Fluvial sediment transport in response to moderate storm flows following chaparral wildfire, Ventura County, southern California

Abstract

Chaparral wildfire has a profound effect on erosion and sedimentation in southern California. The Wheeler Fire in July 1985 burned the entire basin of a tributary (drainage area 2.14 km 2 ) of the north fork of Matilija Creek, near Ventura, California. After the fire, fine gravel was delivered to the channel by the process of dry ravel (dry particle-by-particle sliding of debris under the force of gravity) at a rate of 0.29 m 3 /km 2 /month. The first winter flow (2.1 m 3 /s) following the fire deposited 550 m 3 of fine gravel in the 270-m study reach near the mouth of the tributary. At least 90% of this fine gravel was derived from colluvium delivered by dry ravel processes from hillslopes near the channel. The second winter flow (2.5 m 3 /s) eroded the channel to the pre-fire thalweg. A reduction in particle size and critical shear stress associated with deposition of small gravel following the fire allowed these moderate-magnitude flows to transport large volumes of sediment. Deposits of two debris flows were identified in the tributary basin. Radiocarbon dating of these deposits gives dates of 1045 ± 95 yr B.P. for the older deposit and between 295 ± 35 and 385 ± 84 yr B.P. for the younger deposit. These dates indicate that the recurrence interval of large debris flows in the study basin is at least an order of magnitude greater than the recurrence interval of fire in the area.