A neglected phenomenon: The spatiotemporal evolution of rivers in the city of Luohe, China

Abstract

In the process of urbanization, the low effectiveness of urban water governance has been driven by multiple causes. Among them, a perplexing and misunderstood issue has been that stabilization or even enhancement of urban water areas may often conceal detrimental changes in the ecological function of the urban hydrological system. To address this phenomenon, a typical plains river network city is taken as the research object against the background of China's urban development. Specifically, we studied the spatial evolution of the river system in the city of Luohe, China, over the past 30 years. We attempt to measure the impact of urbanization on the Luohe river system through remote sensing techniques across four dimensions: water area, number and class of rivers, and river grid density. The results show that, (1) from 1988 to 2022, the water area fluctuated and even increased slightly, with a rate of change of + 1.46 %. During this period the area of forest decreased, while the area of urbanized land increased. Given the stable area of the hydrological system it is easy to assume the river has not deteriorated in ecological structure and function. However, our further results show that (2) the length and number of rivers have decreased. (3) The river unit density decline rates were 13.89 %, 12.90 %, 11.11 %, 4.17 %, and 8.70 %, respectively, from 1988 to 2022. This indicates a regional simplification of the structure of the network of river systems in the area. (4) Morans’ I for the river unit density over six periods showed a positively correlated, indicating that the degree of concentration of river density decreased year by year, and the significant clustering distance of density grids increased, and the high-density centroid of gravity of the river network consistently shifted from north to southwest over the analysis period. In summary, our analyses showed that the structure of the Luohe river system has changed to few, wide and concentrated channels. Natural river geomorphology has been transformed to channelized and hardened channels, then to concentration into single main channels, with tributaries combined and concentrated into few main linear channels, explaining why smaller rivers system units have been eliminated and lost through attrition.

Keywords

• River network evolution
• Transition matrix
• Grid density
• Quantity and hierarchy
• Luohe