Dead fish on the river. © Digital Vision   The Damage Dams Do Freshwater ecosystems around the world are gravely threatened. One of the major culprits is dams. Halting and impounding the free flow of water affects the chemical, physical and biological processes of river ecosystems. Some habitats are flooded; others dry up. Changes to the ecosystem may favor some species but be detrimental to many others. Serious, often unanticipated consequences can take years or decades to emerge. Scientists still have a great deal to learn about the intricate physical and biological dynamics of river ecosystems. However, many of the negative consequences of altering these dynamics are becoming increasingly clear.      1 of 11