Characteristics of Flood Regime in Ningxia-Inner Mongolia Reaches of the Upper Yellow River

The Yellow River is famous for its high sand content,frequent flood,frequent channel change,and unique channel characteristics in the lower reaches.These changes and characteristics have been well studied.However,Ningxia-Inner Mongolia reaches of the upper Yellow River have experienced a series of changes in recent years.Much more common changes resulted mostly from the impacts of dams on fundamental fluvial processes,and the flow as well as sediment transport regimes.Decreased water quantity and flood magnitudes led to adjustments of the size and shape of the river channels.In the meantime,the amount of sand from the Qingshui River and Ten Great Gullies which is carried by flooding during the flood periods into the Yellow River is increasing.This made the Ningxia-Inner Mongolia reaches of the upper Yellow River converted into a suspended river.Based on data of discharge of flood seasons,characteristics of flood regime in the Ningxia-Inner Mongolia reaches of the upper Yellow River were analyzed.The Pettitt and Lee-Heghinian statistical methods were used to test trends of changes in the discharge.Results indicate that the time series of discharge in Ningxia-Inner Mongolia reaches of the upper Yellow River can generally be divided into three periods,i.e.,before the year 1969,1969-1986,and after the year 1986.Analysis of characteristics of flood regime of the three periods leads to the findings as following.Before 1969,the flood peaks were much thinner,and had often multiple peaks within a year.During the second period,the flood peaks became fatter than the previous period,and mostly had a single peak within a year.After 1986,the flood was essentially small and medium-sized,and lasted longer than the last two periods.Reasons for these changes in the flood regime from aspects of climate change,water input from tributaries in flood seasons,irrigation and water diversion,and influence of water conservancy projects were expounded.The statistics show that human activities,especially construction and operation of large reservoirs,were the primary reason for the change in the flood regime.Climate change reduced water quantity to a certain degree.Water input from the tributaries had less influence on the flood regime.