The two great basins abut each other, but they are—or were—distinct aquatic worlds. There was no way for a fish (or a mollusk or a crustacean) to climb out of one drainage system and into the other. When Chicago solved its sewage problem by digging the Sanitary and Ship Canal, a portal opened up, and the two aquatic realms were connected. For most of the twentieth century, this wasn’t much of an issue; the canal, loaded with Chicago’s waste, was too toxic to serve as a viable route. With the passage of the Clean Water Act and the work of groups like the Friends of the Chicago River, conditions
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