The Three Hydrological Regimes of Fan-Margin Rivers in Megafan Landscapes
Topics: Geomorphology
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Keywords: megafans, hydrological regimes
Session Type: Virtual Paper
Day: Thursday
Session Start / End Time: 4/8/2021 11:10 AM (Pacific Time (US & Canada)) - 4/8/2021 12:25 PM (Pacific Time (US & Canada))
Room: Virtual 3
Authors:
Murray WILKINSON, Texas State University - San Marcos
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Abstract
Megafans are features >80 km long, i.e.,orders of magnitude larger than alluvial fans (sensu stricto <10-20 km long). In a recent global identification effort 272 have been identified (versus the prior published total of 121). Major megafan-forming (“mountain-fed”) rivers drain major feeder basins and construct sediment cones by avulsing to different radial courses on the cone. Megafan-forming rivers are usually considered the major rivers of basins (aside from the trunk river of the basin to which they are tributary).
However, “fan-margin” rivers draining the depression between neighboring megafans periodically reach far greater discharges:--
Regime 1—Low, permanent discharge from streams draining hillsides (interfan/”foothills-fed” streams) between valleys of the megafan feeder rivers. This interfan drainage combines with small discharges from streams draining both flanking megafan (“plains-fed”/“fan-fed” streams).
Regime 2—Fan-margin river discharge is significantly augmented when one of the major megafan-forming rivers avulses into the fan-margin depression (mountain-fed rivers are usually significantly larger than Regime 1 flow).
Regime 3--Maximum discharge occurs when the megafan-forming rivers on BOTH flanking megafans avulse into the fan-margin river. The fan-margin river then carries the flow of two major megafan-building rivers, clearly a significantly greater discharge than that of the individual megafan-forming rivers.
Regimes 2 and 3 can persist for hundreds of years before the megafan-forming river avulses away from the fan-margin river.
Such periodic discharge extremes appear to be the norm in the fan-margin rivers of multi-megafan landscapes (e.g., Ganges Plains in northern India and Chaco Plains in South America).
