Abstract: Ganjiang river is the largest river system of Poyang Lake Basin. The change of streamflow and sediment in Ganjiang river has an important influence on hydrological process of Poyang Lake. However, the current research on streamflow and sediment of Ganjiang river is mainly based on Waizhou hydrostation, the control station at downstream. That may leads to an incomplete understanding about hydrological characteristics of entire basin. This study systematically examined the changes of discharge and sediment load and their possible causes, which may provide reference for water and sediment resource management of river basin, according to the long-term measured data recent 60 years at six representative stations. Those stations are four stations (Hanlinqiao station, Bashang station, Julongtan station, Xiashan station) at upstream, Jian station at midstream and Waizhou station at downstream. The non-parametric Mann-Kendall test and moving t-test were applied to detect the change in annual streamflow and suspended-sediment. Results indicated that: (1) Great variability at annual and interannual scales on discharge was analyzed at upper four stations, Jian station and Waizhou, with variation coefficient as 0.31, 0.29, 0.28 respectively. 1970s and 1990s were regarded as abundant water time, as average runoff at major stations during those period were 103%-113% than average from 1953-2013, while the other were flat or low water years. No significant trends and abrupt points were detected on streamflow, and annual changes were more slightly obvious in upper stream than downstream.(2) In-phase relations between sediment load and streamflow were observed on shorter time scales before 1992, which may imply a considerable influence of the runoff on sediment transport. However, obvious different tendency were detected between streamflow and sediment during 1993-2013 owing to great reduction of sediment at all stations. Detailed examination shows annual sediment load change was more dramatic compared to runoff, with variation coefficient ranging from 0.46-0.62. An abrupt point of sediment at the upper four stations, Jian station and Waizhou station came out in 2002, 1995 and 1995, with relative change rate at 52%, 71%, 67% respectively. (3) The flow duration curves and monthly average sediment concentration were analyzed to explore possible causes of sediment reduction. There were no significant changes on flow duration curves, while monthly average sediment concentration after abrupt points has decreased to 30%-49%, which acts as a major cause for the decreasing annual sediment. Both decreases in annual sediment and in sediment concentration at the upper stream can be linked with recent human activities, mainly including soil erosion control and reforestation. Although the upstream sediment decrease has certain influence on the downstream, reservoir construction, especially Wanan reservoir which capacity was 2.2 billion cubic meters and operated from 1993, was a main cause of the decreasing trend of sediment at Jian and Waizhou station. The average annual deposition of suspended-sediment in Wanan reservoir was estimated to 400×10⁴t. In addition, soil erosion control and large amounts of sand mining activities at downstream were contributed to the decreasing trend of sediment at Waizhou station. (4) The impact of soil and water conservation measures on reduce sediment was slow and hysteretic, while influences of sediment reduction of Wanan reservoir is prompt and significant. In concluded, a significant decreasing trend was observed on annual sediment load, while the runoff into the Poyang Lake from Ganjiang river maintained steady. Those contributes to reduce amount of sediment deposition and healthy development of Poyang Lake.

Key words: the Ganjiang River, water and sediment characteristics, Mann-Kendall, T test