Remotely Sensed Predictions and In Situ Observations of Lower Congo River Dynamics in Support of Fish Evolutionary Biology
Abstract
Ongoing research into the evolution of fishes in the lower Congo River suggests a close tie between diversity and hydraulic complexity of flow in the channel. For example, fish populations on each side of the rapids at the head of the lower Congo are within 1.5 km of one another, a distance normally allowing for interbreeding in river systems of comparable size, yet these fish populations show about 5% divergence in their mitochondrial DNA signatures. The proximal reason for this divergence is hydraulic complexity: the speed and turbulence of water moving through the thalweg is a barrier to dispersal for these fishes. Further examination of fish diversity suggests additional correlations of evolutionary divergence of fish clades in association with geomorphic and hydraulic features such as deep pools, extensive systems of rapids, alternating sections of fast and slow current, and recurring whirlpools. Due to prohibitive travel costs, limited field time, and the large geographic domain (approximately 400 river km) of the study area, we undertook a nested set of remote sensing analyses to extract habitat features, geomorphic descriptors, and hydraulic parameters including channel forming velocity, depth, channel roughness, slope, and shear stress. Each of these estimated parameters is mapped for each 1 km segment of the river from the rapids described above to below Inga Falls, a massive cataract where several endemic fish species have been identified. To validate remote sensing estimates, we collected depth and velocity data within the river using gps-enabled sonar measurements from a kayak and Doppler profiling from a motor-driven dugout canoe. Observations corroborate remote sensing estimates of geomorphic parameters. Remote sensing-based estimates of channel-forming velocity and depth were less than the observed maximum channel depth but correlated well with channel properties within 1 km reach segments. This correspondence is notable. The empirical models used to derive channel features were developed on alluvial systems and flume studies, but the lower Congo is bedrock-constrained. This research is a novel application of remote sensing of hydraulic properties (velocity, discharge, channel forming depth, shear stress, channel roughness) in support of fish evolution research. These methods will prove useful for studying river dynamics in other large rivers.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H53O..04G
- Keywords:
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- 1813 HYDROLOGY / Eco-hydrology;
- 1825 HYDROLOGY / Geomorphology: fluvial;
- 1855 HYDROLOGY / Remote sensing;
- 1856 HYDROLOGY / River channels