Alain Laraque

Yields of suspended sediment and dissolved solids from the Andean basins of Ecuador

Abstract Water discharge and suspended and dissolved sediment data from three rivers (Napo, Pastaza and Santiago) in the Ecuadorian Amazon basin and a river in the Pacific basin (Esmeraldas) over a 9-year period, are presented. This data set allows us to present: (a) the chemical weathering rates; (b) the erosion rates, calculated from the suspended sediment from the Andean basin; (c) the spatio-temporal variability of the two regions; and (d) the relationship between this variability and the precipitation, topography, lithology and seismic activity of the area. The dissolved solids load from the Esmeraldas basin was 2 × 106 t year-1, whereas for the Napo, Pastaza and Santiago basins, it was 4, 2 and 3 × 106 t year-1, respectively. For stations in the Andean piedmont of Ecuador, the relationship between surface sediment and the total sediment concentration was found to be close to one. This is due to minimal stratification of the suspended sediment in the vertical profile, which is attributed to turbulence and high vertical water speeds. However, during the dry season, when the water speed decreases, sediment stratification appears, but this effect can be neglected in the sediment flux calculations due to low concentration rates. The suspended sediment load in the Pacific basin was 6 × 106 t year-1, and the total for the three Amazon basins was 47 × 106 t year-1. The difference between these contributions of the suspended sediment load is likely due to the tectonic uplift and the seismic and volcanic dynamics that occur on the Amazon side. Editor Z.W. Kundzewicz Citation Armijos, E., Laraque, A., Barba, S., Bourrel, L., Ceron, C., Lagane, C., Magat, P., Moquet, J.-S., Pombosa, R., Sondag, F., Vauchel, P., Vera, A., and Guyot, J.L., 2013. Yields of suspended sediment and dissolved solids from the Andean basins of Ecuador. Hydrological Sciences Journal, 58 (7), 1478–1494.

Gradual diversions of the Rio Pastaza in the Ecuadorian piedmont of the Andes from 1906 to 2008: role of tectonics, alluvial fan aggradation, and ENSO events

The successive courses of the Rio Pastaza in the upper Amazonian Puyo plateau (Ecuador) during the past century have been followed using historical maps, aerial photographs, satellite imagery, topographic and river long profiles, and field studies. The abrupt change in direction of the Rio Pastaza from transverse to longitudinal was a result of two avulsions occurred between 1906 and 1976 at the braided-meandering transition of the former alluvial plain. These avulsions are related to aggradation at the toe of a braided piedmont fan prograding on to a hinterland-dipping topographic slope formed by ongoing tectonic backtilting. The main avulsion proceeded by annexation of a south-dipping depression created in front of the cordillera by backtilting of the plateau. A partial and gradual avulsion having occurred upstream of the former site between 1976 and 2008 is marked by the progressive predominance of a newly formed inner branch. Tectonic backtilting enhanced aggradation upstream of the initial site while it offered the newly avulsed channel a still more favorable way along the cordillera by creating a westward lateral slope. The correlation between ENSO events and the occurrence of the 1976–2008 avulsions strongly suggests that the triggers of the avulsions were the floods caused by the high water and sediment discharges associated with ENSO (La Niña) events contrasting with the regular monthly discharge and the lack of actual ‘normal’ floods during the inter-ENSO periods.