Geomorphic change along the lower Clackamas River is occurring at
a millennial scale due to climate change; a decadal scale as a result River
Mill Dam operation; and at an annual scale since 1996 due to a meander
cutoff. Channel response to these three mechanisms is incision.
Holocene strath terraces, inset into Pleistocene terraces, are broadly
synchronous with other terraces in the Pacific Northwest, suggesting a
regional aggradational event at the Pleistocene/Holocene boundary. A
maximum incision rate of 4.3 mm/year occurs where the river emerges from
the Western Cascade Mountains and decreases to 1.4 mm/year near the
river mouth. Tectonic uplift, bedrock erodibility, rapid base-level change
downstream, or a systematic decrease in Holocene sediment flux may be
contributing to the extremely rapid incision rates observed.
The River Island mining site experienced a meander cutoff during
flooding in 1996, resulting in channel length reduction of 1,100 meters as
the river began flowing through a series of gravel pits. Within two days of
the peak flow, 3.5 hectares of land and 105,500 m3 of gravel were eroded
from the river bank just above the cutoff location. Reach slope increased
from 0.0022 to approximately 0.0035 in the cutoff reach. The knick point
from the meander cutoff migrated 2,290 meters upstream between 1996 and
2003, resulting in increased bed load transport, incision of 1 to 2 meters, and
rapid water table lowering. Ninety-six percent of the total migration
distance occurred during the first winter following meander cutoff.
Hydrologic changes below River Mill Dam, completed in 1911, are
minimal but a set of dam-induced geomorphic changes, resulting from
sediment trapping behind the dam, have occurred. Degradation for 3 km
below the dam is reflected by regularly spaced bedrock pools with an
average spacing of 250 m, or approximately 3.6 channel widths.
Measurable downstream effects include: 1) surface grain-size increase; 2)
side channel area reduction; 3) gravel bar erosion and bedrock exposure; 4)
lowering of water surface elevations; and 5) channel narrowing. Between
1908 and 2000, water surface elevation dropped an average of 0.8 m for 17
km below the dam, presumably due to bed degradation.