Excess Sedimentation Affects River Channels
Excess sedimentation occurs in rivers and streams that head on volcanoes when huge volumes of volcanic sediment are washed far downstream by lahars and muddy floods. They can occur over months to years during and following eruptions. Excess sedimentation causes channels to become unstable; flood capacity is decreased due to infilling, channel aggradation occurs (vertical raising of river beds), and bank erosion may increase. At Mount Hood, deposition of excessive sediment may increase the sizes of the Sandy or Hood River deltas and form downstream islands, which could severally impact the Columbia River shipping channel. Additionally, bridges and roadways may be affected by flooding events with increased sediment load from the rivers and streams around them.
The last dome-building eruption of Mount Hood (1781 to about 1793 C.E.) fed over 75 million cubic meters (100 million cubic yards) of sediment into the Sandy River headwaters. Within only a few years the lower 24 km (15 mi) of the Sandy River upstream of Troutdale became choked with sediment and the pre-eruption valley floor was buried under about 23 m (75 ft) of sand and gravel. The White River valley was similarly affected by this eruptive activity.