Four flume experiments have been performed in which the behaviour was investigated of a bed of fine silt under running water with and without an excess of material introduced into suspension.

In the presence of excess material, bed features were formed in the tangential stress range 0.1 < θ < 0.4, and the bed was found to remain plane in the ranges 0 < θF < 0.1 and 0.4 < θF < 1.3. In the absence of excess material the stable condition over the entire range appeared to be that of the plane bed.

Dual values of the threshold stress for grain movement were postulated to explain these results, and a relationship has been found between the magnitude of the tangential stress and the upstream slope of the features.