Spatial–temporal evolution of the distribution pattern of river systems in the plain river network region of the Taihu Basin, China

Abstract

Understanding the spatial–temporal distribution characteristic and evolution tendency of river systems is important in integrated river basin management. In this paper, six indicators were adopted to describe the physical characteristic of river systems. The spatial–temporal evolution of the distribution pattern of river systems in the plain river network region of the Taihu Basin (PRNRTB) during the 1960s–2000s was analyzed using exploratory spatial data analysis (ESDA) and the gravity centre model. The impacts of urbanization were then investigated. Results indicated that the global distribution patterns of box dimension and river density were all statistically significant spatially clustered, and river sinuosity was partially spatially clustered, while water surface ratio, main river area length ratio and river development coefficient were not spatially clustered. Moreover, the water surface ratio was the most stable parameter, box dimension was also relatively stable parameter, but the others were unstable. Meanwhile, the mutual transformations of hot and cold spots of river systems especially water surface ratio, river development coefficient, and river sinuosity were frequent. The hot spot regions were mainly located in the northeast region. In addition, most gravity centres migrated gradually from southeast to northwest with low distance, except those of river development coefficient and river density. These aforementioned differences were caused by the rapid urbanization. Based on the integrated river basin management suggestions for PRNRTB, the river density, box dimension and river sinuosity should be maintained and restored across the whole watershed, and the river conservation and restoration should focus on countermeasures against the reduction of river development coefficient, and main attention should be paid on the river systems conserving and restoring in Haiyan and Haining in southeast PRNRTB.

Introduction

River systems are important natural heritage sites. The formation and development of river systems is influenced by many natural factors, such as geology, topography, soil, hydrology, climate, and vegetation. Moreover, river systems are usually altered to meet the needs of people in water, energy, transportation, recreation, storage and discharge (Costanza et al., 1997, Nilsson et al., 2005, Doledec and Statzner, 2008, Jia and Chen, 2013). The influences of urbanization on river systems have been widely recognized as the most significant among all human activities. Approximately 60% of river systems have been changed profoundly because of urbanization (Sear and Newson, 2003). These changes have strongly threatened the ecological integrity and ecosystem functions of river systems (Dudgeon, 2006). Thus, an increasing need to investigate the impacts of urbanization on river systems for the sustainable planning, management and conservation of rapidly urbanized river basins has been realized (Karr, 1999, Norris and Thoms, 1999, James and Marcus, 2006, Junior et al., 2010, Pinto and Maheshwari, 2011).

The impacts of urbanization on river systems have been introduced and investigated as a broad and specific question since the mid-20th century (Lane, 1955, Strahler, 1956). Considerable progress has been achieved in rapidly urbanized regions around the world over the past 60 years (Chin, 2006). These studies documented that urbanization could change the quantity, morphology and structure of river systems. River lengths and water surface areas have generally deteriorated in rapidly urbanized regions. Main rivers have also been widened because of the increase in runoff volume caused by river channel dredging and impervious surface increase, while the tributaries have been narrowed gradually and even disappeared because of river channel sedimentation and urban occupation (Vanacker et al., 2005, Gregory, 2006). Moreover, spatial distribution of the urbanization impacts has been observed, and found that the impacts typically decrease from the city to suburbs (Yuan et al., 2006).

The variations of river systems can be considered as spatial–temporal processes. These procedures are controlled in spatial and time scale by many natural and social factors, particularly urbanization. Moreover, the spatial distribution of urbanization and its effects on river systems has the characteristic of agglomeration and dispersion. Previous studies have generally focused on the numeric characteristic of river system variation. These studies assumed the existence of mutual independence in spatial distribution (Yang et al., 2004, Xu et al., 2013). However, studies on the spatial association and spatial heterogeneity of the variations of river systems are lacking. Understanding the spatial–temporal distribution characteristic and the evolution tendency of river systems is necessary for sustainable river basin management. Exploratory spatial data analysis (ESDA) has been used successfully in the variation studies of economic patterns, ecological environments and social issues (Le Gallo and Ertur, 2003, Buttafuoco et al., 2005, Anselin et al., 2007, Ye and Wu, 2011, Rincón et al., 2013). However, the methodology of ESDA in the evolution of river systems research is still at the beginning. Moreover, the plain river network region of the Taihu Basin (PRNRTB) is one of the regions with the fastest rate of urbanization in China. Rapid urbanization has caused significant changes in river systems, such as water degradation, flood disaster and other ecological and environmental problems. Therefore, many major indicators are selected to describe the physical characteristic of river systems. ESDA and the gravity centre model are employed to explore the spatial–temporal evolution of the distribution pattern of river systems in PRNRTB. The aim of this study is to analyze the evolution characteristics and laws of the distribution pattern of river systems, to discuss the possible impacts of urbanization, and finally to provide scientific basis and decision-making references for integrated river basin management in PRNRTB.