Abstract: In the series of annual runoff and sediment load recorded at Toudaoguai station in the period of 1950-2009, three abrupt changes in the years 1969, 1986 and 1996 were identified. According to the three abrupt changes, the hydrological series in the Inner Mongolian reach were divided into four periods. Summer flood peaks were identified from the series of daily water discharge and sediment concentration recorded at six hydrological stations along the Inner Mongolian reach, and statistical analysis was carried out on the frequency distribution of water discharge and sediment load as well as the sediment rating curves of the flood peaks. It was revealed that the water discharge and sediment load of the flood peaks decreased gradually from the earlier to the later periods, with both their percentages in the annual total and the water discharge corresponding to the peaks of product of the water discharge frequency and sediment load being reduced by about two times from the period 1954-1968 to 1996-2006. These changes could be attributed to the flow regulation by large dams and to the flow reduction by climate change, water diversion in the Ningxia-Inner Mongolian reach and other human interferences. It was found that changes in water and sediment input before the 1990s only altered the relation between water discharge and sediment load coarser than 0.005 mm at the entrance of the Inner Mongolian reach, with a tendency of coarser sediment being entrained by the larger flows. In the Inner Mongolian reach, no tendency was identified in the changes of relation of water discharge and sediment load due to the adjustment of sediment scour and deposition before the 1990s. As a result of the great variations of water and sediment inputs in recent years, although there was not an observable change in the sediment transport regime, the sediment rating curves of either the total load or the load of each particle size group became steep, showing an increasing tendency of sediment transport capacity of the larger flows.

Key words: flood peak, sediment rating curve, the Yellow River