The great outdoors has a diversity problem. Can it be fixed?

Late one evening in October 2019, Don Baugh beached his kayak on the rocky shore of Pyne Poynt Park, a 15-acre postindustrial green space abutting a polluted backwater in Camden, New Jersey. Mr. Baugh runs an environmental nonprofit called Upstream Alliance, and he’d spent the day on the water pointing out sewage outflows to staff from New Jersey’s Department of Environmental Protection.

Now it was dusk, and Mr. Baugh wanted to load his boat and leave before dark. Across the Delaware River from Philadelphia, Camden is one of America’s poorest...