Hyperconcentrated flows often occur in the middle and lower Yellow River (MLYR) and its tributaries, within which the main sediment source originates from the Loess Plateau of China due to serious water erosion. Little is known about the properties of river sediment that is transported by hyperconcentrated flows, particularly with respect to the mineral composition and size distribution. Samples of sediment and loess were collected in the northern, middle, and southern Loess Plateau and the mainstream and tributaries of the MLYR. A total of 18 loess samples and 24 river sediment samples were analyzed to determine their sediment size distribution and mineral composition. The bottom loess samples reflected the original sedimentary features of the Loess Plateau, and the median particle size reduced, and the clay content increased from the north to the south of the study region. The surface loess has been weathered under the action of wind and rainfall, and the clay particle content in the surface loess samples was higher than that in the undisturbed bottom loess. Erosion of the surface soil due to rainfall and surface runoff means that fine particles (mostly clay) have been washed away. The median diameter of surface loess particles was a little larger than that of the bottom loess particles where water erosion dominates. The particle size became coarser with increasing distance from the estuary in the MLYR, which reflects depositional sorting in the river channel. Significant logarithmic relations were found between the median diameter of the sediment particles and the i) non-clay mineral content and ii) clay mineral content. Thus, clay and non-clay mineral compositions can be conveniently estimated from the particle size distribution.