Distribution and residence times of large woody debris along South River, Shenandoah Valley, Virginia.

摘要

South River is a gravel-bed stream located in the Shenandoah Valley. The river is characterized by pool-riffle series with bedrock controlled long pools and a riparian zone dominated by agriculture. These long pools have low slopes, 0.00051 to 0.00082, and range from 650 m to over 2 km in length. Large woody debris (LWD) within South River primarily occur as isolated deposits along channel margins, with occasional small jams. Most LWD is less than 10 meters in length, averaging 5.6 meters. LWD does not have a major impact on channel morphology because all pieces are shorter than the average channel width of 20-30 m. Frequencies of LWD in pools are higher, averaging 5.4 pieces/100 m, than those in riffles, averaging 3.4 pieces/100 m. Poor correlation exits between frequencies of LWD and slope. Recruitment rates of wood into the channel from bank erosion are relatively low, averaging less than 1 tree/year/100 m of channel length; recruitment rates increase downstream (R2 = 0.137; p = 0.006). Residence times of LWD range from 1 year to 20 years. Wood enters specific reaches either by falling in from adjacent banks, in situ, or being transported from upstream. LWD are supplied to pools, long pools, and riffles differently, with relative percentages of transported pieces about 40%, 65% and 80%, respectively. Based on the different relative percentages of transported wood, LWD appears to deposit in pools during the peak of a flood and in riffles as the flood recedes. However, further investigation is required to verify the hypothesis.