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During the late Pleistocene, at least one episode of lake expansions occurred in the internally draining high plateau region of Bolivia. Some researchers have advocated that a wetter climate associated with a change in atmospheric circulation caused the development of the large paleolakes, while others have hypothesized that deglaciation contributed to the water source for the expanding lakes. From estimates of the potential meltwater stored in the glaciers during their maximum extents, we conclude that insufficient meltwater was available to fill the large paleolakes. However, the meltwater hypothesis remains viable south of the main plateau region where, in five small drainage basins, the volume of available glacial meltwater was 3 to 16 times greater than the volume of water in the paleolakes. Pollen, dunes, and other eolian features indicate that the region surrounding the Altiplano was much drier during at least one interval of the Late Pleistocene. Although the timing of the dry period with respect to the paleolakes is still unknown, a pluvial explanation for the existence of paleolakes seems unlikely. Decreased evaporative loss, however, remains a possible explanation. To understand what factors could have been associated with a decrease in evaporation rates over the drainage basin, an evaporation model is developed based on the energy balance and bulk-transfer methods. The model indicates that a 10 °C drop in air temperature or a doubling in cloudcover could have caused the paleolakes to reach their highest levels. Alternatively, a 50% increase in precipitation rate could have also maintained the paleolakes.

The northeastern edge of the Bolivian Eastern Cordillera is an example of a tectonically active plateau margin where orographically enhanced precipitation facilitates very high rates of erosion. The topography of the steepest part of the margin exhibits the classic signature of high erosion rates consisting of high-relief V-shaped valleys where landsliding is the dominant process of hillslope erosion and bedrock rivers are incising into the landscape. The authors mapped landslide scars on multitemporal aerial photographs to estimate hillslope erosion rates. Field surveys of landslide scars are used to calibrate a landslide volume versus area relationship. The mapped area of landsliding, in combination with an estimate of the time for landslide scars to revegetate, leads to an erosion rate estimate. The estimated revegetation time, 10-35 yr, is based on analysis of multitemporal aerial photographs and tree rings. About 4%-6% of two watersheds in the region considered were affected by landslides over the last 10-35 yr. This result implies an erosion rate of 9 ± 5 mm yr −1 assuming that 90% of a single landslide reaches the river on average. Classified Landsat Thematic Mapper images show that landslides are occurring at approximately the same rate all across an approximately 40-kmwide swath within the high-relief zones of the cordillera.

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