Bierman 9627896 Large mountain ranges exhibit striking regional scale morphologic heterogeneity which is reflected in the dominance of different geomorphic processes in different parts of the range. The spatial and temporal distribution of these processes and affects the rate and manner in which sediment is evacuated from the range, which feeds back on tectonic processes and affects the behavior of rivers beyond the range front, where human populations are often concentrated. The objectives of the proposed study are to document and mechanistically explain major downstream, inter basinal, and temporal trends in morphology and geomorphic process within a large, important mountain range: the Eastern Cordillera of the Bolivian Andes. Fluvial incision creates relief, which strongly modulates the type and rate of geomorphic processes acting on hillslopes, and the relative ability of a river to transport its sediment load governs loci of incision and deposition, behavior of the river, and therefore morphology of the valley. Understanding spatial and temporal patterns of morphology and geomorphic process in large mountain ranges therefore requires the ability to quantitatively describe fluvial incision into mountain ranges, as well as to link hillslope and fluvial characteristics resulting from incision to specific geomorphic processes. These requirements dictate the methodology to be employed in this study, which involves four major steps. First, the modern pattern of geomorphic regions must be documented within two large drainage basins, the Beni and Pilcomayo, in the Eastern Cordillera. These basins share similar rocktypes and general tectonic histories, but north-south variation in precipitation and total shortening (and therefore fluvial incision history) has produced strikingly different morphologies. Regions dominated by particular hillslope and fluvial geomorphic processes will be identified from field observations and image interpretation. Each region will be characterized morphometric ally to determine: 1) which indices best reflect the occurrence of particular processed; and 2) what general trends characterize large mountain drainage basins. The second major task of the study is to develop and apply a channel network incision model calibrated with field data to illustrate how channel slopes vary and relief is generated through time. In this model, downcutting rate is proportional to stream power, and the spatial and temporal pattern of uplift is constrained by regional structural geology studies, paleoelevation studies, and a geodynamic model. Third, information derived from the fluvial incision model will be used in "off-line" calculations to determine 1) how hillslopes change shape and local relief grows and diminishes through time; and 2) where incisional and depositional regimes occur and how fluvial regime affect valley morphology. For example, the geometry and rate of hillslope lowering by deep-seated, bedrock landsliding will be calculated using successive Culmann wedge stability analyses on steep hillslopes produced by a known rate of fluvial incision. Calculations will be made at selected times during a fluvial incision model run and at selected places down the network. Finally, the fluvial incision model and the predictions made in off-line calculations will be checked with several sources of independent data. The general geomorphic patterns and morphometric characteristics of simulated landscapes will be compared to those of the modern Beni and Pilcomayo basins. Predicted long-term surface lowering rates will be compared with exhumation rates determined from fission track analysis of igneous and metamorphic rocks from both basins, while predictions of the rates determined from fission track analysis of igneous and metamorphic rocks from both basins, while predictions of the net rate of sediment export from drainage basins of various sizes will be compared with rates obtained from analysis of cosmogenic isotopic abundance in alluvium. The proposed study ha s many applications ranging from defining the nature of local geologic hazards to anticipating shifts in gobal erosion and sedimentation patterns due to climatic change.
