Base level and lithology affect fluvial geomorphic evolution at a tectonically active area

The evolutionary history of fluvial geomorphology is the consequences of combined effects of tectonic, climate, lithology and base level. Previous researches had emphasized tectonic impacts on the fluvial system at the tectonically active region, while lithology and base level get little attention. In addition, the resistance of lithology may cause knickpoint and control the evolutionary history of landscape in relatively stable areas, and difference in local base-level is sufficient to induce drainage reorganization. Nevertheless, it is still unclear how far the lithology and base level affect the evolution of fluvial landforms in tectonically active areas.

In this study, researchers chose the area in the NE Tibet Plateau (Laohu and Hasi mountains) (Figure 1) where the development of fluvial landform is affected by both the activity of the Haiyuan Fault and the aggradation/incision of the Yellow River. Here, they aim to untangle the role of lithology and base level on fluvial processes in tectonically activity region. And, the geomorphic indices, i.e., drainage pattern and χ anomalies, were calculated and investigated.

Because of basin-mountain structures controlled by the tectonic activities of the left-lateral Haiyuan Fault, the study area formed the Laohu and Hasi mountains (Figure 1), and two radial drainage systems developed surrounding these mountains (Figure 2). however, since some broad valleys exist upstream and canyons develop at the river mouth (Figure 3), local fluvial landforms cannot always be explained distinctly by the tectonic movements. While, other factors, such as lithology and local base level, may play crucial roles in regional landform at various spatial scales.

Our results show that different base levels and/or bedrock lithology have significant impacts on the drainage reorganization (figure 3) and development of the specific fluvial landform at different spatial scales in this tectonically active environment. Firstly, instead of flowing into the southward river with the short path, channels from the southeastern side of the Laohu Mountain have changed their courses from around south to north direction before flowing into the Yellow River (Figure 2). This flow direction change is triggered by river piracy and drainage network reorganization due to a significant altitude difference of local base-levels (the confluences of different tributaries to the Yellow River) (Figure 1). Secondly, lithology differences lead to the formation of the alternative distributions of canyons and wide valleys. In addition, long rivers with higher steepness index invade shorter ones, thereby enhancing their drainage area and further increasing the erosive capacity. This process of positive feedback may gradually transform an unstable parallel river pattern to a stable dendritic one.