Can 19th century agricultural practices explain 21st century fluvial sediment yield in a tropical watershed?

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

Abstract

Suspended sediment in high concentrations is one of the most common contaminants affecting water quality in rivers around the world. The sediment degrades public drinking supplies, aquatic habitats, hydroelectric power generation, reservoir storage capacity, and recreational uses of river systems. Mitigation of this ubiquitous problem requires understanding of the sources of sediment as well as the means by which it is introduced into channels and entrained.

Current land-use practices in the 26.4-km² Cayaguás watershed, Puerto Rico, are typical of small montane drainage basins on the densely populated, 9,000-km² island. Although most of the watershed is in pasture and secondary forest, annual average fluvial sediment yield is high, at 2,600 Mg/km². Much of the sediment is derived from near-channel deposits of alluvium and colluvium eroded by intensive subsistence farming during the 19^th and early 20^th century when as much as 95 percent of the Cayaguás watershed was deforested. Soil augering, mapping from aerial photographs, and GIS-based estimates of the surface area of footslopes and floodplains indicate that these deposits may total 300,000 Mg/km². This would be sufficient stored material to supply annual average fluvial sediment yield for 115 years. The great availability of stored sediment on footslopes and floodplains will likely maintain high sediment yield well into the 21^th century in spite of government efforts to re-forest hillslopes and institute other soil conservation measures. A direct social impact of this problem is reservoir siltation downstream. About one-half of the storage capacity of San Juan's principal reservoir has been lost since impoundment in 1954. Expensive mitigation is now in progress a s sediment is being dredged from the reservoir at the cost of $10 per cubic meter.

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