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INTRODUCTION
The self-purification capacity of rivers is an important property to maintain river water quality and ensure the ecological stability of rivers. Its influence factors include the flow state of the river, the distribution of aquatic plants on the bank slopes, sunshine duration, climatic conditions, sediment composition, the species and quantity of aquatic animals and microorganisms in the water (Sabater et al. 2002; Vagnetti et al. 2003). Yet river morphology is a key factor among them. In recent years, in China's rapid urbanization process, the improvement of land use efficiency and flood safety has been one-sidedly emphasized regardless of river protection in some areas. Some rivers have suffered from some ‘destructive’ construction projects; for example, levelling river channels, straightening river bends and rigidifying river channels. These projects have dramatically altered the natural shape of rivers and led to environmental issues including water quality deterioration and biodiversity loss (Che et al. 2012).
River sinuosity is one of the most important indicators of river morphology (Mueller 1968). Variations in river sinuosity lead to changes in water flow characteristics (Han & Endreny 2013). With an increase in the degree of river meandering, water can flow over to the beaches of the river to form a longitudinal hyporheic exchange flow, increasing the volume of exchange between underground water and ground water (Peterson & Sickbert 2006; Cardenas 2008, 2009). River sinuosity has a considerable impact on the ecological environment. For example, the biomass and number of fauna in meandering river reaches are higher than those of manually straightened river reaches (Lorenz et al. 2009). Restoration of straightened river bends significantly improves the biological habitats and ecosystems along the river (Moerke & Lamberti 2003). It was also shown that the flow of meandering rivers had an impact on water quality. Dwivedi et al. (2017) found that hyporheic flow in meandering rivers played an important role in the migration of nitrogen in river systems. A preliminary study on the relationship between river sinuosity and water quality indicated that in the sinuosity range of 1–1.07, river reaches with a high degree of river meandering showed a higher capacity for water body self-purification (He 2014).
The review of past studies indicates that river sinuosity-related investigations have mainly been concentrated on the morphology and ecology...