UNDERSTANDING DYNAMICS AND PATTERNS OF RIVER INCISION IN SOUTHERN TAIWAN

Taiwan is a youthful mountainous island, rapidly rising out of the ocean due to the collision of the Philippine Sea and the Eurasian Plate. As such, Taiwan provides an excellent opportunity to study fluvial response to the initial phases of mountain building. In addition to recent and rapid uplift, Taiwan has a warm, wet climate which causes accelerated erosion rates speeding up the timescale of fluvial adjustment to tectonic forces. Geomorphic evidence suggests that the high central mountains have experienced longer uplift, and uplift is propagating towards the southern end of Taiwan. If uplift is most recent to the south, then we expect a south to north trend in landscape development and maturity of hillslope-river systems along this gradient, such that initially river incision may outpace hillslope denudation.

We will use a combination of terrace mapping and dating of strath terraces using optically/infrared stimulated luminescence (OSL/IRSL) to investigate the relationship between incision and uplift. Our methods include identifying, mapping, and dating strath terraces perched at varying heights above the active channel from three drainages on a north-south transect in Southern Taiwan identified from a 5-meter resolution slope map. The terraces will help us understand varying spatial trends in order to piece together the uplift and drainage development story. Dating terraces with OSL/IRSL will provide an age estimate of the last time the terrace sediment was exposed to light, giving an age of deposition. Getting a suite of ages associated with different strath terrace heights will help us understand the rates of stream incision along the N-S transect and test if there is a climatically driven component to the timing of strath terrace formation. Preliminary OSL/IRSL ages from 190 m and 12 m terraces on the Sandimen River in southern Taiwan suggests an incision rates of 1.3 mm/yr from the higher terrace and 10mm/yr from the lower terrace. While preliminary and possibly influenced by the Sadler-like effect, these initial finding may suggest a dynamic landscape. Additional age control and terrace mapping from other catchments in southern Taiwan will help test tectonic and climatic driven hypotheses and reveal spatial and temporal patterns in fluvial response to tectonic drivers.