Morphological organization of a steep, tropical headwater stream: The aspect of channel bifurcation

摘要

Channel characterization and instream wood surveys demonstrate the spatial complexity of an ungauged, tropical headwater stream (HWS) in the Tilaran Mountains of Costa Rica. Throughout the stream course, we characterized 29 individual reaches based on their morphological organization and instream wood occurrences. Bifurcated reaches were the most numerous and consistently distributed. They also had the highest values of wood abundance and loading and contained the majority of unattached instream wood. We selected two bifurcated reaches for ultrahigh resolution terrestrial laser scanning (TLS) surveys. Orthographic TLS produced millimeter-resolution point cloud data sets from which 2 cm, planimetric-resolution digital elevation models (DEMs) of the channel surface were derived for both reaches. Cross-sectional surveys of these DEMs, using the River Bathymetry Toolkit (RBT), produced hydraulic geometries at two flow levels (high and low) for every meter-length of channel reach. We input these geometries into a one-dimensional, analytical model to demonstrate the downstream evolution of mean stream power through varying morphology elements. Mean stream power variation displays a regular pattern through the vertical modulation of step-pool sequences, but this regular pattern is lost through channel widening and bifurcation. Reach geometric data indicates that increasing width-to-depth ratio is the most notable feature of channel bifurcation and corresponds to a reduction in slope and roughness variation. This primary understanding of channel dynamics denotes the complex organization of a tropical HWS and promotes further research into tropical HWS systems.