Rising temperatures and changing rain and snow patterns will force increasingly difficult adjustments with reduced flows in major rivers — including the Colorado and Rio Grande — and more people counting on that water, the White House announced Tuesday.

The federal government’s top environmental officials unveiled a report that calls climate change “a growing risk.” They project wide impacts from flood control to depletion of groundwater.

“One of the greatest challenges we face is dealing with the impacts of climate change on our nation’s water, which is really the lifeblood of our economy,” Deputy Interior Secretary Michael Connor said.

While states manage river flows, federal authorities are encouraging better, collaborative strategies aimed at protecting agriculture, urban drinking water and production of electricity.

The 307-page “Managing Water in the West” report makes projections including:

• A 5- to 7-degree Fahrenheit spike in temperatures by the end of the 21st century.

• Mixed impacts across the Western states with more precipitation over northwestern and north-central areas of the West and less over southwestern and south-central areas — with Colorado split.

• Less snowpack across the West on April 1, the key time for runoff into basins that otherwise are prone to drought.

• Reduced flows in major rivers ( 7 to 27 percent less) between April and July. Those include rivers that start in Colorado — the Rio Grande and Colorado — as well as the San Joaquin in California.

This is the Interior Department’s second report to Congress under a 2009 law requiring efforts to secure water supplies in 17 Western states. It was done by Interior’s Bureau of Reclamation, which runs dams, power plants and canals across the mostly arid Western states.

Federal projects provide 10 trillion gallons of water a year for urban use, and irrigation water for 10 million acres used to grow 60 percent of the nation’s vegetables and 25 percent of fruits and nuts. The electricity from 53 federal plants powers more than 3.5 million households.

Reclamation officials also are charged with managing water in a way that protects the environment.

Natural resources analysts for years have warned of wide impacts of climate-driven changes. For example, when runoff decreases without a corresponding cut in pollutants, maintaining water quality “will become more difficult, especially during periods of low flow,” the report said.

They’ve focused on the Colorado River Basin as one where reductions in spring and early-summer flows may mean less water for irrigating crops and generating electricity at hydropower plants near reservoirs.

The changing snowpack and decreased runoff in Colorado mountains are expected to slow natural recharging of underground aquifers. That trend collides with increased reliance on groundwater to sustain agriculture and cities in California, the report said.

In northwestern states, warmer conditions are expected to put increased pressure on fisheries and raise demand for electricity.

A shift from snow to rain at lower elevations probably will increase flows in the Columbia and Missouri river systems during winter — instead of summer — meaning less water will be available to irrigate crops and more floods.

The Sacramento and San Joaquin river basins in California probably will face earlier runoff, along with increased evaporation, the report said. And that is expected to intensify challenges for food production and sustaining more people in cities.

Bruce Finley: 303-954-1700, bfinley@denverpost.com or @finleybruce