Water regulation of the Xiaolangdi Reservoir of the Yellow River was chosen as a case to investigate variations in concentrations and bioavailability of heavy metals caused by water conservancy projects in rivers.Water and suspended sediment (SPS) samples were collected at downstream sampling sites along the river during this period.Concentrations and speciation of Zn,Cr,Cu,Ni,and Pb in water and SPS samples were analyzed,and their bioaccumulation was studied with Daphnia magna.This study indicated that the exchangeable and carbonatebound fractions of heavy metals in SPS decreased along the studied stretch,and the dissolved heavy metal concentrations increased along the river with 1.6-15 folds.This is because sediment resuspension increased along the river during water regulation,giving rise to the increase of heavy metal release from SPS.The dissolved Zn,Cu,Ni,and Pb concentrations were significantly positively correlated with SPS concentrations,and their increase along the river was greater than Cr.The body burdens of heavy metals in D.magna exposed into samples collected from the reservoir outlet were 1.3-3.0 times lower than those from downstream stations,suggesting that the heavy metal bioavailability increased during water regulation.This should be considered in the reservoir operation.