns. Soaring home prices in California have added to a flow of people leaving for nearby states like Nevada, Arizona and Colorado, where open land, lower tax rates and jobs are attracting new residents.

The fast-growing Las Vegas area lacks the housing supply to meet projected population growth. A 2019 University of Nevada, Las Vegas, study that Cortez Masto’s legislation references projected the population in Clark County would increase 35 percent, to 3.1 million by 2060. That spike will be difficult to accommodate without building in existing communities or public lands.