Authorities rushed to rescue hundreds of people stranded on rooftops and supply drinking water to areas flooded by a collapsed dam in southern Ukraine on Wednesday, in a growing humanitarian and ecological disaster along a river that forms part of the front line in the 15-month war. The collapse of the Kakhovka hydroelectric dam and emptying of its reservoir on the Dnieper River added to the misery the region has suffered for more than a year from artillery and missile attacks.