Channel Containment and Bank Stabilization

Figure 9. East Channel bank revetment construction: 1899-1900
[U.S. Army Corps of Engineers].


The present-day Willamette is restrained by a series of revetments [rock/rip-rap bank stabilization] that have been built on the banks of the river, primarily along the main channel. Between Eugene and Albany one-fourth, or 34 of the 124 miles, of the main channel bank [about 172,000 feet] has been stabilized by revetments since about 1935 [Figure 10]. About 18% of the channel bank has been stabilized between Eugene and the Newberg Pool above Oregon City Falls. The first revetment built by the Corps was constructed across from Corvallis on the East Channel, between 1889 and 1900 [Figures 9 and 13].

Ecology

Most of the revetment construction has been along the outside banks of river bends, the locations where the channel is most active in lateral cutting and moving. Channel constraint has ecological consequences, because river channel migration within the valley floor creates off-channel aquatic zones and gravel bars for cottonwood stands, and delivers large wood to the channel from the banks.

Figure 10. Locations of revetments [bank-stabilizing stone structures] on the Willamette River between Eugene and Albany, built since about 1935, from U.S. Army Corps of Engineers 1986 data [Benner and Sedell, 1997].

Willamette River Wing Dams

The early work by the U.S. Army Corps of Engineers to improve the Willamette River channel for boat travel included snag removal and the cut-off of side channels. The Corps did not report dredging above the Oregon City Falls until 1908. Between 1908 and 1929, the Corps dredged an average of 102,000 cubic yards of material each year. Most of the dredging work was done downriver of the town of Independence [river mile 95].

Initially the Corps scraped the gravel bars to deepen the river channel. The agency also built a number of "wing dams" [Figures 11 and 12] to direct the river flow into the middle of the channel or away from a side-channel.

Figure 11. Skinners Bar dam, built in 1898-99 just downriver of Eugene. Part of the dam cut off the head of a side channel, and the remainder "extended to direct the water into the old channel" [U.S. Army Corps of Engineers, 1899].




Figure 12. Map showing wing dams built at Half Moon Bend in 1876 by the Corps, just downstream of Corvallis [U.S. Army Corps of Engineers, 1877].

Willamette River Historical Floods and Floodplain

Initial Corps survey work for the Willamette River above Oregon City: The adjacent country is flat...the [Willamette] river bottom is from one to two miles in width...[and] is traversed by sloughs and bayous, large and small; and in times of floods is covered by swiftly-running water to a depth of from 5 to lO feet
(U.S. Army Corps of Engineers, 1875).

The 1881 flood peak was 266,000 cubic feet/second (cfs) at the Albany gage: The river experienced during the winter and spring [1881] two very prominent freshets, and three moderate ones. The one which caused the greatest damage... [was] the result of heavy snows in the Willamette Valley, followed by long continued warm rains, and reached its maximum on the 16th of January,.. (U.S. Army Corps of Engineers, 1881).

Corps survey, 2 miles below Corvallis on upriver for 13 miles, after the 1890 flood: During extreme high water the whole valley, with the exception of a few isolated patches, is overflowed. The least distance between banks... within the limits of the survey is two and onehalf miles. The high bank opposite Corvallis is six miles distant (U.S. Army Corps of Engineers, 1891).

Figure 18.

Looking east from the Van Buren bridge in Corvallis during a flood sometime after bridge construction in 1911. View is of area described by the Corps in the 1891 quote [courtesy of Benton County Historical Museum, Harland Pratt collection].







Figure 19. Flood of 1890 at Corvallis, from Marys River confluence. This flood was about a 45-year event at the Albany gage [courtesy of Benton County Historical Museum #16801].

Ecology



River flooding is a process that provides opportunities for extended ecological linkages and interactions between the channel and the floodplain, and the river with its watershed, including nutrient and sediment cycling.

Floods also provide the opportunity for river features to form, such as gravel bars, deep channel pools, in-channel wood, and additional characteristics that create a diverse aquatic habitat for fish and other organisms. Though floods have been thought of as unfortunate and, especially since the 1930s, as necessary to control, they are a very important process for the ecological health of the river and its floodplain.

Major Historical Floods

The Willamette River experienced at least five major floods in the 1800s prior to the 1861 flood, the largest event since Euro-American settlement for which the flow has been calculated. At the Albany gage, the 1861 peak flow has been estimated to have been 340,000 cubic feet/second [cfs], about a 95-year event based on the period of record prior to dams].

Though the size and frequency of floods has been reduced by the dams, much of the land still remains within the floodplain, but is flooded less often. A 10-year flood at Albany, prior to dams, is now about a 100-year event. However, two "100-year" events can occur in the same decade or year; these intervals are statistical probabilities.

Figure 20. Willamette River at Corvallis during the 1964 flood at its peak flow would have been equivalent to about a pre-dam 8-year event, providing frequent ecological linkages between the river and its floodplain. About 83% of today's reservoir storage capacity upriver of Albany was constructed prior to the 1964 flood. [U.S. Army Corps of Engineers photograph, 1964].






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