Historically the region west of the Cascades was densely forested.
In the hills, conifers such as Douglas fir, western redcedar, and western hemlock tended to dominate
the forest. In the river valleys, broad-leafed trees like black cottonwood, red alder, and big-leaf maple were
Native peoples that lived in the region included the Kalapuya, Mollala, and
Chinookan (a group that includes tribes such as the Multnomah, Clatsop, and Clackamas).
Small scale burnings were the only land-altering practice that occurred prior to
the arrival of Europeans. The Native inhabitants would burn sections of forest in order to maintain open
areas that would attract elk and other essential game species.
The first people of European descent to appear in the region were fur trappers and missionaries.
In 1841, the first settlers arrived in Oregon City over the Oregon Trail, starting
a continuing influx of people to the region. Timber industry grew alongside the expanding population as land was cleared for cultivation.
Initially, settlements and agriculture were located in the river valleys, perched between the floodplain and steep hillsides. As the population expanded and
river-engineering ability grew, the settlements stretched
into the floodplains.
Land Uses Today
development, agriculture, and timber are still common
land uses today in the Portland area and beyond. Some land
use issues are discussed here.
Urban and residential areas
With population increasing and few places to expand,
agricultural lands are being converted into residential areas. As farmland is
converted to residential areas vegetation is removed, disturbing and exposing soil that can easily
wash into streams and degrade water quality.
Urbanization results in an increased amount of impervious surfaces such as roads, rooftops, and
lots instead of pervious surfaces like forests, wetlands, and agricultural
lands. Since water is not allowed to percolate through these surfaces, it
runs quickly into the rivers gathering an array of pollutants in its path.
Impervious surfaces decrease the amount
of rainwater available to percolate into groundwater, lowering the amount of
groundwater available to augment summer
streamflows. Increased runoff also results in more frequent flash flooding in waterways.
High streamflow has a scouring effect, eroding inadequately vegetated
riverbanks, which results in increased water turbidity.
Another result of urbanization is the manipulation of stream channels. As population grows, rivers and streams tend to get engineered in order to maximize land use. This alters the natural meander of waterways and can increase stream
velocity - changing the hydrological balance of the river or stream.
The agricultural landscape that supports the human population has benefits and drawbacks
for the watershed. Groundcover crops permit water to infiltrate the soil,
recharging groundwater which helps maintain summer stream flows. However, the single vegetation layer of crops
does not intercept as much water as the multiple layers of forests.
Other aspects of agriculture that can affect watersheds include excess
application of herbicides, pesticides, fertilizers, and other chemicals to crops.
These chemicals can leach into the streams and rivers, leading to algae blooms,
and other potential problems.
Animals used in agriculture such as cattle also can have an effect on watersheds. Animal feces can increase nutrient levels and increase
bacteria. These animals can also trample streamside vegetation leading to erosion.
A vegetated riparian zone is crucial to
watershed health, and farmers can increase stream health dramatically by leaving a vegetation buffer.
These buffers help
decrease chemical and nutrient runoff, as well as sedimentation. A wooded buffer
containing trees and shrubs also provides shade, keeping water temperatures cool and stable.
Forest is the natural state of most of the region. Forested areas with multiple vegetation
best suited to buffer watercourses from drastic changes in water quality
and quantity. This is especially true of
mature or old growth forests.
Roads cutting through forests diminish the health of the streams and rivers, causing erosion and potentially leaking oil and other pollutants into the system.
The cutting of timber also has an impact on watershed quality. Clear cuts have a similar effect as
construction, the removal of vegetation allows soil to erode into waterways
and prevents the slow filtering of rainwater to groundwater that occurs in
living forests. Forest practices such as selective cutting can reduce this
impact as can riparian buffers.