Recent changes in channel-stored sediment in Redwood Creek, California

Madej, Mary

One part of a sediment-budget study of Redwood Creek, California, this research examines alluvial sediment storage, documenting amounts in the creek prior to 1947, after the 1964 flood, and in 1980. It examines forms of storage and their stability, finding that valley width influences the distribution of sediment more than other factors do. Large amounts of sediment were carried in during the 1964 flood, and the same amount was still present in 1980 as sediment was redistributed or added from upper basin erosion. Stream channels form a link between hi 1 1 si ope erosion processes and sediment transport processes in that they temporarily store sediment before transporting it out of the system. Storage of alluvium in the mainstem of Redwood Creek was quantified for three time periods spanning 35 years (1955-1980). An unusual amount of aggradation occurred during the December 1964 flood (a 50-year flood), and increased the total volume of sediment stored on the valley floor by almost 1.5 times to 16 x 10^6 m3 . High landslide activity of the 1964 flood contributed to 5.25 x 10^6 m 3 of sediment to- the mainstream of Redwood Creek, and channel storage increased by 4.74 x 10^6 m 3 . Moderate to high flood flows (2-20 year recurrence intervals) following 1964 eroded sediment in the upper basin and redeposited it in downstream reaches, causing little change in the total sediment on the valley floor. Presently, sediment is stored in several geomorphic compartments, and some compartments (such as recent gravel flood terraces, debris jams, stable alluvial terraces and strath terraces) are only found in particular reaches of Redwood Creek. Potential mobility of sediment stored in Redwood Creek was characterized as active, semi-active, inactive and stable, depending on its distance from and height above the thalweg, and the age and type of vegetation on the deposit. To identify active sediment in the channel bed, the amount of scour and fill occurring during various flows was determined through scour chains and discharge measurement notes. In this gravel -bedded stream, depth of scour increases downstream for equivalent discharges and depth of scour increases with increasing discharge at a given station. Sediment was not distributed uniformly downstream and maximum deposition occurred in areas of ancient deposition. Valley width is a more important control on sediment distribution than channel gradient, sites of sediment input, or drainage area. Attrition of Redwood Creek bed material during transport is high. A tumbling experiment indicated that schist bed material breaks down more quickly than sandstone pebbles. Erosion of bed sediment that had been deposited in the 1964 flood contributed greatly to annual bedload transport in the upper reaches of Redwood Creek for several years after the 1964 flood. Current sediment yields for Redwood Creek are 2,700 metric tons per square kilometer per year (t/km 2 /yr) in the upper basin and 2,200 t/km 2 /yr at the mouth, and bedload constitutes 20% and 11% of the total load at these stations, respectively. Residence times of active and semi-active sediment generally decrease downstream, but increase for stable sediment. Residence times range from decades for sediment in the active channel bed to thousands of years for sediment in stable floodplain deposits. Average velocities of stored sediment are highest for active sediment, and decrease with decreasing activity levels. Recovery time for the channel in response to the 1964 flood has been slow, and total recovery will take more than a century.

Recent changes in channel-stored sediment in Redwood Creek, California

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