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Patricia A. Benner at bennerp@peak.org

Willamette River Channel Loss

The Willamette River channel system has been extensively simplified since Euro-American settlement in the mid-1800s. About forty-five to fifty percent of fiver channel length between the McKenzie River confluence (near Eugene) and Albany has been eliminated. This is a loss of about 90 channel-miles.

Between the McKenzie River and Harrisburg, 60-70% of the channel has been eliminated (Figure 3), and about 40% has been lost between Harrisburg and Albany (Figure 5).

Figure 1. Snag boat Mathloma at 1,000-foot-long cut-off dam at Davis Chute in 1899, about 2 miles downriver of Eugene [U.S. Army Corps of Engineers, 1900].

Figure 2. Cut-off dam [850 ft. long] in 1899, at head of Lambert Slough, downriver of Salem [U.S. Army Corps of Engineers, 1900].



Channel loss between McKenzie River near Eugene, and Harrisburg. [Sedell and Froggatt, 1984].

The U.S. Army Corps of Engineers constructed "cut-off" or "closing dams" that were built across the heads of side channels [Figures 1 and 2]. Other sloughs were closed with cut trees and floating drift, and later dredge spoil. Boat travel benefited from channel simplification that concentrated water flow. River bottomland farming practices also played a large role in the cut-off of smaller channels.




Figure 4. Farming on Kiger Island just upriver of Corvallis, around 1910 [courtesy of Benton County Historical Museum #985-120, 903].




Please Click Map for Larger Image.

Figure 5.
Channel Loss: Harrisburg to Albany
Historical map from General Land Office original survey [Benner and Sedell, 1997].

Ecology

Channel loss has had a significant impact on the ecology of the river system. The side channels and their river banks provided an abundance of quality aquatic habitat that was a refuge for young fish and other organisms. The miles of river bank created the opportunity for leaf and other nutrient inputs from the riparian zone. The tree canopy over the narrow channels shaded the river water from the summer sun.


Willamette River Bottomland
Historical Forests

The low bottom lands within the floodplain along the Willamette River corridor were once predominantly covered by a dense forest [Figure 6]. Scattered within this forest were clearings where fern often grew. This forested bottomland was two to three miles in width or more in some areas. On some of the higher floodplain lands were communities of fir, oak, and sometimes ponderosa pine. Data for 1852-54 map was from the original survey and donation land claim survey field notes.



River Snag and Streamside Trees Removed

The U.S. Army Corps of Engineers reported that downed trees, called "snags," and "drift piles" of wood were historically a common feature in the Willamette River [Figure 7]. However, downed trees were a hazard to boats. Between about 1880 and 1950, the agency removed over 69,000 snags from the channel and overhanging trees from the river banks [Figures 8a and 8b]. Private individuals also removed wood from the banks and channel.
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The sources of this large woody debris included the mainstem river banks within the extensive bottomland forests [see quote below], and much of the wood came from high up in the watershed during floods. Snags and wood jams created diverse channel habitat. Wood also played an important ecological role through its creation and movement of secondary channels, and in floodplain island formation.

Figure 7. Wood jam in the "East River" [East Channel] of the Willamette in 1899 [U.S. Army Corps of Engineers, 1900].

Surveyor's Description of Sources of Large Wood, 1852

The bottoms along the Willamette are heavily timbered with [grand] fir, [big leaf] maple, [Oregon] ash, Balm of Gilead [black cottonwood], and a dense undergrowth of vine maple, hazel, and briers ... there are numerous sloughs that would make the township impossible to survey in the winter [General Land Office Survey T13S R4W, 1852].



Figure 8a and 8b. Snag boat "Mathloma" in operation on the Willamette, in 1899. Cottonwood tree [left] was 120 ft. long by 5 ft. in diameter [U.S. Army Corps of Engineers, 1900].






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