This story is part of a special National Geographic News series on global water issues.

A musty smell repels swimmers from some Iowa lakes in summer, when the bodies of water too often turn brown, green, or blue-green.

Researcher John Downing says his state is 92 percent cultivated, so fertilizers seep off croplands into waterways during rains, prompting algae blooms that can harm drinking water and make it tough to boat or catch sport fish. Less than half a pinhead of phosphorus per gallon of water-"a phenomenally tiny quantity," said Downing-is enough to turn a lake bright green.

A fleck of phosphorus fertilizer costs a farmer almost nothing. "But that half pinhead per gallon can cost society millions in lost recreational value and cleanup costs," said Downing, an Iowa State University professor whose water-monitoring group tests 137 Iowa lakes. "We don't have lakes that we could point to and say: 'Here is a pristine lake that has been unimpacted by people.' "

You wake up to cereal made from midwestern corn. You slip on cotton clothes, get into a vehicle fueled partly by ethanol and dine later on chicken and rice—all made possible by crops from the Mississippi River Basin, a vast area that stretches from Montana to New York and drains all or parts of 31 states.

The part of the basin east of the Mississippi River largely relies on rain to grow crops; farmers on the west side irrigate much, much more. All told, it's among the most productive farming regions in the world.

Trouble is, fertilizer that flows from fields (and cities) takes a toll on local waters and eventually reaches the Mississippi River and the economically important fisheries of the Gulf of Mexico, where nitrogen and phosphorus pollution suffocates marine life and has led to a dead zone larger than the state of Connecticut.

What's Grown

Nearly four out of 10 ears of corn grown in the world come from the Mississippi River watershed. So much corn, soy, and wheat grow here that some communities claim superlatives—Decatur, Illinois, "Soybean Capital of the World;" Sumner County, Kansas, "Wheat Capital of the World;" and Iowa, "Food Capital of the World." The lion's share of the country's corn, grain, livestock, poultry, cotton, sorghum, and soy is grown in the Mississippi basin.

A whopping 60 percent of Americans' "water footprint"—their impact on water resources—is in the Mississippi basin, according to new data from the Water Footprint Network and its partner, The Nature Conservancy. And because of American exports of farm products, the Mississippi watershed also supports part of the water footprint of people around the world. It turns out that the United States is the world's biggest net exporter of "virtual water" because of agricultural sales. With corn and other imports, Mexico receives a Nile River's worth of virtual water annually from the U.S.

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At What Cost?

This comes at a cost, environmentally and socioeconomically. Rice growing in Arkansas over the past century has depleted the groundwater, dropping the Mississippi River Alluvial Aquifer by 90 feet, wrote Cynthia Barnett in her 2011 book, Blue Revolution. The heavily subsidized crop will now grow with the help of river diversion.

That worries fans of the nearby Cache River Wildlife Refuge, where wetlands have dried up as the aquifer dropped. Nine percent of crops that grow in the huge Mississippi system rely on rivers, an aquifer, or lakes for irrigation. Among the thirstiest are rice, cotton, and wheat.

Pollution is also a serious problem. Most oysters that Americans eat come from the Gulf of Mexico and nearly three million recreational anglers fish there, but every year a swath of the Gulf becomes devoid of marine life.

The dead zone serves as poster child for the unsolved water-quality problems of the Mississippi basin. One study found oxygen-starved female fish in the dead zone developed testes. The push to grow more corn for ethanol may send as much as 19 percent more nitrogen pollution to the Gulf, one study says, widening the dead zone. Already, most corn fuels vehicle gas tanks and feeds livestock—not people.

"U.S. water practices and attitudes have allowed freshwater to become the single-most-degraded of all America's major ecosystems—a particular shame when you consider our great water wealth compared with other countries," Barnett said in an interview. "With a shared water ethic, Americans would come to value water in the same way we came to value clean cities, parks, and streets and turned around what once seemed like an intractable littering problem in one generation."

"I don't think there's anybody that any longer denies a serious problem," said Fred Kirschenmann, Leopold Center Distinguished Fellow at Iowa State University.

Solutions?

If fixing the Mississippi River basin's monumental problems were easy, it would've happened by now. Two examples from Kirschenmann:

-If corn growers raised alfalfa instead of soybeans or more corn for part of the year every other year, research shows they could stop all nitrate leaching into the groundwater (plus improve corn yields). But farmers don't. Reason: Alfalfa has a tiny market. Today's intensively raised cattle eat corn, not alfalfa.

-If farmers planted a fall cover crop of, say, winter rye after harvesting corn or soybeans, the rye would take in nitrogen and hold it so that it doesn't leach away and wind up in the Gulf of Mexico. When the rye is tilled into the ground in the spring, it'd provide nitrogen to the soil. But farmers find nitrogen fertilizer cheaper.