LONGITUDINAL VARIATION OF CHANNEL MORPHOLOGY IN THE TROPICAL MONTANE STREAMS OF PUERTO RICO

The results of a study comparing longitudinal changes in stream channel morphology, from the headwaters to the coastal estuary, in five adjacent watersheds draining the subtropical Luquillo Mountains in Northeastern Puerto Rico are presented. Extensive field surveys and high-resolution GIS analysis were used to compare downstream changes in channel slope, cross-sectional geometry, grain size, and channel energetics. At the scale of the entire Luquillo Mountains, channel and slope morphology are closely related to underlying geology and the landscape has many features that are commonly associated with piedmont landscapes. At the watershed scale, upland bedrock and boulder-lined channels display significant downstream hydraulic geometry relations despite the influence of landslides and other non-fluvial processes. In contrast, alluvial coastal plain sections did not follow expected downstream trends in hydraulic geometry. Apparently the upland reaches have had sufficient time to develop a watershed-scale equilibrium-type morphology despite resistant bedrock substrate and the influence of non-fluvial processes. In contrast, the coastal plain alluvial channels have not had sufficient time to develop into standard equilibrium-type forms over the past 500 years of continual changing land use. The results indicate that differences in response times and the method and scale of observation influence the development of systematic downstream changes in channel morphology. Similarly, although hydraulic geometry and notions of equilibrium landforms are useful at large spatial scales, more complex and encompassing spatial models that utilizing geologic and climatic inputs better estimate channel morphology at local and reach-level scales.