Distinctive hydrodynamic properties and ecological responses of multi-thread rivers under different degrees of multiplicity in the Upper Yellow River.

Alluvial rivers that exhibit multi-thread patterns are common in nature and can be the dominant channel morphology in large rivers. However, their ecological properties in response to diverse and dynamic channel morphology has gained limited attention and remained poorly understood. In this study, we adopted an eco-hydraulic model by integrating a hydrodynamic, a sediment-transport, and a habitat-suitability model to assess habitat quality for fish species (Schizopygopsis pylzovi and Platypharodon extremus) in three anabranching reaches with each exhibiting a distinct anabranching morphology in the Upper Yellow River, eastern Qinghai-Tibet Plateau. Based on the hydrologic data and actual channel morphology, we modeled the hydrodynamic and sediment-transport conditions for a period spanning ten years, and simulated habitat conditions under a potentially changing environment with different flow magnitudes and frequencies. The results indicated that the average flow velocity in the low and mid-order anabranching reaches is higher than that in the high-order, complex anabranching reaches. Meanwhile, the bedload transport rate was higher in the high and mid-order anabranching reaches than that in the low-order anabranching reach, demonstrating a greater transport efficiency of multi-thread systems with a greater multiplicity. Consequently, the habitat suitability shows a deteriorating trend over the ten-year modeling period and Schizopygopsis pylzovi shows better habitat status than Platypharodon extremus. The flow magnitudes and frequency also have a significant impact on the distribution of high habitat suitability index among the different river patterns in Upper Yellow River. This study can provide valuable information to optimize ecological outcomes and provide valuable insights for future dam operation strategies and consideration efforts aimed at preserving and restoring riverine ecosystems.