Topographic Patterns in Southern Puerto Rico Watersheds Reveal Landscape Evolution History

Abstract:

A period of accelerated rock uplift occurring ~4 Ma has been previously identified in the Northeastern region of Puerto Rico, in part through the common knickpoint elevation of 600 m in the Rio Blanco Watershed within the Luquillo national forest. The purpose of this research is to identify additional locations on the island illustrating the impact of this uplift event. We investigated patterns of slope, relief, and knickpoint elevation that may be indicative of the same base level change in an area west of Luquillo in the island’s south-central region. The study area consists of one low relief watershed which drains to the north surrounded by relatively smaller watersheds, which drain to the south. The region has numerous knickpoints, many occurring around 600 m that we hypothesize illustrate the same increase in rock uplift rate that impacted the morphology of the Rio Blanco. Identification of additional knickpoints at disparate elevations led to further analysis with Chi and normalized channel steepness that revealed slope patterns unlike that of the Rio Blanco. Changes in channel morphology show no clear connection to local lithology or faulting. From these patterns, we hypothesize that the south draining watersheds are capturing flow from the north draining watershed, leading to divide migration. The driver of this reorganization is unknown and whether it was triggered 4 Ma or by accelerated rock uplift more recently (~2.4 Ma) that has been documented in other regions of Puerto Rico requires further study. However; we hypothesize that multiple changes in rock uplift rate may be the cause of knickpoints at multiple elevations. Using Divide Tools, an extension of Topotoolbox in MATLAB, we identified cross-divide differences in channel elevation, upstream relief, Chi, and slope gradient in headwater streams along unstable divides to determine drainage migration direction. Results support the hypothesis that southern watershed divides are migrating into the northern draining watershed.