| Summary |
The Rio Desaguadero is a transitional river system located on the Bolivian Altiplano. From its headwaters at Lake Huinaimarca to its terminus at Lake Poopo, the Rio Desaguadero changes character several times, displaying morphologies characteristic of a marsh with no distinct channel patterns, of end-member braided and meandering systems, and of non-endmember transitional systems with mixed channel morphologies. These channel morphology changes result from variations in stream gradient, sediment supply, and discharge along the river. The Rio Desaguadero is particularly important because, as the only effluent river from the otherwise closed basin occupied by Lake Titicaca, sediments preserved in its terraces play a key role in delineating Holocene water-level fluctuations of Lake Titicaca. Three distinct facies associations preserved in terrace and modern cutbank exposures reveal dramatic, changes in the hydrology of the basin during the late Holocene. Lacustrine-deltaic, braided river, and meandering river sediments are preserved in multiple Rio Desaguadero terraces north of the Rio Mauri. Radiocarbon dates from organic matter within the terrace sediments indicate that lacustrine and meandering river sediments accumulated in the Parko Kkota and Calacoto areas between 4700 and 4000 years ago. These sediments are overlain by deltaic deposits at Parko Kkota and braided river sediments at Calacoto. The presence of deltaic and braided river sediments atop lacustrine and meandering river deposits indicates increased sediment supply, discharge, and (possibly) precipitation, that were coincident with a higher-than-modern lake level. The downcutting and subsequent exposure of these sediments is most likely a response of a lake-level fall. South of the Rio Mauri, single terraces that expose braided river sediments, similar to those accumulating in the modern Rio Desaguadero, are preserved. This single terrace level suggests that only one episode of fluvial downcutting occurred in this area and that other than fluvial downcutting and lateral channel migration, the Rio Desaguadero south of the Rio Mauri has not changed significantly in the past 7100 years. The constant water and sediment supplied to the southern reaches of the Rio Desaguadero by the Rio Mauri tributary is the cause of this long-term fluvial constancy. The deposition of meandering, braided, and especially lacustrine-deltaic sediments indicates that a higher-than-modern lake level must have occurred between 4700 and 4000 BP. This highstand was followed by at least 2 lake-level drops causing fluvial downcutting and exposure of the previously deposited sediments as river terraces. This interpretation contradicts previous studies (based on sediment cores from Lake Titicaca), which suggest lower-than-modem lake levels during the entire mid-Holocene until, at the earliest, 1650 BP and which can not explain the presence of lacustrine muds on the Altiplano between 4700 and 4000 BP. Hence, this thesis proposes that during relatively higher-than-modern lakestands, the fluvial record represents a better proxy for determining the magnitude of these high lake levels. Additional analyses including more radiocarbon dates and exact terrace elevations and terrace thicknesses are necessary to accurately determine the magnitude of the Holocene highstands of Lake Titicaca |
