INTRODUCTION

The erosion, transport and deposition of sediment within river basins are among the important processes of geomorphological development and have influenced the evolution of river channels, floodplains and deltas (Peng, Chen, & Dong, 2010). Since the 20th century, the sediment fluxes in the lower reaches of rivers have substantially decreased due to reservoir construction, soil and water conservation measures and land use changes (Syvitski, Vörösmarty, Kettner, & Green, 2005; Vörösmarty et al., 2003; Walling & Fang, 2003); examples include the Mississippi and Colorado Rivers in America and the Yangtze and Yellow Rivers in China (Maren, Yang, & He, 2013; Meade & Parker, 1985; Walling & Fang, 2003; Wang et al., 2007). However, a sediment supply shortage in the lower river reaches directly affects the morphology, stratigraphy, and ecological environment of channels, floodplains and deltas (Edmonds, Slingerland, Best, Parsons, & Smith, 2010; Xia et al., 2016; Xu et al., 2016).
With the development of society and the acceleration of urbanization processes, humans have improved their environmental protection consciousness and gradually realized the importance of river ecosystems (Dong, 2004; Gao, Jiang, & Zhang, 2008; Hill, 1979; Hillman, Aplin, & Brierley, 2003). Floodplains, as prominent parts of a river, play a pivotal role in river ecology. Moreover, floodplains have received considerable attention to date because of the valuable social and ecological functions of these systems, such as flood control, sediment and nutrient retention, recreational opportunities, agricultural production, and wildlife habitat (Pierce & King, 2008), and their land resources have increased in value. To protect these valuable resources from flood inundation, many river control works on both sides of the main channel have been built, which considerably limit the inundation space of large floods and lead to an uneven distribution of sediment deposition areas and a more complex riverbed form (Hudson & Middelkoop, 2015; Parker, 1995; Wu, Wang, Ma, & Zhang, 2005), especially along the lower Yellow River (LYR), which is a typical case of this kind. Owing to the characteristics of “insufficient runoff and excessive sediment loads lacking sufficient coordination” (Hu, Chen, Guo, & Yan, 2017; Wang, Zhou, & Li, 2006), long-term deposition has occurred in the main channel, resulting in the continuous evolution of the river morphology and the well-known phenomenon of a “secondary suspended river” in local reaches. The elevation of the channel bed is usually higher than that of the floodplain, while the elevation of the floodplain is also higher than that of the levee backside (Hu & Zhang, 2006; Li, Chen, & Liu, 2009; Liu, 2012; Zhang, Huang, Carling, & Zhang, 2017). In addition, the floodplain domain on both sides of the channel, where many villages are located, is very wide in the LYR and is the base of survival and development for approximately 1.895 million local inhabitants (YRCC, 2013). Currently, the environmental management of the floodplain mainly faces a competition between land protection and economic construction.
The LYR has experienced two distinct stages: continuous deposition before the operation of the Xiaolangdi (XLD) reservoir and continuous scouring after its operation. In recent years, numerous studies have been conducted to investigate the changes in erosion and deposition of the lower reaches (Chu, 2014; Liu, Shi, Zhou, Gu, & Li, 2019; Xu, 2003; Zhang et al., 2017), changes in river channel morphology (Hu, Chen, Liu, & Dong, 2006; Lu, Chen, & Chen, 2000; Wang & Li, 2011; Wang, Wu, & Shen, 2019; Wang, Xia, Zhou, & Li, 2019; Xie, Huang, Yu, & Zhang, 2018) and bank-full discharge (Li, 2019; Wu, Wang, Xia, Fu, & Zhang, 2008; Wu, Xia, Fu, Zhang, & Wang, 2010; Wu, Xia, & Zhang, 2007; Xia, Wu, Wang, & Wang, 2010; Zhang, Zhong, & Wu, 2013), river ecological water requirements (Shi & Wang, 2002; Xia, Yang, & Wu, 2009) and water-sediment relationships (Li & Sheng, 2011; Miao, Kong, Wu, & Duan, 2016; Wang, Fu, Liang, Liu, & Wang, 2017). However, little research information is available on the changes in sediment budget and morphology or the environmental management of the floodplain, especially during the XLD operation since 2000. At present, the following problems occur in the floodplain: i) the tension between the flood flow, detention and sediment deposition functions and the lives and properties of residents in the floodplain; ii) the competition between the rapid economic deployment along the Yellow River and the severe shortage of land resources; and iii) the contradiction between disordered land development, private construction and green ecological development of the floodplain (Zhang, 2017).
This research examines channel changes and sediment distribution in the braided reach, based on the idea of environmental management of floodplains along the lower Yellow River. This study aims to i) quantify the changes in the temporal-spatial distribution of sediment and the morphology of the braided reach of the LYR during the operation of the XLD reservoir since 2000; ii) evaluate the impact of the decrease in sediment supply on the environmental management of floodplains; and iii) propose different floodplain reconstruction modes in terms of the characteristics of different reaches.