Frank Gehry wants to build a park in Los Angeles. Not just a normal park on empty land; that’s for lightweights. Gehry wants to take chunks of the legendarily unlovely Los Angeles River, a 51-mile engineered waterway mostly lined with concrete, and suspend parks over them. It sounds like a pipe dream, or in this case more of a channel dream; it’s expensive, unprecedented, structurally complex, and anathema to many of the locals. But Gehry, 94, has made a career of overcoming such obstacles and, in the process, transforming cities...