Dams are created to help control the flow of water for any one of a number of purposes—to satisfy irrigation needs, to minimize the chance of floods, to generate hydroelectric power or to help create recreational areas. While the benefits to humans are substantial, a steep price is paid when it comes to river habitats and the sustainability of plants and animals that rely on the free flow of water. These habitats, referred to as riparian zones by scientists, are critical in that they contain vegetation that prevents the erosion of soil and that provides shade and safe harbor for both aquatic and land animals. 

The free flow of water is also critical for the propagation of species. For example, in Northern California, Coho Salmon are now threatened with extinction due to a combination of factors that includes damming and water diversion for agricultural purposes. Elsewhere, on the Bio-Bio River in Chile, Academy ichthyologist Dave Neely, has noticed a significant decline in fish populations since a dam was erected in 1996. And, on various dammed rivers in China, India and Brazil, the Academy’s Biodiversity expert, Healy Hamilton, notes a significant decline in river dolphin populations

Academy scientist John McCosker studies Chinook salmon and other fish species that populate the Sacramento River in Northern California. He notes that native species have been devastated by the introduction of non-native species introduced by State resource managers. For example, the population of  Sacramento Perch (the only native fresh water sunfish in the U.S.), is now only a fraction of what it was in the late 1800s, before bass were introduced to the area.

Rivers come in all shapes and sizes. They typically begin in headwaters or small streams that feed a larger channel bounded by banks. Some rivers meander, others branch. And some branching rivers find their way back together again, forming braided rivers. An oxbow lake is a remnant body of water left behind by a sharp hook in a river that changed course.

When rivers dump sediments outside their banks, floodplains blossom. A delta is just a floodplain at the mouth of a river. Sometimes, at their mouths, rivers meet the sea and create large bays of brackish water. These are called estuaries, and can occur even in long, narrow fjords and funnel-shaped rias.

Not all rivers flow to the ocean. Rivers on the eastern slope of the Sierras, for example, drain into the western United States’ Great Basin. The Amu Darya in Central Asia drains to the Aral Sea, a saltwater lake on the boundary between Kazakhstan and Uzbekistan. The Nile River is said to flow backwards—south to north—originating in Uganda and draining into the Mediterranean Sea.