The amount of water in the river is very closely controlled by an agreement called the Colorado Compact. This is an agreement between seven US states and Mexico.
The US Supreme Court finally decided on the division of water in 1963. They decided to allocate a percentage of the available water rather than a total amount based on previous flow.
This allowed for variations in the flow of the river.
The compact however, allowed only for the quantity of the water. There were many arguments over the quality of the water. Mexico complained that the increased salinity meant the water was almost unusable.
After many years of argument the USA finally spent $300 million building the desalination plant at Yuma. This costs over $20 million a year to maintain but aims to improve the quality of the water reaching Mexico.
Some states have increased their water allocation by drawing from groundwater supplies. However this is unsustainable as groundwater levels are dropping and not being replenished.