A flume study was undertaken to determine the source of the large variability in field estimates of γ, the coefficient of proportionality between excess shear stress and concentration of suspended sediment at a reference height above the bed. Accuracy in predicting suspended sediment concentration and particle flux demands accurate knowledge of this coefficient. To avoid error due to incorrect assumptions concerning the eddy diffusion coefficient for mass, reference concentration was estimated from concentration profiles. A linear relationship was found adequate to describe the dependence of reference concentration on excess boundary shear stress. The value of γ was determined to be 1.3×10−4. Values for the coefficient range between only 0.8×10−4 and 4.9×10−4 at the 95% confidence level. We show that the variability in previous field-derived estimates of γ over 4 orders of magnitude is not attributable to unsound theory relating reference concentration to excess shear stress, but is most likely due to improper treatment of the eddy diffusion coefficient for mass and to measurement error.