Considerations for Decision-making: NaturaL Systems
Land Cover
I
Early forest and non-forested upland: higher elevation areas without green vegetation and distinct
TabLe 6
Figure 5
similar areas on the valley floor This category in-
Acres by Land cover type
Acres by Land cover type
Land cover and vegetation can be mapped in numerous ways. The appropriate system for classifying
cludes recently harvested areas, open grassland, non-
vegetation types depends on the needs of potential
Acres
Type
Percent
forested alpine areas and barren areas.
users and resources available. Although the U.S.
I 8,777
3.13
306,662
51.08
Regrowth forest
I 29,443
21,56
Early forest and non-forested upland
I
I 4,825
9.12
Native vegetation, valley floor
8,0623
I .34
Irrigated crops
I 2,023
2.00
7,820
.30
49
0.01
2,733
0.46
Urban (see definition in note)
Forest Service and Bureau of Land Management have
Native vegetation, valley floor: vegetated areas of
detailed vegetation cover information fortheir lands,
the valley floor that did not appear to be associated
gathered through eId surveys, this information is not
with agricultural activities. This category would
available forthe entire watershed. The only land
include wetlands and riparian vegetation associated
cover information readily available forthe entire
for general purposes.
The USGS Willamette Basin Land Cover map was
D
with streams.
watershed was the USGSWillamette Basin Land
Cover developed from satellite imagery and useful
Mature forest
EFM
Irrigated crops: areas that formed distinct fields and
areas that were non-vegetated in June, but were
vegetated in August (the vegetative patterns between
Grass fields, small grains
the June and August images were compared).
Perennial snow
Grass fields, small grains: the grass-seed producing
Water
derived from LandsatThematic Mapper (satellite)
data collected in June and August I 992 and I 993.
Satellite imagery is a broad spectrum of light divided
into bands, each ofwhich reflects different character-
istics ofthe land cover The satellite image was then
refined using three data layers: elevation, slope, and
fields ofthe valley floor as well as hay fields, pastures
and fields ofsmall grains.
Perennial snow: the snowpack on Cascade Range
peaks in August.
soils. These layers were used to resolve discrepan-
cies. For example, an area coded as both forest and
Water: Open water including some intermittent
irrigated crop would be designated forest if it was
open water
high elevation and irrigated crop if it was low elevaAs indicated in Table 6 and Figure 5, the vast majority
Relative Surface
Erosion Potential
The USFS information contained pre-calculated
For more information about these erosion potential
soil to erode, based on slope and soil type. Existing
is based on expected losses ofsurface soil when all
categories, see the Mt. Hood National Forest Soil
and future land uses were not considered when
vegetative cover including forest lifter is removed.
evaluating whether soil was likely to erode.
The interpretation also considered climate, slope, soil
soil and bedrock materials.
Method 2. NRCS Data
the entire watershed. Two sources,the U.S. Forest
Soil types were grouped into three general catego-
In the absence ofa pre-calculated factor for erosion
ries, based on their erosion potential:
potential, soil scientists from the NRCS and the U.S.
amounts of agriculture and urban land making up the
because no single detailed soil inventory exists for
remainder
Urban: areas of urban development with a popula-
tion density of I 000 or more persons per square
Note: Urban areas were not classified from satellite
Service (USFS) and the federal Natural Resources
mile, as defined by the USGS and based on I 990
data. Instead, an urban layer provided by the USGS'S
Conservation Service (NRCS), cover different par-
Water Resources Division was superimposed over the
tions ofthe watershed and have different information
satellite data. "Urban" is defined here as areas that
in their data bases. The Mt. Hood National Forest
have a population ofimore than 1,000 persons per
Soil Resource Inventory (SRI) covers USFS land. Soils
square mile. These areas consist ofmany land uses,
in the remaining area were mapped by NRCS as part
Moderate: Some loss ofsurface soil materials can be
including residential, agricultural, commercial and indus-
ofthe Clackamas County Soil Survey A few areas
expected. Rill erosion and some small gullies or
trial (refer to the Existing Land Use map for more
had no data available because they are outside
detailed information in these areas).
agency survey boundaries.
amount of light and correspond to older forest
stands.
Regrowth forest: forest stands that reflected more
light than the mature forest and correspond with
younger forest stands.
Resource Inventory
characteristics, and hydrologic characteristics of the
This map was derived using two soils data bases
Mature forest: forested areas that reflected the least
occur
information on erosion potential. The interpretation
ofthe Clackamas watershed is forested, with similar
census data.
gullies, or considerable loss from sheet erosion may
This map shows the potential of undisturbed surface
ti 0 n.
The nine land cover categories are defined as follows:
Method 1. Forest Service Data
Forest Service were consulted for the best method
Slight: Little or practically no loss ofsurface soil
materials is expected. Some minor sheet and nIl
erosion may occur
sheet erosion may occur
High: Considerable or large loss ofsurf'ace soil
Because there are two different soils data bases, two
methods were used to represent erosion potential:
materials can be expected. Rill erosion, numerous
of determining erodibility. Two pieces of information,
'K-factor" and "average slope," are available for each
mapped soils unit and were recommended for use in
calculating erodibility.
K-factor is an erodibility factor that is adjusted for the
effect of rock fragments.
Average slope is the average of high and low slope
values within a soil mapping unit.
Figure 6
Relative Infiltration Rate
High potential for surface erosion
Soils having moderate infiltration rates when
For this map, current land use was not factored into
thoroughly wetted and consisting primarily of
infiltration rates, except in highly developed urban
moderately deep to deep, moderately well to
areas. Because oftheir high imperviousness (inability
well drained soils, soils with moderately fine
to be penetrated by water), developed commercial
to moderately coarse textures and moder-
and industrial lands were assigned to the category of
ately slow to moderately rapid permeability
lowest infiltration, regardless ofwhat kind ofsoil was
These soils have a moderate rate of water
underthe buildings and parking lots.
This map shows soil infiltration rates based on soil
type. Soil infiltration refers to the ability of water to
Subwatershed
penetrate the ground surface and pass through the
Upper Clackamas River
soil below. For example, sandy soils have rapid
6%
Hot Springs Fork Collawash River
infiltration rates, meaning that when it rains, the water
20%
Collawash River
will soak rapidly into the ground. Dense clay soils, on
21%
Oak Grove Fork Clackamas River
transmission.
- 3%
the other hand, have slow infiltration rates. On these
Fish Creek
30%
Roaring River
4%
South Fork Clackamas River
soils, any rain that falls may run offto other areas or
Soils having slow infiltration rates when
remain as puddles on the ground.
thoroughly wetted and consisting chiefly of
15%
Slope
soils with a layerthat impedes downward
Slope, orthe change in elevation over distance, is
movement ofwater soils with moderately
shown on this map. Note that the lower portion of
fine to fine texture, soils with slow infiltration
the watershed, west of Estacada, is relatively flat.
due to salts or alkali, or soils with moderate
More rugged terrain occurs in the middle watershed
watertables. These soils may be somewhat
nearthe Cascade mountains. The eastern portion of
poorly drained.
the watershed is high plateau, especially around
Like the Relative Surface Erosion Potential map, this
Middle Clackamas River
38°/o
North Fork Clackamas River
map was derived using two soils databases because
4%
no single detailed soil inventory exists for the entire
Eagle Creek
Goose Creek
watershed.
4%
Deep Creek
Both the Mt. Hood National Forest Soil Resource
Richardson Creek
8%
Timothy Lake. By comparing the slope, elevation and
Inventory (SRI) and the NRCS Clackamas County
Clear Creek
Soils having very slow infiltration rates when
5%
Soil Survey contain the propertyhydrologic soil
Rock Creek
11%
thoroughly wetted and consisting primarily of
group," defined by the U.S. Forest Service as:
Lower Clackamas River
patterns.
clay soils with a high swelling potential, soils
7%
0%
ecoregion maps, it is possible to see basin-wide
10%
20%
30%
40%
50%
60%
Percent with high erosion potential
Used in watershed management planning to esti-
with a permanent high watertable, soils with
As shown in Figure 7, slope measurements can be
mate runofffrom rainfall. Soil properties that influ-
a claypan or clay layer near the surface, soils
expressed in percent. Percent slope is calculated as
ence the minimum rate of infiltration obtained for a
with very slow infiltration rates due to salts
the vertical elevation (rise) divided by the horizontal
bare soil after prolonged wetting are considered.
or alkali, and shallow soils over nearly imper-
distance (run),then multiplied by 100.
These properties include: depth of seasonally high
vious material. These soils have a very slow
watertable, infiltration rate and permeability after
rate of water transmission.
Data for this map came from a USGS Digital Eleva-
The formua, used by NRCS H the unkiersa soI
Figure 6 shows the percentage of land area within
Oss equaton to evauate pHme farmland (K-factor
each subwatershed that has a high potential for
x average slope), produced pont vaues rangng from
surface erosion. For example, in the Middle
to 50. These vaues were cIassfied into three
categories:
tion Model (see Page 6 for more information).
prolonged wetting, and depth to very slowly permeable layers. The influence of ground cover is treated
independently."
Clackamas River subwatershed, 38 percent of the
land area has a high potential for erosion. In the
Rock Creek subwatershed, only
I
percent ofthe land
Slight (less than 2)
The hydrologic soil groups, as defined by the Forest
Figure 7
Service, are:
Slope equation and examples
has a high potential for surface erosion. Like the
Moderate (2-8)
Relative Surlace Erosion Potential map, a high ranking
High (8-50)
Resource managers can use the Reative Surlace
Erosion Potential map to determine general areas
within the watershed that are less likely to erode.
A.
Soils having rapid infiltration rates even when
for erosion potential is based on several factors,
thoroughly wetted and consisting primarily of
including soil type and slope.
deep, well to excessively well drained sands
Percent of slope
rise
run
x 100
and gravels. These soils have a high rate of
water transmission.
30'
30'
Although the soil erosion information is not site-
I 0'
30'
specific, it can help guide decisions about where to
build new roads, where to concentrate development,
or where to harvest timber
Slope = 100 percent
rise
run
Slope = 300 percent
Slope = 33 percent
rise > run
rise < run
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Agricultural. Commercial and Industrial. Refer to the
Existing Land-Use Mapfor detail on land use within
the Urban areas.
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