Figure 2 Sketch map of the study area.

2.2 Changes in the runoff and sediment load

The Yellow River Basin has been greatly affected by human activities in recent decades, and the runoff and sediment load in the LYR have notably changed due to regulation works such as reservoirs, water resource utilization and soil conservation (Li, Fu, Wu, & Wu, 2014), especially the operation of the XLD reservoir since 2000. The flow and sediment regimes entering the LYR can be represented by the hydrological data from the HYK and GC gauging stations during the pre-dam period 1960-2000 and the post-dam period 2000-2017 (see Figure 3); these data reveal decreases in the annual runoff and sediment load during the period from 1960 to 2017. From 1960 to 1973, the annual water discharge and sediment load were approximately 455.69×108m3/yr and 11.76×108t/yr, respectively. Large-scale soil and water conservation measures and a large number of silt dams were built on the Loess Plateau in the 1970s (Liu, Gao, Ma, & Dong, 2018; Xu, 2005), and the annual sediment load decreased to 10.54×108 t/yr, while the annual runoff reached 422.28×108m3/yr during 1974-1985. In the mid-1980s, the Longyangxia reservoir (see Figure 1) was built on the upper Yellow River and came into operation in 1986; the annual water discharge reached approximately 257.86×108m3/yr, and the annual sediment load decreased to 6.18×108 t/yr between 1986 and 1999. The XLD reservoir has been in operation since 2000, and by October 2016, the bank-full discharge had increased from 1,800 m3/s during the pre-flood period in 2002 to 4,000 m3/s. The annual water discharge of the LYR was maintained at approximately 236.31×108m3/yr, while the annual sediment load amounted to only 1.03×108 t/yr during 2000-2017.