Late Holocene Aggradational Processes and Rates for Three Alluvial Fans, Cascade Foothills, Washington

Aggradation histories going back to the mid Holocene were compiled by subsurface investigation for three alluvial fans in the Cascade foothills, northwest Washington. Sygitowicz, Radonski, and Hardscrabble Creeks originate on the east slope of Stewart Mountain in steep, wooded watersheds (0.36 to 5.5 km2) and drain to the South Fork Nooksack River valley where they have built small (0.28 to 0.61 km2) post-Pleistocene alluvial fans. Ten trenches, excavated to a depth of 3 to 5 meters on the three fans, exposed well-preserved strata and paleosols. The processes of strata deposition were identified by correlation with the known mode of modern fan deposits. Gravelly debris flow and hyperconcentrated flow processes (totaling 39 percent and 37 percent of average vertical exposures, respectively) are responsible for most of the ancient fan aggradation. Woody debris flows, common in the twentieth century, make up five percent and stream deposits comprised twelve percent of the total vertical exposures measured. Nineteen radiocarbon ages were obtained from six of the ten trenches. The results indicate that all three fans have been aggrading from the mid Holocene (6,000 years B.P.) to the present, and the rates of fan aggradation increase for five of six trenches in this time period. The increase in fan aggradation corresponds to an increase in the frequency of debris flow despite the overall thinning of the debris flow deposits with time. The average recurrence interval is 868 years for ancient debris flows to be deposited at a given point on a fan surface. A change to a cooler, moister climate in the late Holocene, as indicated by climate models and pollen studies from this region, is likely to have contributed to the increased sedimentation seen in the fan strata during this time