S. Gregory L. Ashkenas Historical Willamette River Channel Change D. Oetter P. Minear K. Wildman Introduction have carved numerous channels through these sediment deposits. The Throughout the western United States, large rivers played a key role in northern or downstream end of the Willamette River is a simple meandering early settlement. In the decade after 1850, the U.S. General Land Office river channel with lower gradients than the upper river section. Complex (GLO) surveyed the landbase and rivers of much of the West. Engineers braided channels are more localized, and lateral changes in the river channel surveyed most habitable lands, and delineated the vegetation, stream chan- are limited. The Willamette Falls exerts a major control on the river, and the nels, and wetlands along monumented section lines. The Willamette River 45 miles (72 km) below Willamette Falls is extremely low gradient and network and its riparian vegetation were mapped in this first rigorous survey controlled by the backwatering effect of the Columbia River. Given these of the West. In 1895 and 1932, the U.S. Army Corps of Engineers surveyed differences in basin geomorphology, it is not surprising that the channels of the entire length of the river for navigation. These maps provide an excellent the upper Willamette River have changed more dramatically than the simpler record of the historical locations and configuration of the Willamette River meandering channel of the lower river. and its numerous side channels, tributaries, and islands. The river is com- posed of three channel types: primary channel (unbraided, or the portion of During the period from 1850 to 1995, the total area of river channels the channel with the most flow), side channel (channels connected to the and islands decreased from 41,000 acres to less than 23,000 acres (Table 4, mainstem at both ends; this includes the small channels that form islands), Fig. 19) and the total length of all channels decreased from 355 miles to 264 and alcoves or sloughs. The latter are “blind” channels, i.e., they are con- miles (Table 5). The proportion of primary channel increased for the Wil- nected only at one end to a primary, side, or secondary channel (see Off- lamette River as a whole, reflecting the elimination of islands and side channel Habitats, pp. 26-27). channels in efforts to open the main channel for navigation. More than one- half of the area of small floodplain tributaries and more than one-third of the Changes in the River Channel alcoves and sloughs were lost by 1995, and area of islands diminished by The morphology of the Willamette River differs greatly along its 170 roughly two-thirds (Table 4, Fig. 18). Decreases in channel length were mile (273 km) length in the Willamette Valley. Large hills and volcanic cones similar in trend but lower in magnitude for most channel types (Table 5, Figs. within the valley floor have blocked the river in places, confining the position 24-27). These changes in the channels and floodplains differed along the of the river and creating extensive storage basins. These same landforms Willamette River mainstem. The lower reach stretches from Portland to dammed huge lakes that inundated much of the Willamette Valley floor after a Newberg, the middle from Newberg to Albany, and the upper from Albany to series of floods during the end of the last glacial period. Deposits of sedi- Eugene. Overall, loss of channel complexity was least in the lower river ments and landform constraints created the template on which the modern section, variable and intermediate in the middle section of the river, and Willamette River and subsequent human land use developed. greatest in the upper river section (Figs. 18, 19). Gradient differs among these three sections of the river, but the overall landforms constraining the The southern end of the river from Eugene to Albany is an extensively channel have the greatest influence on the history of channel change. The braided channel, containing numerous side channels and islands. Tributaries, upper river is a broad unconstrained floodplain. From Albany to Newberg, such as the Middle Fork Willamette River, Coast Fork Willamette River, the river runs through a series of basaltic outcrops and mountains within the McKenzie River, Long Tom River, Marys River, Calapooia River, Santiam floodplain, causing the river to bounce back and forth between these resistant River, and Luckiamute River, have delivered large amounts of sediment into a hills. The Willamette is confined within a basaltic trench from Newberg to depositional basin created by the blockages of the Salem hills. Past floods the mouth. These larger landforms have shaped the river and limited the degree of channel complexity and its historical change. TOTAL RIVER AREA - PERCENT COMPOSITION OF CHANNEL Channel Types Total area TOTAL LOWER MIDDLE UPPER Primary Alcoves Side Islands ha acres 1850 1850 35.2 1.6 8.9 54.2 16543.9 40880.0 1895 42.8 1.7 9.7 45.7 15281.7 37761.1 1932 45.9 1.1 9.5 43.5 13295.7 32853.7 1995 55.0 1.9 7.1 36.0 9197.3 22726.4 TOTAL RIVER AREA - % CHANGE VS. 1850 Channel Types Primary Alcoves Side Islands Total Area 1895 1895 12.3 -0.9 0.3 -22.1 9.5 1932 4.7 -46.5 -14.7 -35.5 -0.9 1995 -13.3 -35.1 -55.6 -63.1 -22.3 Table 4. Changes in area of channel features from 1850 to 1995 for the Willamette River from Portland to Eugene. Changes in island area are also documented. Calculation of the changes since 1850 is based on actual areas, not the percentages shown in the table above. 1932 TOTAL RIVER LENGTH - PERCENT COMPOSITION OF CHANNEL Channel Types Total Length Primary Alcoves Side km miles 1850 51.2 7.8 41.0 571.3 355.0 1895 55.1 8.9 36.0 492.2 305.8 1932 55.8 6.6 37.6 487.2 302.7 1995 1995 64.8 11.8 23.4 424.1 263.5 TOTAL RIVER LENGTH - % CHANGE VS. 1850 Channel Types Primary Alcoves Side Total Length 1895 -7.3 -0.9 -24.5-13.8 1932 -7.1 -27.8 -21.8 -14.7 Main Channel Side Channels 1995 -6.1 13.1 -57.7 -25.8 Alcoves Islands Table 5. Changes in the length of the mainstem Willamette River, from Portland to Eugene, between 1850 and 1995. This table Figure 18. Proportion of the channel composed of main (or primary) shows the absolute changes in length of each of these channel river, side channels, alcoves, and islands. The change in the percent types as well as the percent change from 1850 to the present. composition of the river is shown for four mapping dates (1850, 1895, Calculation of the changes since 1850 is based on actual lengths. 1932, and 1995), as well as for three different reaches of the river. 18 PNW Ecosystem Research Consortium WATER RESOURCES Willamette River Historical Channels Portland LOWER REACH Oregon City Newberg Newberg Newberg Newberg 18501895 1932 1995 MIDDLE REACH Salem Albany Albany Albany Albany Corvallis UPPER REACH Harrisburg Junction City Springfield Eugene Eugene Eugene Eugene Scale 1:650000 0km 10km 20km 30km 40km 50km 0mi 10mi 20mi 30mi Figure 19. The channels of the Willamette River in 1850, 1895, 1932, and 1995. More detailed maps for these dates are presented in Figures 220-227, pp. 166-173. Three different river reaches are indicated with lines. The upper river reach extends from Eugene to Albany. The middle river reach extends from Albany to Newberg. The lower river reach extends from Newberg to the mouth of the Willamette River where it enters the Columbia N River in Portland. Near-river roads and railroads are shown in red on 1895 and 1932, 1995 UGBs are shown in tan. S Willamette River Basin Atlas 19 2nd Edition Historical Willamette River Channel Change Major Changes in the Lower Reach of the Willamette River: Portland to Newberg Willow swamp The lower reach of the Willamette River from Portland to Newberg has Conifer-dominated remained relatively constant geomorphically over the last 150 years. The woodland position of the river channel today is essentially identical to the river in 1850. Douglas fir-white Total area of river channels and islands remained largely unchanged from oak woodland 1850 to 1995 (Table 6, Fig. 18) and the total length of all channels increased Ash-mix deciduous slightly from 43 miles to 47 miles (Table 6). Though alcoves and sloughs riparian forest were lost, area and length of side channels increased and islands are largely Douglas fir-bigleaf unchanged (Table 6). Overall, the lower section of the Willamette River is maple/alder/dogwood woodland controlled by resistant geology and has experienced little historical change in complexity. Sandy bar LOWER REACH - PERCENT- COMPOSITION OF CHANNEL AREA Water bodies Channel Types Total area Primary Alcoves Side Islands ha acres 1850 85.9 0.6 6.4 7.1 1714.7 4237.0 1895 81.6 0.2 9.7 8.5 1813.7 4481.6 1850 1932 83.5 0.0 8.5 8.0 1952.0 4823.5 1995 83.0 0.1 10.0 6.9 1694.1 4186.1 Unvegetated, LOWER REACH - % CHANGE IN AREA VS. 1850 rocks Channel Types Agriculture, Primary Alcoves Side Islands Total Area cultivated yellow 1895 0.5 -72.2 60.1 27.3 4.1 1932 10.7 -100.0 51.2 27.9 12.8 Agriculture, clearing 1995 -4.6 -80.0 53.6 -3.0 -1.1 Agriculture, LOWER REACH - PERCENT COMPOSITION OF CHANNEL LENGTH fringe & agriculture Channel Types Total Length Alcoves Urban, Primary Side km miles road & railroad 1850 87.5 3.5 9.0 68.5 42.5 1895 82.3 0.5 17.2 73.0 45.4 Forest & woodland, mixed conifer, 1932 81.1 0.0 18.9 71.6 44.5 deciduous & brush 1995 79.7 0.7 19.6 75.9 47.1 Forest & woodland, conifer present, LOWER REACH - % CHANGE IN LENGTH VS.