Influence of solar activity on breaching, overflowing and course-shifting events of the Lower Yellow River in the late Holocene

The Lower Yellow River (LYR) has been characterized as a frequently breaching, overflowing and shifting river in historical periods. Understanding the factors that influence the LYR variations is critical for river management and disaster prevention. This study constructed a spatio-temporal data base of the LYR's breaching and overflowing events (BOEs) and course-shifting events (CSEs) occurring in the late Holocene. The data base and corresponding solar activity data were analyzed to determine the overall influence, temporal influence and spatial influence of solar activity on the LYR. Results showed that 75.5% of the LYR CSEs and 61.7% of the LYR BOEs occurred in sunspot number decline phases of 11 yr solar cycles, suggesting that the LYR changed more frequently during the sunspot number decline phases. The underlying mechanism of this phenomenon was further interpreted as the high correlation between sunspot decline phases and heavy rainfall in the middle reaches of Yellow River (MYR). Five of the six heavy rainfall years over the last 60 years and 14 of the 16 well-known heavy rainfall records from 132 BC to AD 1933 in the MYR occurred in sunspot decline phases. Heavy rainfall in the MYR promoted the increase of the LYR runoff and the sediment rate and then raised the possibility of the occurrence of BOEs and CSEs. The study also found that the frequency of BOEs was positively related to the fluctuation amplitude of the sunspot maximum intensity in long time series. The flow directions of the LYR courses were found to affect the influence of solar activity on BOEs. The highest correlation between sunspot decline phases and BOEs was presented during the lifetime of eastward flows while the lowest during the lifetime of northward flows. In addition, human activities may undermine the impact of solar activities on the LYR changes