For this month:

From Matthew Larsen:

A new article. If anyone wants a copy, send me an e-mail. I can send it via
e-mail as a MSWord file, or mail a paper copy.

Larsen, M.C. and Santiago-Román, Abigail , 2001, Mass wasting and sediment
storage in a small montane watershed: an extreme case of anthropogenic
disturbance in the humid tropics in Dorava, J. M., Palcsak, B.B., Fitz
patrick, F. and Montgomery, D.eds., American Geophysical Union Water
Science & Application Volume 4, Geomorphic Processes and Riverine Habitat,
p. 119-138.

Abstract: By the peak of land-use conversion for subsistence cropping and
plantation agriculture in Puerto Rico in the 1940's, 94 percent of the
original forest cover had been eliminated. In a small (26.4 km2) upland
watershed that typifies this land-use history, field surveys and
examination of aerial photographs indicate that more than 2,000 landslides
have occurred since about 1820 when forest clearing began. The landslides
are attributable to a combination of three factors: a highly weathered
bedrock (Cretaceous granodiorite), episodic heavy rainfall, and almost two
centuries of intense land-use practices. On average, landslide scars number
140/km2 in the Cayaguás watershed, equal to 80 landslide scars/km2/100 y.
The volume of hillslope material eroded by landsliding is estimated at
660,000 m3/km2 (870,000 Mg/km2). If all colluvium was transported from the
catchment, then the volume is equivalent to a mean surface lowering of the
entire watershed by 660 mm, or 3.8 mm/y. Soil augering, field observations
at construction sites, road cuts and stream banks, mapping from aerial
photographs, and GIS-based estimates of the surface area of footslopes,
indicate that colluvium may total 149,000 Mg/km2. If mobilized, this would
be sufficient stored material to supply the annual average fluvial sediment
yield for as long as 129 years. The great availability of colluvial and
alluvial sediment on footslopes, floodplains, and in channels will maintain
high sediment yield well into the 21st century in spite of government
efforts to reforest hillslopes and institute other hillslope soil
conservation measures.

Matthew C. Larsen, Hydrologist
USGS Caribbean District Chief
GSA Center, 651 Federal Drive
Guaynabo, Puerto Rico, 00965-5703, USA

phone: 787-749-4433, fax: 787-749-4301
NOTE: new cell phone number: 787-548-0750
http://pr.water.usgs.gov