bedmaterial

Bedmaterial refers to the sediment that forms the bed of a stream or river. It comprises the grains that are in direct contact with the bed and, under enough shear stress, can be entrained, rolled, or slid along the bed. This material is typically coarser than the particles carried in suspension and largely determines the texture and stability of the channel bed.

In fluvial sediment transport, bed material is distinguished from the wash load. The bed material load includes bed grains that are transported by sliding, rolling, or saltation along the bed, and, at higher flows, may contribute to suspended load as bed-material–size grains. Wash load consists of finer particles that travel primarily in suspension and originate from sources outside the bed, such as banks, hillslopes, or upstream supply from finer fractions.

The grain-size distribution of bedmaterial is often described by measures such as the D50 of the bed surface or subsurface material. Bedmaterial dynamics respond to flow regime: floods can mobilize larger grains, reshape the bed surface, and promote channel adjustment and bedform migration. Armoring, incision, and changes in sediment supply are linked to how readily bedmaterial is entrained and replaced.

Understanding bedmaterial is important for predicting channel morphology, sediment budgeting, and habitat conditions, as it influences sediment transport capacity, bed stability, and the provisioning of downstream sediment. Field assessment typically involves bed-material sampling and analysis, using sieves or grain-size analysis, and may include bedload measurements to estimate transport rates of bed-material grains.