Flood control dams are intentionally created so that water has somewhere additional to flow when excessive rains arrive. The idea behind them is that, unchecked, the water from particularly rainy seasons will flood rivers around which settlements are built or do other undesirable damage; however, by building a dam to which these excessive waters can flow, those waters will flow safely away from the endangered areas. The Mount Morris Dam in New York is one of the most prominent examples of a flood control dam. The dam was constructed in response to many years of flooding in the areas around it, causing millions of dollars in damage every few years. The intention of the dam was to stop the water that would otherwise have flooded the town areas and keep it, then slowly release it over time in a way that would be harmless to the civilizations downstream.
When described only in terms of their intended functions, flood control dams sound desirable. However, they can lead to very significant problems. The main problem is a challenge to the surrounding ecosystem. Natural ecosystems evolve slowly and naturally over a period of one million years as thousands of different creatures slowly attain a sort of homeostasis with one another; bringing in a massive river blockage can through those food chains and systems into sheer chaos. That is not only a problem from an environmentalist angle, but it can also introduce problems for humans as well, altering the food supply or sending carnivores looking for food on human settlements. Dams also have the challenge of aggregating sediments. This is especially a challenge for reservoirs because as sediments gather in the reservoir, the amount of usable water decreases more and more. These sediments also may have played roles in systems further downstream, and gathering them all up in one place can have immensely negative repercussions for other systems in the area. The effects can be extremely far-reaching; recently, actions taken with a reservoir in northern Georgia were seen impacting farms on the Gulf of Mexico.
