More than half of the world's large rivers are fragmented and regulated by dams, including all the largest and the most biologically diverse rivers, according to new research from the University of Umea in Sweden and the Nature Conservancy in the United States. The study shows that flow in 172 of the 292 largest rivers is regulated by dams, and that this number would be larger if irrigation were included. There are dams in the world's 21 largest rivers and in the eight rivers that are biologically and geographically most diverse.