If you want to clean up the largest pollution spill in the country, one unaltered by decades of work and billions of dollars, you need to spend a lot of time making tiny measurements. Most of them will only confirm the depressing trend: More and more contaminants are winding their way from farms into rivers and streams.

But in one small watershed in northern Indiana, those measurements have revealed an undeniable improvement in one of the most stubborn environmental crises in the world.

After 13 years and a million dollars in state, nonprofit, and federal funding, the data show a clear decline in nitrogen and phosphorus flowing out of this watershed during the critical springtime thaw. These two nutrients fertilize crops, but when they wash into the water, they fertilize algae blooms and cause a host of problems. In other words, the chemicals we rely on to grow food often end up poisoning the planet and threatening the lives of many species on it, including ours.

As far as I can tell — and I spent a lot of time looking — there’s only one place in the country where conservation measures have found a fix for this dilemma: the Shatto ditch.

The Shatto ditch drains 5 square miles of northern Indiana. It begins in low hills and travels 8 miles, with water seeping up from the ground and trickling down pipes protruding from the earth into the ditch. Then its water flows to the Tippecanoe River, and from there to the Wabash River, the Ohio River, and finally to the Mississippi River, carrying fertilizer washed off fields along the way.

It all started as a modest project by a collection of nonprofits, local agencies, and farmers to clean up the Tippecanoe River, one of the most important rivers in the United States for ecological diversity, and one of the last remaining healthy homes for the Midwest’s shellfish. It’s home to nearly 50 species of mussels, including threatened and endangered species with fantastic names: sheepnose, clubshell, fanshell, rabbitsfoot, and snuffbox.

In 2006, The Nature Conservancy proposed widening the Shatto ditch to keep these mussels from getting smothered under silt. The wider ditch would act more like a natural creek, with a floodplain that would allow high water to spread out and slow down during storms. The slower the water moves, the less sediment it carries.

Read the full article about the Shatto ditch by Nathanael Johnson at Grist.