Objective As a special type of informal green space (IGS) with important recreational value in mountain city, river beach is one of the main space carriers for residents to carry out waterfront activities and enhance the characteristic recreational experience of mountain city, which has become a popular recreational destination in recent years. In the face of the current surge in recreational demand, there exists contradiction in the use of river beach: some river beaches are overcrowded with poor recreational experience, while some others are rarely visited with untapped recreational potential. In order to explore the rational utilization of river beach IGSs, it is necessary to identify and classify them from the perspective of recreation opportunity.

Methods First, on the basis of reviewing relevant literature, this research clarifies the definition of river beach IGS in mountain city. Next, the research selects 20 spatial units of river beaches with the most concentrated recreational resources and recreational behavior in the range of two rivers and four banks in Chongqing. Based on the existing research, the environmental status of river beach and the big data of social media, the recreation opportunity index system of river beach IGS in mountain city is preliminarily established by qualitative analysis. The research conducts a field investigation of each spatial unit and sends questionnaires to recreation participants to explore the importance evaluation of environmental variables based on their personal recreation experience. Finally, this research carries out a factor analysis on questionnaire data to extract key environmental factors that best represent the characteristics of the recreation environment, which are superimposed with the environmental conditions, activity information and experience results of each spatial unit to divide the types of the recreation environment of river beach, based on which the research constructs a comprehensive recreation opportunity spectrum, and puts forward targeted suggestions on the governance of river beach IGSs and their surrounding areas in Chongqing.

Results 1) The research identifies four types of recreation environments of river beach IGS (natural experience type, daily life type, humanistic characteristic type and urban tourism type) and corresponding characteristics. At the urban interface, the disturbance of these four types of recreation environments on river beach IGS changes from low to high, forming a continuous sequence of recreation opportunities. 2) Considering the relationship between the recreation environment, recreation activity and recreation experience of each river beach, the recreation opportunity spectrum and recreation opportunity map of river beach IGS in the two rivers and four banks of Chongqing are constructed based on four environmental types and ten environmental factors. The four types of recreation environments correspond to different recreation experience. Specifically, the main recreation experience of the natural experience type of recreation environment enables recreationists to immerse themselves into in the natural wildness, release the pressure of life, and enhance the relationship with friends and relatives; the daily life type of recreation environment can satisfy the needs of recreationists for daily recreation, outdoor exercise and solitude, while enhancing their relationship with relatives and friends; the humanistic characteristic type of recreation environment enables recreationists to immerse themselves into the characteristics and of humanistic customs of mountain city, thus integrating into local atmosphere; the urban tourism type of recreation environment enables recreationists to appreciate the landmark tourist destination of mountain city and feel the lively river beach culture. 3) Based on the recreation opportunity spectrum of river beach IGS in the two rivers and four banks of Chongqing that is constructed from the perspective of users' use experience, this research proposes optimization strategies to improve the quality of the recreation environment of river beach from both the overall and classified dimensions. Due to the special land use of river beach, it is impossible to excessively construct the internal physical environment of river beach, so the optimization of river beach should focus on the management and maintenance of river beach and the planning and design of surrounding urban interface. Of the aforesaid optimization strategies, overall optimization strategies include the overall planning of recreation opportunities, the coordination of peripheral supporting construction and the construction of a complete recreation management system. The optimization strategies for each type of recreation environment of river beach are as follows: the natural experience type of recreation environment should focus on developing reasonable measures for protection and restoration of ecological environment; the daily life type of recreation environment should focus on improving the walking accessibility and the attractiveness of artificial space; the humanistic characteristic type of recreation environment should focus on excavating local cultural connotation and improving supporting recreational facilities in the interface between river beach and the city; the urban tourism type of recreation environment should focus on setting up temporary facilities, improving the quality of night view, strengthening public security management and environmental sanitation, and promoting the efficient traffic dispersal during peak hours of night recreation.

Conclusion This research discusses the definition and characteristics of rive beach IGS in mountain city, identifies four types of recreation environments, including natural experience type, daily life type, humanistic characteristic type and urban tourism type, constructs the recreation opportunity spectrum of river beach IGS in Chongqing, and puts forward feasible suggestions for the planning, construction and management of river beach IGS in mountain city and surrounding areas from the perspective of improving recreation experience. This research is of important reference significance for improving the blue and green space system of mountain city dominated by formal green space and supplemented by IGS, and also expands the application scope of IGS and the research on recreation opportunity spectrum.