Knickpoints within the Suoshui watershed and the implication for the age of sandstone pillars in Zhangjiajie, China

• Knickpoint retreat is modeled with a variable erosion coefficient K.
• The erosion coefficient K is related with the channel steepness.
• Knickpoints record the incision history of the Suoshui basin in 0.45 Ma.
• The formation of sandstone pillars in Zhangjiajia started in the early Pleistocene.

Using DEM and topographic and geological maps alongside field surveys, this study investigated the characteristics and retreat processes of the knickpoints along the mainstream and major tributaries of the Suoshui watershed. Seven disequilibrium transient knickpoints are found along the mainstream, and 0–6 are on the trunk of each of the major tributaries. The knickpoints are labeled from KP0 to KP7 from the lowest to the highest according to their spatial position. The average drop across each knickpoint is in the range of 9–19 m, and the cumulative fall from KP7 to KP0 is about 107 m. Using a brute force two-parameter search, the coefficient m in the stream power incision model for the Suoshui watershed is found to be 0.525, and the coefficient K is variable with an inverse relationship with channel steepness. Using the stream power incision model with these parameters, the time for the highest knickpoint (KP7) to retreat from the mouth of the Suoshui River to its present location is found to be ca. 0.454 Ma. Based on the relationship between the cumulative fall and migration time of knickpoints, the average downcutting rate of the Suoshui River is estimated to be 0.224 mm/a in the past 0.454 Ma. By comparing the downcutting rates with the height of typical sandstone pillars in the Suoshui watershed, the formation of these pillars might commence in the early Pleistocene.