I was paddling down the Big Gunpowder Falls near Sparks, Maryland, when I saw a great blue heron standing on a log in the river, tall and elegant. As I drifted closer, it launched into the air and flew over my head, its six-foot wingspan and knife-like beak all the more impressive at close range.Nearby, atop the riverbank, was a house. I thought: what is the economic value of this heron to that homeowner? Would he be able to sell his house for $505,000 instead of $500,000 if a buyer saw the heron before agreeing to the price? Or maybe the location and the view of the river are all that matter in the fast-moving world of real estate transactions.