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Spillway discharge leads to total dissolved gas (TDG) supersaturation in the plunge pool, and these gases cannot dissipate to restore equilibrium after traveling tens or even hundreds of kilometers downstream of the spillway. Supersaturated TDG is detrimental to fish and may cause gas bubble disease. The negative impact is enhanced when cascade reservoirs are in operation. Field observations of the TDG concentrations were performed in the lower reaches of the Dadu River in the summers of 2009 and 2012. The relationships between the TDG concentration and various impact factors were analyzed. Increasing the unit discharge rate and downstream water depth can accelerate the production of TDG supersaturation, a ski-jump was a better pattern for dissipating energy than an underflow, and the power generation system did not contribute to the TDG production. The TDG dissipation showed that the dissipation coefficient increased with an increasing flow rate. An investigation was conducted to explore the cumulative effect of a hydropower cascade. A controlled cascade reduced the TDG production by reducing the occurrence of a discharge at the downstream cascade is beneficial. Based on the field observations, which were focused on operational regulation, mitigation measures for TDG supersaturation for the watershed management of a hydropower cascade were suggested and discussed.

The urbanization process of the Hanjiang Basin is speeding up and facing many challenges. It is necessary to take active actions to realize the balance between urbanization development and water environment protection. This paper aims to establish a comprehensive evaluation model of urbanization and water environment (UAWE) and explore the development of UAWE in the Hanjiang Basin in China from 2010 to 2018. In terms of methodology, these evaluation models are combined with the Coupling coordination degree model (CCDM), Exploratory Spatial Data Analysis (ESDA), and Geographically and Temporally Weighted Regression (GTWR) to measure and analyze the coupling degree (CD) and spatiotemporal heterogeneity of UAWE. Our findings include the following: (1) From 2010 to 2018, in all regions of the Hanjiang Basin, urbanization showed a gradual upward trend. Among Them, the comprehensive urbanization index (CUI) of Xiangyang increased by 25.37%. (2) The comprehensive water environment index (CWEI) presents a fluctuating growth, with the rate of increase in Jingmen's reaching 17.44%. (3) There is heterogeneity in time and space between cities in the Hanjiang Basin, and the positive interaction effect of UAWE is mainly focused on the middle and lower reaches of the Hanjiang River. (4) Coupling coordination degree (CCD) in the basin shows an increasing trend, and the areas with high CCD are mainly concentrated in Xiangyang and Xiantao. (5) ESDA shows that the CCD among cities in the basin has spatial correlation and heterogeneity, and the regions with similar CCD tend to be distributed centrally in space.

Supersaturation of total dissolved gas (TDG) is caused by high dam discharge, sudden increases in water temperature and extra photosynthesis of aquatic plants, which may lead to gas bubble disease and even cause mortality of fish. It is imperative to explore treatment measures to mitigate the negative impact of supersaturated TDG for the protection of aquatic organisms. Based on the knowledge of the promotion effect of the aeration bubbles on the mass transfer rate between gas-liquid interfaces, a series of experiments in an aeration column were carried out under different aeration conditions to explore the promotion effect of aeration on the dissipation process of supersaturated total dissolved gas. A quantitative relationship between the dissipation coefficient of supersaturated TDG and the aeration conditions, such as gas flow rate, water depth and pore size, was established. Within the variation range of the aeration conditions, the dissipation coefficient exhibits an exponential increase with the gas flow rate. For a certain gas flow rate, the dissipation coefficient decreases with increasing water depth and diffuser pore size. The exploration of the mass transfer of supersaturated TDG in the aeration tank column provides insight into the quantitative effect of aeration on the dissipation of TDG. This would be useful guidance for further research on the mitigation measures of supersaturated TDG.

Urban rivers represent an important source of freshwater. Accelerated urban development has resulted in imbalances in the water ecological environment and even eutrophication. Moreover, both natural and anthropogenic sources result in frequent heavy metal pollution in urban rivers. However, the combined impact of eutrophication and heavy metal pollution on the diversity and structure of the river microbial communities has not been adequately addressed. The microbial community distribution and predicted functions were examined in six water and sediment samples from the Laojingshui (LJS) River using metagenomic sequencing. The results showed that there were distinct differences in the microbial composition along the river. Redundancy analysis (RDA) revealed that the redox potential (Eh) was the most influential factor, explaining 76.5% of the variation (p = 0.002), and the heavy metals Zn and Cu explained 4.5 and 3.9%, respectively (p 0.05). The results revealed that high nitrogen and phosphorus concentrations may have affected the proliferation of opportunistic plant species, such as Eichhornia crassipes, but Eh and heavy metals may have had greater impacts than N and P on the microorganisms in the water and sediment. The sensitivities of Deltaproteobacteria, Acidobacteria, Gemmatimonadetes and Nitrospira were most significant under Zn and Cu contamination when accompanied by eutrophic conditions. The expression ratio of the CYS (Cystain) gene might explain why the spatial distribution of each metal differed. This study suggests that heavy metals in eutrophication water continue to be the main factors determining the composition of microbial community, so the treatment of eutrophic water still needs to attach great importance to the complex pollution of heavy metals.