On the highly erodible catchments of Draix (French Alps), concentrations of suspended sediments up to 900 g/L have been observed during floods. To investigate the behavior of such flows, an experimental study is conducted in a flume with suspensions of natural sediments at different concentrations. Surface observations suggest that with increasing concentration, the flow evolves from a turbulent to a laminar regime, with surprisingly stable surface patterns at high concentrations. Velocity profiles for the lower concentrations are logarithmic, whereas at higher concentrations, they show a clear unsheared zone that is haracteristic of a yield stress fluid behavior. The thickness of this unsheared zone is used to estimate the yield stress of the hyperconcentrated suspensions. According to these experiments, there is a transition in the flow behavior for concentrations higher than a critical value which is around 600 g/L. This suggests that discharge measurements should be reevaluated for floods of high concentration in Draix streams.