Subterranean Fishes of the World

5269

2009

ABSTRACT: The lower Congo River is one of the deepest, most powerful, and most biologically diverse stretches of river on Earth. The river’s 270 m decent from Malebo Pool though the gorges of the Crystal Mountains to the Atlantic Ocean (498 km downstream) is riddled with rapids, cataracts, and deep pools. Much of the lower Congo is a mystery from a hydraulics perspective. However, this stretch of the river is a hotbed for biologists who are documenting evolution in action within the diverse, but isolated, fish popula-tions. Biologists theorize that isolation of fish populations within the lower Congo is due to barriers pre-sented by flow structure and bathymetry. To investigate this theory, scientists from the U.S. Geological Sur-vey and American Museum of Natural History teamed up with an expedition crew from National Geographic in 2008 to map flow velocity and bathymetry within target reaches in the lower Congo River using acoustic Doppler current profilers (ADCPs) and echo sounders. Simultaneous biological and water quality sampling was also completed. This paper presents some preliminary results from this expedition, specifically with re-gard to the velocity structure and bathymetry. Results show that the flow in the bedrock controlled Bulu reach of the lower Congo is highly energetic. Turbulent and secondary flow structures can span the full depth of flow (up to 165 m), while coherent bank-to-bank cross-channel flow structures are absent. Regions of flow separation near the banks are isolated from one another and from the opposite bank by high shear, high veloc-ity zones with depth-averaged flow velocities that can exceed 4 m/s.

2

6th IAHR Symposium on River, Coastal, and Estuarine Morphodynamics