Sediment eroded from a wilderness area upstream enters a California river; the natural balance of erosion and deposition replaces sediments carried downstream with sediments of similar quantity and particle size from upstream.

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Agents of Natural Change in Watersheds

Natural Change Processes: Erosion and Sediment Deposition
Despite the fact that erosion can dramatically affect water quality, it is a natural process. Sediment erosion, transport and redeposition are among the most essential natural processes occurring in watersheds. In fact, cutting off sediment supply (as when an impoundment intercepts cobbles, gravels and smaller particle sizes normally transported downstream as a stream's bedload) can lead to dramatic shifts in streambed composition and stream downcutting, among other effects.

Although sediment may enter a river from adjacent banks, most is transported from upstream sources. Sediment supply can be a major factor in determining channel morphology, which in turn determines many of the primary physical and biological characteristics of a river. During the 1964 floods of Northern California and Southern Oregon, for example, channel widths doubled and channel beds aggraded up to 4 meters in response to increased sediment loads. Sediment transport depends on the interaction between upland topography, which represents the source of sediments, and discharge regime, which supplies the kinetic energy for their distribution in the river channel. The vegetation cover, climate, soil type, and slope gradient can all influence the location and nature of erosion. Dramatic changes can occur, for example, when intense rainstorms cause landslides or when a stand-replacing fire changes the vegetation cover on a hill slope. It is important to note that in most parts of the watershed erosion, transport and deposition do not occur at constant rates, but are highly active during events such as flooding, windstorms, ice scour, and seasonal or drought-induced exposure of soils.

The relationship of transport capacity to sediment supply determines the channel response to inputs of sediment. Widening and aggradation of the channel, pool filling, and braiding can all occur when sediment supply overwhelms the ability of the river to transport it. Channels with too-high sediment supply tend to be unstable and have multiple active channels separated by bars.