Suspension-effect density currents formed by both natural and non-natural materials have been investigated in three series of laboratory tests. The lock exchange method of current formation has been shown to be unsuitable for suspensions of high settling velocity and a more effective, dropping method of release has been devised. The adoption of a new linear proportionality equation for correctly correlating the variables acting on the system has demonstrated the significance of the densimetric Froude-Reynolds number for suspension-effect current studies.Quantitative plots of extension against time and non-dimensional time have been prepared for a wide range of concentrations of different materials and the expected increase in both velocity and total extension with increasing concentration verified. For suspensions of constant initial concentration, increase in material particle size has been shown to result in underflows of increased velocity but reduced extension. Mixtures of coarse and fine material in the one suspension have also been investigated.Viscous scale effects have been examined by using two sizes of geometically similar flumes and a short range congruency type of diagram prepared for suspensions of a natural river sediment.Suspension effect currents are compared with the simpler solution effect currents and the greater influence of viscosity on the former discussed together with the suitability of using solution current congruency diagrams for model/prototype studies of suspension-effect currents.