Boats skimmed along the Potomac River this last weekend of August—power boats, yellow and red kayaks, boxy green canoes, sleek white sculls.  I settled into the latter late Sunday afternoon, dropping oars into the warm water.  Many in Washington are still out of town—on vacations or home visiting constituencies—but in their place are tourists exploring the nation’s capitol. The heart of the city beats on in festive cadence.

Baking in the summer sun, I eased leisurely down the river—past the Kennedy Center, the Watergate apartments, past the Georgetown waterfront where outside cafes were filled with people eating and bicyclists walking their bikes, past the new park along the river, then under Key Bridge, where...