Dams help to make modern life easier by providing reliable water sources for irrigation and drinking water, flood control, and hydroelectric power. While dams can improve society there are also important consequences of building dams.
Reliable sources of water are vitally important to any city and dams can help provide that. Reservoirs can hold water to last a population through a drought or in coastal areas prevent salt water from reaching drinking water supplies. One example is the coastal city of Wilmington, NC. The main source of drinking water for Wilmington is the Cape Fear River. In times of drought salt water can travel up the Cape Fear River but the water intake is located behind Lock and Dam #1 on the river. This prevents saltwater from reaching the main water source for the city.
Some river systems flood after heavy rain and some flood seasonally. Reservoirs behind dams can hold this water so floods are less likely to happen downstream. This can reduce property damage although in regions that are seasonally flooded there are usually not many permanent buildings that would be damaged.
According to the United States Geological Survey (USGS) about 7% of power in the United States is provided by hydroelectric dams while that figure jumps to 19% worldwide. Hydropower is a significant source of renewable energy.
Rivers are like conveyor belts, they pick up sediment from one area and deposit it in another. This process is greatly influenced by the speed of the water, the faster the water moves, the more the river can carry both in size and volume. When a dam is built this process is halted and sediment builds up behind a dam because the water in a lake moves very slowly. This can eventually lead up to the filling in of a reservoir. Downstream of the dam the river is starved of sediment which increases erosion and reduces the amount of sediment deposited.
When dams are built they completely change the characteristics of the water body they are built on, free flowing rivers are transformed into lakes. This is a significant change of the ecosystem and the organisms that use this ecosystem will change accordingly. This means fish that thrive in free flowing water will be replaced by fish that prefer slow moving or still water. This can also lead to invasive species taking over.
While dams preventing flooding can be looked at as a benefit it can also be looked at as a disadvantage. When rivers overflow their banks they are carrying sediment and nutrients with them. They distribute these over the floodplain which creates fertile land. This is why a lot of farming has historically been done on floodplains. Without this process man made fertilizers are sometimes used which can cause other problems. River flooding also creates natural levies. These levies are created because when the river overflows its banks the water immediately slows down and drops sediment it is carrying right along the riverbank. This creates raised areas parallel to the river channel which can help prevent future flooding.
River systems can be drastically different from one another and because of these differences the effects of building a dam will be different between rivers. This is an overview of common positive and negative consequences of building dams.