Considerations for Decision-making: Human Impacts
Estimated PopuLation Density
This map dispays popuIatan density, or the number
of persons per square mile of land, for the Clackamas
River watershed. Demographers at Nietro used
block-level data from the I 990 U.S. census to estimate I 995 population numbers. Census blocks are
the smallest geographic units used by the U.S. Census
Bureau to report summary census data. Census
blocks are delineated along major roads and other
physical features and typically do not coincide with
watersheds or other geographic units. Towns and
communities are often split into several blocks.
Estimating population totals in the Clackamas watershed was difficult because census blocks often over-
lapped the watershed boundary Demographers
calculated population density by taking the proportion of each census block falling within the watershed
and estimating a percentage ofthe total block population. This analysis assumes that populations are
evenly distributed within census blocks, yet population is usually clustered in urban areas. Based on this
method, the total I 995 population within the watershed was estimated to be 63,702.
transport pollutants such as oil from streets and
urban reserves in the Rock Creek and Richardson
parking lots to waterways. Several studies have
Creek subwatersheds, these areas will likely see
TabLe 7
shown a link between the percent impervious area
larger increases in impervious area than shown on
and watershed health (Watershed Protection Tech-
this graph. Comprehensive plans are also somewhat
Estimated percent of impervious surface
by Landuse* type
niques, I 994).
general and do not include designations for future
Property class description
This map shows the percent oftotal impervious
surface based on land-use categories from the
Forested or non-developed subwatersheds: For
property classification for assessment (RCA) codes in
these areas, found mostly in the upper Clackamas
as well as building roofs. Even areas with compacted
or hard soils, gravel roads and certain crops or
vegetation can form a partially impervious surface,
allowing waterto run offto nearby storm drains or
streams.
When land becomes more impervious through
paving and building, water can no longer soak slowly
into the ground after a rain storm. Instead, the water
flows quickly into waterways, where it can raise water
flows and cause flooding. The runoffwater can also
E 0/
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(roads and rights ofway are calculated separately)
codes represent land use as recorded at the time of
estimated using a formula ofone percent impervious
0 to 0.25 acre
35%
the last tax assessor's appraisal. To estimate impervi-
area per mile of road per square mile (Booth). Thus,
0.26 to 0.5 acre
30%
ous area, each land-use type was assigned a percent-
two forested areas could have different percentages
0.5 I
age ofimperviousness, listed inTable 7. All roads
of impervious surface. For example, we estimated
greater than I .0 acre
were assigned an impervious value of9O percent. As
that the Roaring River subwatershed, a nearly
development occurs, land use and the amount of
roadless area, has only one percent impervious area.
impervious surface will change.
The Fish Creek subwatershed, with a greater road
density, has three percent impervious area. In the
Figure 8 (see next page) estimates the current
Eagle Creek subwatershed, the more developed
percent of impervious surface area in each
areas near Estacada are partially balanced by the
subwatershed, compared to what the subwatershed
Salmon-HuckleberryWilderness, giving this
is expected to have in the future. The percent
subwatershed an overall impervious area of nine
impervious surface area was calculated in one of two
percent.
ways, depending on whether the subwatershed was
primarily developed or primarily forested.
We were not able to estimate any changes in per-
residential areas, shown on the bottom ofthe graph,
comprehensive plans. However any future road
we estimated the current percent of impervious
include paved areas such as streets and parking lots,
Single-family residential, vacant
watershed, the percent impervious surface was
urban subwatersheds, as they are not subject to local
easily penetrated by water Impervious surfaces
5%
Single-family residential, improved
Developed subwatersheds: For these urban or rural
An impervious surface is any surlace that cannot be
Open space
the Clackamas County assessor's database. RCA
cent impervious surface forthe forested or non-
Impervious Area )
% impervious 2
roads.
building would raise the percent impervious area.
to
I .0 acre
average
Commercial land, vacant
Commercial land, improved
Industrial land, vacant
25%
20%
27.5%
5%
90%
5%
Industrial land, improved
90%
Department of Revenue appraised industrial
90%
Tract land, vacant
5%
Tract land, improved
20%
Farm land, vacant
20%
Farm land, improved
20%
Forest land, vacant
5%
Forest land, improved
2%
Multi-family residential, vacant
5%
surface area using the values shown inTable 7. Each
Multi-family residential, improved
65%
subwatershed's overall percent impervious area was
Mobile home parks
60%
weighted. In other words, if I 0 percent of a watershed is rural residential, 30 percent is commercial and
Current Zoning
60 percent is single-family residential,the overall
This map shows zoning designations, which are the
imperviousness value is based on the amount of land
legally established, permitted uses of land. Zoning is
within that subwatershed that is made up of each
administered by each local government, and mdi-
land-use type.
vidual cities and counties define their own unique
zoning codes. To provide compatibility among data
Figure 8 also estimates how the percent impervious
from different jurisdictions, the map groups zoning
area for each subwatershed could change in the
types into general categories.
future, based on land-use classifications in each
jurisdiction's comprehensive plans (see also the
Note: The forestry/forest management zoning category
Future Land-Use Plan map). Because these corn pre-
consists ofsevera/ different uses, inc/uding timber
hensive plans have not been updated to reflect new
harvest, recreation and wi/d/ife habitat.
Recreation land, vacant
5%
Recreation land, improved
20%
Miscellaneous
20%
* Land uses as defined by the Clackamas County property classification for assessment (PCA) system.
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