Tropical geomorphology and geomorphic work: A study of geomorphic processes and sediment and water budgets in montane humid-tropical forested and developed watersheds, Puerto Rico

U.S. Geological Survey, GSA Center, 651 Federal Blvd. Guaynabo, Puerto Rico, 00965, USA

Abstract

The low latitude regions of the Earth are presently undergoing profound, rapid landscape conversion. An appreciation of the impacts of these land-use changes demands careful analysis of watershed-scale hydrologic and geomorphic processes in an attempt to manage what is largely an economically-driven, often disorganized process. The investigation of land use history, hillslope erosion, fluvial sediment transport, and the development of sediment and water budgets contributes to improved understanding of the impacts of development.

Four small, montane, humid-tropical watersheds in the Luquillo Experimental Forest and nearby Río Grande de Loíza watershed, Puerto Rico, were paired to compare and contrast the geomorphic effects of land use and bedrock geology. Land use history was characterized in detail for the period of aerial photographic coverage, 1937 to the present, when forest cover was in a state of recovery, and more generally for the period 1820 to 1936, when forest cover was in decline. Annual water and sediment budgets for the period 1991 to 1995 were developed for each basin using sheetwash, soil creep, treethrow, landslide, fluvial suspended and bed sediment, streamflow, rainfall, and groundwater flow data. Sediment storage in channel beds, bars, floodplains, and colluvium was assessed.

Two of the watersheds, with primary and secondary forest cover and underlain largely by Cretaceous volcaniclastic rock, were relatively resistant to erosion. In these two watersheds, suspended sediment yield (120 to 140 Mg/km²/y), landslide frequency, (4 to 21 landslides/km²/10 0 y) and sheetwash (5 to 30 Mg/km²/y), were lower than in the two watersheds underlain by intrusive bedrock. In a forested, intrusive-bedrock watershed, suspended sediment yield, landslide frequency, and sheetwash were 525 Mg/km²/y, 43 landslides/km²/100 y, and 20 Mg/km²/y, respectively. In an agriculturally developed intrusive-bedrock watershed, these three parameters were 746 Mg/km²/y, 62 landslides/km²/100 y, and 105 Mg/km²/y, respectively, demonstrating that the erosional effects of h uman disturbance are great. Even in the two watersheds with primary forest cover, located in the Luquillo Experimental Forest, the effects of anthropogenic disturbance were significant as 43 to 71 percent of landslide-caused erosion there was associated with road construction and maintenance.

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