Mass wasting in the humid tropics: The agriculturally-developed Cayaguás watershed, Puerto Rico--An extreme case

U.S. Geological Survey, GSA Center, Suite 400-15, 651 Federal Drive, Guaynabo, Puerto Rico, 00965-5703, USA

Abstract

During the peak of land use conversion for pasture and subsistence cropping in Puerto Rico, 95 percent of original forest cover had been eliminated. This period, during the 1930's and 1940's, coincided with several large magnitude hurricanes each of which delivered up to 1,000 mm of rainfall in 1928 and 1932. Through examination of 1937 and 1995 1:17,000-scale stereo aerial photographs, more than 1,000 mass-wasting scars have been mapped and may be attributable to a combination of factors: hurricane-associated rainfall, intense land use practices, and the highly weathered substrate (granodiorite). An additional 1,000 scars are visible but not yet mapped.

Average scar frequency is 90/km² in the 25.6 km² watershed. The scars were caused mainly by shallow soil slips, debris flows, and slumps, generally measuring less than 100 m in length, 30 m in width and 4 m in thickness. Field verification of scars indicates good correspondence between aerial-photograph determined dimensions and ground-based measurements, ie. length measurements n = 105, r² = 0.85. In contrast , the nearby forested Icacas watershed is underlain by comparable bedrock (quartz diorite) but landslide frequency is only 20 scars per km² attesting to the dramatic effects of land use conversion in the Cayaguás watershed.

Most of the scars in the Cayaguás watershed appear to date from the early 20th century indicating that massive hillslope erosion occurred on a time scale of only a decade or two. The mass of material eroded from the watershed during this period was as much as 13,000 Mg/km²/year during this period. This is far more than the current fluvial sediment load (2,000, ± 1,000 Mg/km²/year) exported from the watershed. Extensive deposits of the eroded sediment remain in storage as colluvium on footslopes and alluvium along stream channels. This stored material is the likely source for much of the current fluvial sediment load which is therefore likely to remain high well into the 21st century. This high sediment load contributes to a major sedimentation problem in a downstream reservoir which supplies 50 percent of the water supply for the city of San Juan.

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