Distributary Channels, Fluvial Channels and Incised Valleys

Janok Bhattacharya¹ and Cornel Olariu^2
1 University of Houston, Houston, TX
² University of Texas at Dallas, Richardson, TX

Many deltas show several orders of branching resulting in a wide range of sizes and shapes of distributary channels. At the largest-scale, trunk rivers become distributive at the point where the river becomes unconfined. Good outcrop exposures of incised trunk rivers can be used to estimate paleodischarge. Intermediate-scale delta plain channels tend to be few in number and may be separated by wide interfluves. The smallest-scale "terminal distributaries" lie close to or within the delta front, especially where they extend offshore. Because discharge decreases downstream, terminal distributary channels tend to be narrow and shallow, rather than wide and deep.

The number of active terminal distributary channels appears to be a function of wave-climate, tidal-energy, river discharge and water depth. Large rivers feeding into shallow water basins (e.g. Volga, Lena, Atchafalaya) show numerous orders of branching, with dozens to hundreds of simultaneously active outlets. These are typically stable for periods of a few years to a few decades, and experience frequent avulsions.

Wave-modified deltas, like the Nile, show only a few outlets. Waves carry sediment away from the river mouth and inhibit progradation and avulsion. Tide-dominated deltas show a bewildering alternation of tidal channels and terminal distributaries. Tidal processes may allow terminal distributary channels to be stable for thousands of years, such as seen in the Mekong and Mahakam deltas in southeast Asia.Predicting time-scales over which multiple terminal distributary channels are active is critical for determining downstream controls on avulsion and for accurate numerical modeling of deltas.