This summer I reached a personal milestone: my first triathlon. I swam in the Merrimack river, and rode and ran through the streets of Lowell, Massachusetts. The Mill City triathlon is named after the massive brick buildings that line the banks of the river. In the 1850s, those mills made Lowell the largest industrial center in the United States, producing 50,000 miles of cloth each year. Today the mills are converted into comfortable lofts, offices and a museum. The powerful river that drove the machines of the American industrial complex is now clean enough for a swim. It’s hard to imagine this transformation while swimming in the warm water of the Merrimack.