S. Gregory L. Ashkenas
D. Oetter P. Minear K. Wildman
Longitudinal Patterns - Vegetation
Floodplain Vegetation
While channel alteration has been extensive and ecologically detrimental
in many reaches of the Willamette River, changing the native floodplain forest
of the Willamette River was even easier than changing the channel. Early
settlers used the cottonwood and ash forests as a source of lumber and firewood, and riverboat operators created log decks along the river to store fuel
cut from the floodplain forests. Development of farms, towns, and cities along
the Willamette River accompanied a clearing of the floodplain forest to create
tillable fields and provide firewood. The narrow corridor of remaining forest
along the Willamette River is now vulnerable to loss by channel migration or
bank erosion during high floods. Historically, the wide floodplain forests were
not sensitive to the local losses of forest by erosion and channel change, but
today’s narrow forest margins cannot withstand flood events as they did in the
past.
Figure 179. Floodplain forest along the mainstem Willamette River.
The Data
As described on pages 40-43, maps of the river in 1850, 1895, 1932,
and 1990 provide a scientifically quantitative measure of the types and extent
of riparian plant communities along the Willamette River and its floodplain.
We use the floodplain axis as a quantifiable context for tracking changes in
floodplain vegetation just as we did for channel complexity on the preceding
pages. Within each 1-km slice, we measure the length and area of floodplain
vegetation and the percent of the floodplain occupied by forests. This longitudinal display of the vegetation within the floodplain creates a linear
illustration of floodplain plant communities, a chart of the changing biological conditions of the river and its floodplain. We focus on the floodplain
forest because this was the dominant riparian plant community prior to
settlement. Prairies, non-forest wetlands, and oak savannas were a minor
component of the mainstem riparian areas (see pp. 40-43).
Patterns
We compare the longitudinal patterns of floodplain forests along the
Willamette River in 1850 and 1990 to determine the potential for restoration
(Figs. 181-86). In the graphs on the facing page, it is important to note
differences in the values along the vertical axis of the graph. Loss of forests
was so extensive along the entire length of river that the bars of the histogram would be barely detectable for 1990 if we used the same values on the
vertical axis that were used for 1850. The axis for area of forest in 1990 is
one-fifth the scale of the axis for 1850, and the axis for length of forest in
1990 is the same as the scale of the axis in 1850.
Potential for Restoration
The potential for restoration efforts successfully to restore floodplain
forests differs along the Willamette River. Historical patterns of forests
provide a context for evaluating potential recovery of floodplain forests
through restoration action. The difference between the graphs of channel
complexity in 1850 and 1995 provides a longitudinal measure of the relative
loss or gain in channel complexity (Fig. 174, p. 134). Figure 180 below
depicts the change in forest patterns described above. In contrast to restoration of channel complexity, both the upper and middle river sections have the
greatest potential for future recovery of floodplain forests. The middle reach
of the Willamette River offers unique opportunities because it historically
exhibited some of the most extensive floodplain forest and more of the
floodplain was vegetated by forests than any other reach of the river (Fig.
185). Some of this land is managed by the Oregon Parks and Recreation
Department, and park managers are actively restoring some of the side
channels and floodplain features that have been modified over the last 150
years. Restoration of forest communities in the public parks is an important
opportunity for ecologically sound restoration (p. 140).
Slice Number
0
In 1850, the pattern of the length of forested margins along the
Willamette River was almost identical to the pattern of the area of forested
margins (Figs. 181, 183). By 1990, the floodplain forest had been fragmented so that length of floodplain forest is greatly diminished and bears
little resemblance to the pattern in 1850. Isolated patches near Eugene and
downstream of Salem still show higher extents of forested length, but their
values are less than 10% of the values observed in 1850.
In 1850, forests occupied roughly 20-80% of the total floodplain area
(Fig. 181). The wide floodplains upstream from Albany had woody forests
over 20-40% of their area ca. 1850. The middle reach from Albany to
Newberg had variable floodplain widths, but forests occupied 40-80% of the
floodplain area. The narrow floodplain below Newberg was variable, with
20-60% of the floodplain in woody forest. By 1990, less than 10% of the
area inundated under historical floods is occupied by forests. Only the
floodplains between Portland and Salem exhibit forests that approach 2040%, and even that level is far less than historical extents in this section of
the river.
136
20
30
40
50
60
70
80
90 100 110 120 130 140 150 160 170 180 190 200 210 220
0
Change in Forest Area (ha/1 km)
The overall shape of the graph of area of forest along the channel in
1990 is similar in shape to 1850, but the values for area of 1990 forest are
75% to 90% lower than 1850 values (Figs. 181, 182). The broader floodplain
of the upper river still contains the greatest area of forest, and the maximum
area of forest in any single 1-km slice of floodplain is at the Santiam River
and Luckiamute River confluences with the Willamette River.
10
-100
-200
-300
-400
-500
-600
-700
Floodplain Distance
Figure 180. Net increase or decrease in area of floodplain forest per 1 km of
floodplain distance between 1850 and 1990.
PNW Ecosystem Research Consortium
Figures 181-186 (facing page). Longitudinal
patterns of floodplain forests along the mainstem
of the Willamette River. Note change in scale of
vertical axis in Figures 181-84.
RIVER RESTORATION
Floodplain Vegetation
Scale: 1:1000000
Portland Metro
0 mi
1995 Urban Growth Boundaries (UGBs)
10 mi
20 mi
Slices which intersect UGBs
0 km
10 km
20 km
30 km
S
N
Salem/Keizer
Eugene/
Springfield
Albany
Corvallis
N
S
1000
hectares
0-26
27-64
65-133
134-192
193-233
234-298
299-362
363-979
800
600
400
Figure 181. Area of floodplain
woody vegetation ca. 1850.
225
220
215
210
205
200
195
190
185
180
175
170
165
160
155
150
145
140
135
130
125
120
115
110
105
95
100
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
5
10
0
200
200
hectares
0-3
4-12
13-19
20-27
28-35
36-49
50-64
65-173
150
100
Figure 182. Area of floodplain
woody vegetation 1990.
225
220
215
210
205
200
195
190
185
180
175
170
165
160
155
150
145
140
135
130
125
120
115
110
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
50
0-1.4
1.5-2.0
2.1-2.5
2.6-3.5
3.6-5.3
5.5-8.5
8.6-12.6
12.7-24.2
20
15
10
225
220
215
210
205
200
195
190
185
180
175
170
165
160
155
150
145
140
135
130
125
120
115
110
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
0-0.2
0.3-0.5
0.6-0.9
1.0-1.2
1.3-1.6
1.7-2.1
2.2-2.8
2.9-8.6
20
15
10
Figure 184. Length of floodplain
woody vegetation along water’s
edge 1990.
225
220
215
210
205
200
195
190
185
180
175
170
165
160
155
150
145
140
135
130
125
120
115
110
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
0
100
Percent
0-19.7
19.8-25.7
25.8-29.9
30-34.8
34.9-45
45.5-56.7
56.8-67.2
67.3-93.5
80
60
40
225
220
215
210
205
200
195
190
185
180
175
170
165
160
155
150
145
140
135
130
125
120
110
115
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
20
0
100
Percent
0-2.0
2.1-3.1
3.2-4.4
4.5-5.9
6.0-7.4
7.5-10.3
10.4-15.6
15.7-41.3
80
60
40
Portland Metro
Salem/Keizer
Albany Corvallis
225
220
215
210
205
200
195
190
185
180
175
165
170
160
150
155
145
140
135
130
125
120
110
115
105
95
100
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
20
0
Figure 186. Percentage of floodplain area in woody vegetation
1990.
0
25
kilometers
Figure 185. Percentage of floodplain area in woody vegetation
ca. 1850.
0
25
kilometers
Figure 183. Length of floodplain
woody vegetation along water’s
edge ca. 1850.
0
0
Eugene/Springfield
Note: 1 hectare equals 2.47 acres, 1 kilometer equals .62 mile
Willamette River Basin Atlas
2nd Edition
137