Science World
Science World for grades 6–10 brings science to life with fascinating feature articles and hands-on activities that reinforce science concepts and help students build test-taking and critical-thinking skills.

Rough Waters

One of the world’s most turbulent rivers is home to a wide array of fish species. Now, large dams are threatening their future.

For most of its length, Africa's Congo River is calm. The continent's largest river slowly meanders over a high, flat plateau, collecting water from 3.6 million square kilometers (1.4 million square miles) of land. Near the cities of Kinshasa and Brazzaville, the water forms a large pool. Then, the water plunges off the plateau to form the lower Congo River.

Over the next 350 km (217 mi), the water descends more than 280 meters (918 feet) to reach the Atlantic Ocean. This extreme drop in height combined with the huge amount of water in the river creates powerful currents. "The largest rapids on Earth are found in the lower Congo," says Melanie Stiassny, an ichthyologist at the American Museum of Natural History in New York. Stiassny studies the fish that live in these wild waters.


The violent rapids make studying much of the lower Congo tough. Only a few areas have calm enough water to travel by boat, so Stiassny gets help from world-class river kayakers. They travel with instruments on their boats to collect data such as the water's speed and depth. Even these expert boaters can safely ride on only a small portion of the river-the rest is too dangerous.

The lower Congo River has one of the most diverse fish populations in the world. More than 300 species of fish are found in this small stretch of river. In a similarly long portion of the Mississipppi River, you would be lucky to find 20 species of fish.

Roughly 80 of the fish species found in the lower Congo are endemic, or found no place else. One such species is Lamprologus lethops. Stiassny thinks this blind fish lives in river canyons as deep as 160 m (525 ft) below the surface.


Stiassny has discovered that the diversity of fish in the lower Congo is likely the result of the river's extreme conditions. She has been analyzing the DNA (chemical that carries hereditary information) of different fish populations in the river.

Members of a single species of fish should have similar DNA. However, Stiassny has found significant differences in the DNA of individuals of the same species living in different areas. In one case, individuals living on one side of the river had significantly different DNA than that of the same species living on the opposite side.

Stiassny thinks that the river's powerful currents are acting as a natural barrier for the fish. They can't swim across the currents to interact and mate with one another. "The genetics have diverged so much that we can say there is no connection between the two populations," says Stiassny. Over time, the separation might cause populations of the same species to develop new characteristics and become different species.


Many of the unique fish that live in the river may soon be at risk of disappearing. Two hydroelectric dams have been built already on the lower Congo to generate energy, mostly for mining. There are plans to build an even larger dam. "That could be the end of the ecosystem as we know it," says Stiassny.

She hopes the information she
has been gathering will be used to make predictions about how different dam designs might affect the river. That information could be used to create a dam that preserves important fish habitat.

Britt Norlander

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