Discharges of large amounts of sand and gravel into streams and rivers can occur in conjunction with disturbances such as wildfires, landslides, volcanic eruptions, mining and removal of dams. Such sediment pulses are common in many forested mountain streams in the western United States. They are of concern to land managers because they influence sediment input in downstream reaches, water supplies and reservoirs; affect the stability of infrastructure features such as bridge crossings and buried pipelines; and limit the availability of aquatic habitat for species like salmon. Predicting how channels will react to sediment pulses is important in stream restoration work.

"If we can predict the natural tendency of rivers to change following a disturbance, we can work with the rivers to speed recovery of stream channels," said Dr. Mary Ann Madej, a research geologist with the USGS Western Ecological Research Center in Arcata, Calif.

Rivers typically exhibit certain patterns of channel response, depending on the size of gravel and boulders in the river channel, presence of large wood in the river bed, channel gradient and time since disturbance, said Madej. Writing in the August 2001 issue of the journal Water Resources Research, she reports on the changes in steep mountain river channels following disturbances.

"The effect of sediment pulses on a channel may not always be detectable by a distinct wave of sediment moving downstream," said Madej. "More subtle changes in channel structure and organization can also accompany sediment pulses. By discovering these changes and understanding how a channel redevelops over time, we will be able to better understand how habitat for both aquatic invertebrates and fish evolves. An understanding of the trajectories of physical recovery in disturbed stream systems will help predict the biological response to disturbances."

Madej proposed a conceptual model to look at the developmental sequence of channel changes following sediment pulses. She evaluated her model using streams in the Redwood Creek basin in northern California and additional research sites, which allowed her to test the model against different kinds of disturbance. Madej found that after a pulse, the streams and rivers studied commonly displayed decreases in water depth, in channel complexity and roughness, and in the regular spacing of gravel bars, pools and riffles. Through time, the structure and organization of the river channel redeveloped. Depending on stream characteristics, recovery time varied from two years to more than two decades.

Madej likened channel structure to the architecture of a single building and channel organization to the arrangement of individual buildings in a community. Although previous studies have used the "single building" approach, Madej's research has focused on the "community" approach by examining channel organization throughout a watershed. She found as bars and pools redeveloped, channel roughness and complexity increased, leading to more diverse habitats in streams.

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