Abstract: The presence of concentration gradients shifts the effective location of the reported mean velocity away from the sampling bin's center in sediment-laden flow measurement with particle tracking velocimetry.Under typical velocity and concentration profiles for open channel flows,the bias error proves to be constantly less than zero,leading to an underestimate of the actual velocity.The error is closely related to three variables which are the relative location of the flow,the size of the sampling bin,and the suspension index of the sediment particles.Numerical analysis shows that the magnitude of the error follows a first-decrease-then-increase pattern from the bottom to the water surface along the vertical.