LATE QUATERNARY/HOLOCENE STRATH TERRACES ALONG THE CLACKAMAS RIVER, OREGON

Well-preserved Holocene strath terraces, inset into Pleistocene strath terraces, have been mapped and dated along the Clackamas River, Oregon; and provide insight into both the Quaternary and modern geomorphology of the river and region. A three-kilometer-wide strath terrace, referred to as the Estacada Formation (Qt₃), has an average age of ca. 10,025 yrs BP. This strath terrace is broadly synchronous with other strath terraces identified in the Oregon Coast Range, Willamette Valley, and Olympic Mountains, suggesting a regional aggradational event at the Quaternary/Holocene boundary. Terrace ages obtained from this study also suggest that there may be a time lag between glacial retreat in the Cascade Mountains and valley aggradation in the lower Clackamas basin.

Incision rates determined from the Qt₃ strath terrace generally increase upstream and reflect an increase in longitudinal profile concavity since the early Holocene. A maximum incision rate of approximately 4.3 mm/year occurs where the river emerges from the Western Cascade Mountains and decreases to 1.4 mm/year near the confluence with the Willamette River. A younger strath terrace (Qt₈), with an average age of ca. 1,125 yrs BP, yields a maximum incision rate of approximately 4.9 mm/year. These rates are an order of magnitude greater than those reported for other Cascade rivers. Bedrock erodibility, rapid base-level change downstream of the strath terraces, or a systematic decrease in Holocene sediment flux may be contributing to the rapid and consistent incision rates observed. Vertical incision of up to 3 meters has been documented on a time scale of decades to centuries in response to short-term changes, such as meander cutoff events.

The longitudinal profile and volume of gravel contained in the Qt₃ strath terrace suggest a sediment flux significantly greater than modern rates, resulting in a base level near the mouth of the Clackamas as much as 15 meters higher than the modern level. This has implications for the history of salmon migration in the region. Willamette Falls, a 13 meter waterfall on the mainstem Willamette River just above the mouth of the Clackamas, may have been buried in alluvium during the early Holocene, effectively removing it as a barrier to anadromous fish passage at that time.