Sediment transfer in the Rio Pacuare watershed and neighboring basins of Costa Rica

This study examines factors related to sediment yield, sediment source areas, and sediment transfer processes on the Atlantic slope of the Cordillera de Talamanca in Costa Rica. Data were gathered from existing sources and from the author's field observations to describe and analyze sediment transfer processes in steep, humid Costa Rica. First, a broad scale analysis of 18 watersheds was carried out to investigate general relationships between sediment yield and watershed physical factors. Then, a more detailed, field-based analysis and three sediment budget scenarios were developed for the 651 km² Pacuare River basin.

Globally, past research sets up the expectation that forested basins yield little sediment compared to agricultural basins. Land cover has commonly been cited as the determining factor in sediment yields in the tropics. Results of this study demonstrate that the expected relationship between land cover and sediment production is weakened by other controlling factors in the steep humid tropics of the Cordillera de Talamanca, where basins that have very little human intervention and are almost completely forested show high sediment yields. The influence of deforested lands on sediment transfer in the steep, humid area investigated in this study is not as important as is commonly suggested for tropical watersheds.

Although a detailed sediment budget could not be developed, an approximate accounting of sediment processes for the Río Pacuare watershed shows that surface wash erosion can only provide a small portion of the sediment that leaves the basin. Consistent with previous research stressing the importance of mass wasting in basin denudation, this research shows that mass wasting processes contribute more than half the sediment exported from the Pacuare drainage basin. However, in contrast to previous emphasis on mass wasting's role in long-term basin denudation, this analysis indicates that landslides mobilize considerable quantities of sediment even over relatively short time spans. In the Talamanca region of Costa Rica, storms and earthquakes of sufficient magnitude to trigger sediment-producing landslides each have recurrence intervals of about five years, suggesting that in any given year there is about a 40 percent probability of an event that will trigger landslides.