Expanding Croplands Mississippi Headwaters, Minn.

The Mississippi begins as a stream flowing from a lake in a forest wilderness, crystal clear and toxin-free. Contaminants are accumulating in rivers and drinking water below those headwaters, after burgeoning global demand for food led farmers to convert tens of thousands of acres of forest, grassland and dormant fields into cropland.

Farmers near the Straight River in recent years have installed sprinklers on once fallow land to grow fertilizer-hungry row crops in sandy soils. Irrigated acres near the river increased 81% in the 24 years ending in 2016.

Concentrations of nitrate are rising in the Straight. During the summer, they measure at least 100 times higher than in nearby rivers.