Two standard modes of laboratory sediment transport flume operation are sediment‐feed and recirculating. In the case of uniform sediment, the final equilibriums achieved in the two modes are equivalent and independent of initial conditions, even though the specified flow and sediment constraints differ. When the sediment consists of a mixture of sizes, however, this symmetry breaks down. In a sediment‐feed flume, both the feed rate and size distribution are specified by the user, and must be identical to those of the bed load at final equilibrium. The final state thus becomes independent of initial conditions. In a recirculating flume, however, the equilibrium size distribution of neither the bed load nor the bed surface material can be specified in advance. As a result, the final state depends on the initial conditions and can include the possibility of partial transport. The equilibriums themselves are equivalent in that they obey the same laws of flow and sediment transport. Their a‐priori predictability, however, differs due to the difference in governing constraints. When fractional transport is scaled by the bulk size distribution, the two flume modes may produce very different transport relations, even for the same sediment. A consistent approach requires that the transport be scaled by the size distribution of the bed surface. Read More: http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9429(1993)119%3A11(1192)