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.