Streams move downslope under the influence of gravity, the passage of water is called stream flow. Several factors control the amount of sediment that can be carried by a stream: 1) volume of stream flow, 2) the stream gradient, 3) shape of the stream channel, and 4) kinds and volume of sediments available for erosion in a drainage basin. During floods, the volume and rate of stream flow increases, and erosion along the stream bed mobilizes sediments that accumulate during times of decreasing stream flow. Erosion carves the sides of stream channels, contributing sediments to streams and allowing the channel to migrate over time. Turbulence in the often violent or unsteady movement and mixing of air or water, or of some other fluid.; a most important factor influencing sediment transport in a stream.
A flood is an overflowing of a large amount of water beyond its normal confines—inundating areas that are typically dry at least part of the year). Floods are linked to seasonal precipitation patterns (such as monsoons or spring snow melt) or catastrophic events (such as intense rainstorms, collapse of natural or manmade dams or levees).
Sediments are moved by streams in different ways. Fine grained particles and dissolved components are carried in suspension (called the suspended load). With increasing turbulence, the suspended load increases. Particles that are to heavy to be carried in suspension roll, bounce, and hop along the stream bed; this process is called saltation. This moving mass of solid material is called the bed load. During floods it is often possible to hear the roar created by rocks cracking into each other as they tumble along the stream bed.
The shape of a stream channel and the stream gradient controls the amount of sediment that can be transported down stream. In straight channels, stream water moves as laminar parallel vectors, but with increasing speed and when objects hinder flow, the water becomes turbulent, constantly mixing. With increasing speed and turbulence stream water can carry more sediment (and larger particles) is suspension and as bedload.