Thousands of small dams built for mills and factories still stand on rivers where the industry they served closed a century ago. Many produce no power, hold no useful water and need repairs their owners cannot justify. They do, however, block migrating fish and trap the gravel that spawning depends on. Removing such a dam is often cheaper than repairing it, which is why the number taken out each year has risen sharply without any change in environmental law. An owner's liability for a structure that fails has moved the numbers more than any conservation argument.