Dynamic state of drainage divide and river network of Bohol, a tropical island in the Central Visayas Region, Philippines

Abstract

Drainage systems, consisting mainly of river networks and drainage divides, evolved through geologic time as governed by erosion, tectonic uplift, and climatic processes. The present topographic features of these functional landforms provided footprints of their evolution. Divide migration and river reorganization were indicative of their dynamic state which has implications on the delivery and transport of water, sediments, and nutrients from the upstream areas to downstream areas, and influences biodiversity and speciation across divides in response to gain or loss of drainage areas. The utility of topographic metrics to predict this dynamic state on synthetic and natural landscapes has been tested in previous studies. Application of the topographic metrics in a tropical island was aimed at understanding natural land surface variability critical for disaster risk assessment and sustainable resource management. The present study conducted a topographic analysis to understand the dynamic state of river networks and drainage divides of a tropical island using divide stability tools. Four across divide contrasts were tested, namely: chi (χ), mean gradient, mean local relief and channel bed elevation to decipher the state of tropical island drainage systems. The TopoToolbox implemented in Matlab facilitated the routine analysis with a 10-m digital elevation model as input. Outputs from Matlab were further analyzed in a GIS platform. The four metrics showed that the divide between major drainage systems was stable, however, anomalies indicated by differences of χ-values were found at the headwaters where mountain areas marked the boundaries between the major and small drainage systems. On these areas, divide migration was predicted to move from the areas with lower χ values to the areas with higher χ values. The χ – inferiority map gave a more detailed potential divide movement. This divide migration entailed either an increase in the drainage areas of the aggressor catchment or a reduction in the drainage area of the victim catchment. Literature suggested that divide and river network reorganization will continue until equilibrium is attained within the landscape.