Rapid recession of fault-scarp waterfalls: Six-year changes following the 921 Chi-Chi Earthquake in Taiwan

Recession of waterfalls is a significant agent in bedrock river erosion and development, but field data on waterfall recession rates have been limited due mainly to the lack of information on initiation age and recession distance of waterfalls. This study examines recession rates of fault-scarp waterfalls that were formed along the Chelungpu surface fault at the 921 Chi-Chi Earthquake on September 21, 1999 at central Taiwan. Our field measurements reveal the 6-year-mean recession rate of the waterfalls to be 3.3-58.2 m/y, which is considerably higher than any other waterfall recession rate in the world. In order to explore the factors affecting the observed high waterfall recession rate, relationships between the waterfall recession rate and relevant physical parameters (comprising erosive force and bedrock resistance) are examined using a previously proposed empirical model. Applying field measured parameters at the rivers, the previously established equation provides an estimated recession rate of 0.6-3.9 m/y, which is much lower than the actual recession rate. The discrepancy between the actual and equation-derived recession rates is likely due to abundant abrasive material in the rivers, which may considerably enhance waterfall recession.