A turbidity current is idealized as a time-varying, one-dimensional boundary layer flow. A full Reynolds stress model appears to give significantly better predictions than one- and two-equation turbulence closures. Model verification is obtained from stationary downslope experimental flows. Verification of the model is also supported by data from stratified boundary layers, represented by Monin-Obukhov relations, and density interface flows. ¿ American Geophysical Union 1992