For the Standing Rock Sioux tribe, whose lands straddle the North and South Dakota border, river water means drinking supplies. For Illinois farmers, it's irrigation for their crops, according to Bloomberg.

Rivers also power hydroelectric plants, provide recreation for boaters and give coal companies inexpensive access to export markets with barges to New Orleans.

Balancing these competing demands on the nation's water resources has never been easy. Global warming, linked to near-record low water levels on the Mississippi River this year as well as last year's severe floods along the Missouri River, is making the task even harder.

"You end up pitting one constituency against another, and then you mud-wrestle over the right balance," said Ben Grumbles, president of the Washington-based environmental group U.S. Water Alliance.

It may also mean more disputes such as the one that erupted in recent weeks in the Midwest. Shippers and political leaders from along the Mississippi River's midsection asked the U.S. Army Corps of Engineers to adjust its dams so more Missouri River water would flow into the drought-shrunken Mississippi, keeping barges moving on the nation's busiest waterway. States along the upper Missouri opposed that and the Army Corps refused. Read more.