The component of a stream (or river)'s sediment load that cannot be transported by the stream's normal flow. If the river can't carry it, you're likely to find it on the bottom of the river, making up the river bed. But bed load is not always hidden: If you've ever walked across a stream on 'stepping stones', you've been stepping on the stream's bed load.
Bed load is a consequence of the stream's sorting the individual particles that make up its sediment. Some particles are made of minerals that easily dissolve in water. Some particles (mostly clays) can be kept in suspension by the electrostatic properties of water. Others are simply picked up by the current and swept along by brute force. The velocity a stream needs to carry a particular particle is a complicated function of the particle's shape, size, and density, as well as the stream's density and viscosity, both of which are dependent on the stream's velocity and sediment load.
When a particle reaches a certain size, the water cannot pick it up. So how did a particle too big for a river to carry get downstream?
Bed load is transported by:
- Saltation, that is, a particle bouncing or rolling along the bottom. Sand and silt grains jump and roll along the bottom, forming ripples and sandbars.
- Floodwaters. When a stream is forced to carry an unusually large amount of water, its velocity, viscosity, and sediment carrying capacity increase in a nonlinear fashion. A stream that can carry only clay and silt in suspension becomes able to carry sand and possibly pebbles and gravel in suspension. The flood can pick up and roll cobbles and boulders that are ordinarily embedded in the bottom.
Bed load is the primary shaper of the river within its channel. Islands form in the middle of a stream, made out of sand and gravel the river dumped there. The inshore bars the river forms as part of the process of meandering have sand and gravel at their base.
Bed load typically accounts for less than 10% of a stream's sediment load. But some 'bed load streams' have far more bed load than others. The streams and rivers flowing down off the Piedmont into the Atlantic Coastal Plain are fast enough to carry sand, but look less like rivers and more like stone fields with water flowing betwen the stones. Rivers in the dry Great Plains (such as the Platte or the Canadian Rivers) often form wide, braided channels, which wind around countless sandy islands.
Particularly catatrophic floods can carry particularly large objects. Piles of house-sized boulders can be found in the channeled scablands of eastern Washington State, and because of it, it's fun to think of the Columbia River as the world's most massive bedload stream.