Long-Term Sediment Generation Rates for the Upper Río Chagres Basin

Abstract

In situ-produced cosmogenic 10Be was measured in 17 sediment samples to estimate the rate and distribution of sediment generation in the upper Rio Chagres basin over the last 10 to 20 kyr. Results indicate that the upper Rio Chagres basin is generating sediment uniformly. Nuclide activities suggest basin-wide sediment generation rates of 143 and 354 tons/km/yr (avg. = 234 ± 74 tons/km/yr; n = 7) for small tributary basins and 248 to 281 tons/km/yr (avg. = 267 ± 97 tons/km/yr; n = 3) for large tributary basins. The weighted average of all tributaries is 269 ± 63 tons/km/yr; n = 10). A sample collected upstream of Lago Alhajuela suggests that the entire basin is exporting sediment at a rate of 275 ± 62 tons/km/yr. These cosmogenic nuclide measurements all suggest that the upper Rio Chagres basin (when considered on scales 350 km2) is generating sediment at ∼270 tons/km/yr. This long-term (1 −20 kyr) sediment generation rate that is equivalent to the estimate derived from suspended sediment yield measured below the upper Rio Chagres- Rio Chico confluence from 1981–96 (289 tons tons/km/yr). Such similarity implies that decadal and millennial sediment yields are similar. Thus, short-term sediment yields and long-term sediment generations are in balance, implying steady landscape behavior over time. The background sediment yield suggests that it would take ∼3,600 years to completely fill Lago Alhajuela, the reservoir for the Panama Canal. Taking into account the present day 2- to 3- fold increase in sediment yields for adjacent human-impacted Rio Boqueron and Rio Pequeni basins, the filling time is reduced to ∼2,000 years. However, it would only take between 250 to 600 years to reduce the reservoir capacity (69% of maximum) enough to drain the entire reservoir for precipitation conditions similar to the 1982 El Nino event. Such models highlight the importance of proper watershed management in order to reduce the sedimentation of Lago Alhajuela.