Dana Cronin / Harvest Public Media

When it rains on Joe Rothermel’s central Illinois farm, most of the water drains into the nearby East Branch Embarras River. There, it begins a journey south through the Wabash, Ohio and Mississippi Rivers. As it flows through more and more farmland, fertilizer runoff -- which once nourished crops -- compounds the water’s nutrient load, resulting in a dead zone off the coast of Texas and Louisiana.