Rivers are important parts of the hydrologic system because they return water to the ocean. Rivers are also conduits for transporting sediment. The ability of a river to move sediment depends on the velocity of water in the river and the size of the sediment particles. Water moving at a low velocity can move only small fine particles such as clay, silt and sand. Water moving at a high velocity can move large coarse particles such as cobbles and boulders as well as small particles.
Most river channels contain water flowing at several velocities.
Under normal flow conditions coarse sediment such as gravel is deposited in the deeper part of the channel called the talweg and the fine sediment such as silt and clay are deposited in the shallow water on the slopes of the point bars. Sedimentary Structures Figure 3 shows the bedding pattern of sand in a small excavation into a point bar. When the particles come to rest as sediments on the bottom of the river they form distinctive patterns. The shape and size of these sedimentary structures are determined by the size of the particles, the velocity of the water flowing over them, and the direction of flow. Characteristic bedding patterns associated with point bars are large scale trough cross bedding, horizontal bedding and small scale trough cross bedding.