Wearing white sneakers, a cowboy hat and overalls, Bernie Fowler walks into Maryland’s Patuxent River every June to see how deep he can go and still see the tops of his shoes. As a young man he could see his feet on the river bottom as he stood chest-deep to net blue crabs. Now in his nineties, he ventures into the river to assess the water clarity. Fowler has been collecting this data point for the past 29 years and counting, calling it “sneaker depth.”