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Observations on Climate and Sediment Discharge in Selected Tropical Rivers, Indonesia

By
C. Blaine Cecil
C. Blaine Cecil
U.S. Geological Survey, Reston, Virginia 20192, U.S.A.
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Frank T. Dulong
Frank T. Dulong
U.S. Geological Survey, Reston, Virginia 20192, U.S.A.
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Ronald A. Harris
Ronald A. Harris
Brigham Young University, Provo, Utah 84602, U.S.A.
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James C. Cobb
James C. Cobb
Kentucky Geological Survey, Lexington, Kentucky 40506, U.S.A.
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Harold G. Gluskoter
Harold G. Gluskoter
U.S. Geological Survey, Reston, Virginia 20192, U.S.A.
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Hendro Nugroho
Hendro Nugroho
Department of Geology, UPN Veteran Yogyakarta, Yogyakarta, Indonesia
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Published:
January 01, 2003

Abstract

Factors that influence fluvial sediment discharge in warm climates (catchment-basin size, relief, gradient, tectonic setting, bedrock lithology, and rainfall) can readily be evaluated in fluvial systems of Indonesia. In equatorial Sumatra and Seram, rainfall, catchment-basin size, relief, and gradient are similar, whereas bedrock geology and tectonic setting differ. The relief and rainfall in equatorial Borneo is similar to that of Sumatra and Seram, but gradient, catchment-basin size, and tectonic setting differ. All factors, except rainfall, are very similar for Timor and Seram. A pronounced dry season in Timor and Java distinguish those islands from the wet climates of Seram and Sumatra, respectively.

The nature of stream channels (braided or meandering), stream bed materials, the degree of fluvial estuarine fill, deltas, and the nature of coastlines were used to evaluate sediment discharge. In addition, reconnaissance-level stream sampling was conducted for solid-suspended-sediment concentrations, solute concentrations, and pH in rivers in equatorial regions in Sumatra and Borneo, in Seram at 3° S, in Irian Jaya at 4° S, and in West Timor at 10° S. Rainfall in Sumatra, Borneo, and Seram exceeds evapotranspiration for all months of the year (> 100 mm/month and > 2.4 m/yr, perhumid climate). In contrast, in Timor 85 percent of all rainfall (1.4 m/yr) occurs during a four-month rainy season (dry subhumid climate).

The absence of a fluvially derived bed load, river-mouth deltas, the lack of fluvial fill of estuaries, and mud-dominated coastal zones in the perhumid regions are indicative of a very low fluvial sediment discharge. Very low sediment concentrations (10 mg/l suspended and 10 mg/l solute) in modern rivers in the perhumid equatorial region of Indonesia are consistent with this observation. In contrast, sediment discharge in dry subhumid climates of Indonesia is very high, as indicated by coarse-grained braided-stream bed materials with cobbles transported to the coast, the complete fluvial fill of estuaries, the formation of river-mouth deltas, and coarse-grained beaches. Very high sediment concentrations (2100 mg/l suspended and 340 mg/l dissolved) during rainy-season discharge in modern rivers in dry-subhumid regions of Indonesia (Timor) are consistent with this observation. The dominant variable affecting fluvial sediment discharge among the islands of Indonesia, therefore, appears to be the degree of seasonality in rainfall regardless of tectonic setting, relief, or catchment-basin size. Solute concentrations in humid and perhumid climates are indicative of bedrock geology. Chemical weathering of massive Miocene limestone thrust sheets in high mountainous areas of Seram and Irian Jaya results in solute concentrations that approximate the solubility of calcite (∼ 50 mg/l). Humid and perhumid areas without significant limestone bedrock geology have solute concentrations that approximate that of rainwater (∼ 10 mg/l).

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Contents

SEPM Special Publication

Climate Controls on Stratigraphy

C. Blaine Cecil
C. Blaine Cecil
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N. Terence Edgar
N. Terence Edgar
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SEPM Society for Sedimentary Geology
Volume
77
ISBN electronic:
9781565762145
Publication date:
January 01, 2003

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