VARIATION OF DAILY WATER AND SEDIMENT DISCHARGE IN THE YELLOW RIVER LOWER REACHES IN THE PAST 55 YEARS AND ITS RESPONSE TO THE DAM OPERATION ON ITS MAIN STREAM

Based on the daily data of water and sediment discharge recorded at Huayuankou and Lijin hydrographic stations in the Yellow River lower reaches during the past 55 years(1950-2004),characteristics of the daily water and sediment discharge and its variation at the two stations were analyzed and compared through 5 phases corresponding to the periods of operation of 4 large dams on the main stream.The result shows that the characteristics of the daily water and sediment discharge changed significantly in a stepwise way in coincidence with the period of operation of the 4 large reservoirs on the main river stream.The natural character of the daily water and sediment transport processes before the reservoir construction has been totally changed into a human-control one after the operation of Xiaolangdi reservoir.The major change after that was as follows:water flood peak and sediment peak disappeared;difference in terms of water and sediment discharge between flood season and non-flood season became ambiguous;sediment load during the water and sediment regulation period accounts for above 2/3 of its annual amount,and the river bed of the lower reaches turns to be eroded rather than accumulated.Comparison of the daily records between the two stations illustrates that the channel of the lower reaches plays a notable role both in flood peak-shaping and restraint and in sediment trapping during the water and sediment transport process from Huayuankou to Lijin.The difference in daily water-sediment discharge between the 2 stations from March to June increased rapidly for the last 30 years and reached as high as 20 times,indicating that the water consumption in the lower reaches is concentrated in this spring irrigation period and is a major cause for water reduction in the lower reaches.In recent years,even the runoff and sediment load of the Yellow River decrease sharply,however,the sediment concentration at Lijin during the water and sediment regulation period after flood season still reaches high enough to form sediment gravity flow in the estuary.Variation of the daily water-sediment process in the lower reaches during the past 55 years shows that the characteristics of the daily water and sediment transport processes in the lower reaches have been greatly changed by human activities.