CHANNEL DYNAMICS IN A CRITICAL WEDGE: DEFORMATION, HYDRAULIC GEOMETRY, AND STREAM INCISION PATTERNS ALONG THE PEIKANG RIVER, CENTRAL TAIWAN

The Western Foothills of central Taiwan comprise a series of west-verging thrust sheets. A wealth of modern GPS measurements and structural data are available for this region, but Late Quaternary/Holocene deformation patterns remain poorly constrained. Numerous rivers flow through the region in a generally westward flow direction that crosses major structural features, allowing an opportunity to use fluvial geomorphology to document deformation patterns. We present data on stream power and fluvial incision rates that provide constraints on Late Quaternary/Holocene tectonic activity. The study focuses on a ~45 km stretch of the Wu river and its principal tributary, the Peikang Hsi. Geomorphic data, including channel width, slope, and grain size, are derived from a combination of field mapping and DEM analysis, and are used to estimate unit stream power along the channel. Estimates of rock strength suggest that lithology plays only a minor role in controlling stream power. Correlation and dating of strath terraces provide estimates of late Quaternary incision rates. Incision rates, terrace deformation, and estimated stream power all correlate with mapped structures. Estimated incision rates vary from ~4 mm/yr near the Shuilikeng Fault to ~0.35 mm/yr in the axis of a mapped syncline, where two-year stream power drops to ~50 W/m². Basal shear stress for this discharge at this site is ~40 Pa. Maximum two-year stream power occurs upstream of the Meiyuan fault where it reaches ~900 W/m² and a basal shear stress of ~230 Pa. Ratios of the basal shear stress from the two-year flood to the critical shear stress required to move the median grain size on the channel bed are near unity despite strong variation in bankfull shear stress. These geomorphic signals give insight into the activity of various structures along the stream's path, including the unstudied Meiyuan fault at the eastern edge of the Western Foothills. The observed order-of-magnitude variation in stream power and Shields stress mainly reflect changes in channel width rather than gradient, which underscores the importance of bedrock channel-geometry dynamics.