The seven Colorado River Compact states agreed last week to send less water to Lake Mead.
The decision follows the news that Lake Powell is at its lowest point since the 1960s, threatening power generation at Glen Canyon Dam.
The proposal will allow Lake Powell to reduce the volume of water released to Lake Mead, although it will still release 7 million acre-feet to the downstream reservoir. 480,000 acre-feet will be retained at Lake Powell.
The decision is the first of its kind, according to KTNV Las Vegas.