HAKAI: Rule number one: lean the opposite way from your partner, unless you both want to take a swim in the tepid Mystic River. While one person reaches over the side of the canoe and swirls a batch of bright green leaves like spaghetti, the other keeps the boat balanced. Then the person leaning over the gunwale yanks up the plant and tosses it into a basket. This is how you save the river.

I listen to the directions while standing on a sunny riverbank with my husband and in-laws, along with a few dozen other volunteers in Massachusetts. Eight kilometers southeast of us, this waterway finds Boston and empties into the Atlantic Ocean. The Mystic inspired the song “Over the River and Through the Woods” as well as one of those Hollywood movies where everyone fakes a Boston accent, but today we’re seeing it under siege—from a plant. Read more.