California is insisting on its legal claims under a compact dating back to 1922 as the river faces unprecedented strain because of climate change and population growth in the Southwest.

On one side are six states, including Arizona and Nevada, where growing cities such as Las Vegas and Phoenix are in an existential battle to avoid exhausting their supplies from the Colorado River.

A bipartisan group of Western representatives, excluding officials from California, urged President Joe Biden to support the proposal offered by the six states in a letter Wednesday morning.

People walk by a formerly sunken boat standing upright into the air with its stern buried in the mud along the shoreline of Lake Mead, Jan. 27, 2023, near Boulder City, Nev. | John Loche/AP Photo

Their plan relies on a new tool to preserve some water for Arizona and Nevada users by accounting for evaporation and leaks along the river as it flows downstream to California.

“We agree there needs to be reduced use in the Lower Basin, but that can’t be done by just completely ignoring and sidestepping federal law,” said J.B. Hamby, the chair of the Colorado River Board of California and an Imperial Irrigation District director.