--> Monsoonal and Ephemeral River Systems: Facies Model and Controls

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Monsoonal and Ephemeral River Systems: Facies Model and Controls

Abstract

This paper is a synthesis of 52 modern and ancient examples of monsoonal and subtropical river systems, including 4 original field datasets. Monsoon rain is the main surface water supply to many of the world's rivers. Some such rivers occur completely within the tropical monsoon climate zone, whereas others have parts of their drainages in temperate climate zones, or on high elevations and receive some of their water discharge from snow melt. Yet others, have their upstream drainages in the tropical monsoon climates, but flow through subtropical drylands. Some rivers in sub-humid to arid subtropics are ephemeral and receive monsoon rain and transmit discharge only during abnormal or strengthened monsoon seasons and the associated cyclonic flow. Common to all these rivers is that they frequently experience high-magnitude floods that last from just hours to months, with a yearly (seasonal) to decadal (ephemeral) recurrence frequency. The high-magnitude floods are caused by intense monsoonal rainfall events that may contain most of the annual precipitation. Rivers may transmit lower magnitude flood or non-flood discharges during the rest of the year or the rest of the monsoon season, or the river beds may remain dry for the most of the year, or even for a decade. These rivers distinctly contrast with perennial rivers, where the annual discharge range (highest-lowest discharge) is relatively small compared to its annual discharge mean. Some monsoonal rivers have discharges 40–50 times greater during the summer monsoon. It is this highly seasonal discharge with a characteristically rapid rise, sharp peak and rapid decline of the flood hydrographs that define the monsoonal and sub-tropical river form and function, as well as the facies and facies successions, as up to 85–95% of their annual sediment transport and deposition occurs during these events. The paper presents specific recognition criteria that differentiate the highly seasonal river deposits from perennial river facies models. The internal variability within the highly seasonal rivers is presented as a continuum with end members. The monsoon regions, and the bordering subtropical regions reside on each side of the equatorial perennial rainfall regions, approximately between ca 10–35° N and S. However, multiple greenhouse world examples occur at considerably higher latitudes, suggesting hydrological cycle, poleward moisture transport and climate zones different from the current ice-house world.