Xiangxi River is a typical tributary of Three Gorges Reservoir (TGR) in China. Based on field observations in 2010, thermal stratification was significant in most months of the year. Through field data analysis and numerical simulations, the seasonal and spatial variation of thermal stratification as related to the impact of the operation of TGR were investigated. Thermal stratification was most pronounced from April to September in the Xiangxi River tributary. Air temperature (AT) and water level (WL) were the two dominant variables impacting thermal stratification. AT affected the surface water temperature promoting the formation of thermal stratification, and high WLs in TGR deepened the thermocline depth and thermocline bottom depth. These results provide a preliminary description of the seasonal variation and spatial distribution of thermal stratification, which is important for better understanding how thermal stratification affects algae blooms in Xiangxi River.

In-stream structures, such as channel spanning logs and weirs, can enhance hyporheic exchange in streams. Hyporheic exchange is important for stream ecosystem function, and restoring this function is a goal of many stream restoration projects. However, studies on the connection between in-stream structure size, hydrogeologic setting, and hyporheic exchange remain inadequately characterized. In this study, we combined flume experiments and numerical simulations to systematically evaluate how in-stream structure and its hydrogeologic setting impacted the hyporheic vertical exchange flux, Q, the solute penetration depth, D_p, and the solute flux, QS, in the hyporheic zone. The results showed that stream water downwells into the riverbed upstream of the weir and upwells downstream. Exchange rates were greatest near the weir and decay with distance upstream and downstream. Model results indicated Q, D_p and QS had a positive exponential relationship with the weir height, h, the flow velocity, u, and the sediment intrinsic permeability, k. While model results indicated that u was the most important factor determining Q, D_p and QS, followed by h, while only h reached a certain value, the hyporheic exchange would increase with the height and vice versa. Hyporheic exchange generally was sensitive to changes in k, only the magnitude of k varied from 10⁻⁸–10⁻¹⁰m². This finding suggests that a rethinking of the currently applied restoration techniques is required to better consider in-stream structure size, hydrological conditions and natural substratum dynamics in river restoration.

The hyporheic zone (HZ) plays an important role in the river ecosystem, and hyporheic exchange and solute transport in the HZ are important ecological functions. However, the relationship between the design parameters of river structure and solute transport is still poorly understood. In this study, we combined flume experiments and numerical simulations to systematically evaluate how in-stream structures impact the solute transport depth (D_P), hyporheic vertical exchange flux (Q), and solute flux (Qs). The results showed that the in-stream structure had a significant influence on solute transport in the HZ and could obviously increase the intensity of hyporheic exchange and promote solute transport. Model results indicated that D_P, Q, and Qs increased with the ratio of ground height to underground height of structure (H/D) and structure number (N), while Q, D_P, and Qs increased with the structural spacing (S) to begin with; then, Q remained constant, and D_P and Qs decreased as S continued to increase. This study deepened our understanding of the influence of in-stream structural design parameters on HZ solute transport, which is helpful to provide a theoretical basis for ecological restoration projects in the river HZ.

1. In natural and restored streams, in-stream structures serving as geomorphic features are ubiquitous. In-stream structures are mainly constructed of rock or wood and include channel-spanning weir...