Holocene Floodplain Development of the Lower Sycan River, Oregon
Lind, Pollyanna, 1970-
Water and pumice accumulated behind a dam that, upon failure, scoured the c1aydominated floodplain and deposited pumice sands across the Sycan Valley. The pumice originated from the eruption of Mount Mazama (approximately 7660 ybp), and dam failure occurred very shortly afterwards. In response to the flood the lower Sycan River underwent episodes of channel aggradation and degradation. This study presents the history of channel evolution for the lower Sycan River from 11,000 years ago to present, based on floodplain stratigraphy and radiocarbon chronology. Seven primary periods of channel and floodplain development are identified: I. Early Holocene Dynamic Equilibrium; II: Sycan Outburst Flood; III. Initial Channel Formation; IV. Degradation & Widening; V. Aggradation & Lateral Migration; VI. (Secondary) Degradation & Widening; VII. Modern Dynamic Equilibrium. The active floodplain of the modern lower Sycan River is flanked by terraces of the rapidly abandoned Sycan Outburst Flood deposits.