Earth Surface Processes and Landforms
An extensive survey and topographic analysis of fi ve watersheds draining the Luquillo Mountains in north-eastern Puerto Rico was conducted to decouple the relative infl uences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels. The Luquillo Mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic infl uence on the stream channels. However, the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment. GIS-based topographic analysis was used to examine channel profi les, and survey data were used to analyze downstream changes in channel geometry, grain sizes, stream power, and shear stresses. Results indicate that the longitudinal profi les are generally well graded but have concavities that refl ect the infl uence of multiple rock types and colluvial-alluvial transitions. Non-fl uvial processes, such as landslides, deliver coarse boulder-sized sediment to the channels and may locally determine channel gradient and geometry. Median grain size is strongly related to drainage area and slope, and coarsens in the headwaters before fi ning in the downstream reaches; a pattern associated with a mid-basin transition between colluvial and fl uvial processes. Downstream hydraulic geometry relationships between discharge, width and velocity (although not depth) are well developed for all watersheds. Stream power displays a mid-basin maximum in all basins, although the ratio of stream power to coarse grain size (indicative of hydraulic forcing) increases downstream. Excess dimensionless shear stress at bankfull fl ow wavers around the threshold for sediment mobility of the median grain size, and does not vary systematically with bankfull discharge; a common characteristic in self-forming ‘threshold’ alluvial channels. The results suggest that although there is apparent bedrock and lithologic control on local reach-scale channel morphology, strong fl uvial forces acting over time have been suffi cient to override boundary resistance and give rise to systematic basin-scale patterns.
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