Download Physiographic Framework of the Willamette Lowland (Ben Shivers)

Physiographic Framework of the Willamette Lowland Aquifer System
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Abstract
This paper focuses on the physiographic framework of the Pacific
Northwest, specifically the geology of the Willamette lowland aquifer system and
its natural vegetation. The Willamette Lowland is a 5,680 sq. mi. area positioned
between the Oregon Coast Range to the west, and the Cascade Range to the
east. The Willamette aquifer system is separated into the five hydrogeologic
units, these include: (1) the basement confining unit, (2) the Columbia River
basalt aquifer, (3) the Willamette confining unit, (4) the Willamette aquifer, and
(5) the Willamette silt unit.
These units have variable porosity and permeability, and establish the
groundwater resource base for a large percentage of Oregonians. Large
Pleistocene glacial-outburst floods, coupled with volcanic activity, have covered
the Willamette Valley with unconsolidated surficial deposits as thick as
250 feet. Fertile soils at the surface have given birth to approximately
11,764,000 hectares of forested area in Oregon and Washington, and the
agricultural zones we see today.
Ben Shivers
ES 407 Environmental Geology
Depositional History of Unconsolidated Material
• The depositional period of the Willamette aquifer system spans from the
tertiary to the quaternary periods ending in the Holocene series.
Natural Vegetation of Willamette Valley
• Glacial outburst floods from lake Missoula in Montana washed down the
Columbia river and drained into the Columbia basin area and the Willamette
valley.
• The glacial floods deposited a variety of material ranging in size from silt to
boulders, and gave birth to one of the most fertile agricultural regions in the
world.
• Between Oregon and Washington there are 11,764,00 hectares of forested
land.
• Coniferous forests dominate the foothills and valleys of the Willamette valley,
and can grow as tall as 75 meters and live to be 500 years old.
Primary Tributaries of Willamette River
Geologic and Hydrogeologic Setting
• The Cascades are an active volcanic mountain range that are fueled by the
Cascadia subduction zone and the partial melting of the Juan de Fuca plate.
• The Volcanic and tectonic activity of the pacific Northwest has led to the uplift
of rock units to the east and west of the Willamette valley and lowland aquifer.
• The aquifer is approximately 130 miles long by 45 miles wide, encompassing
5,690 square miles of the Willamette valley, and contains low permeability
volcanic rocks and marine sedimentary rocks from surrounding highlands
(Gannet and Caldwell 1998).
Fig. 2 Depositional history of Willamette aquifer (Gannett and Caldwell,1998).
Thickness of Unconsolidated Deposits in Willamette River Valley
Fig. 4 Upper and Lower tributaries of Willamette river (Westfly, 2008).
Conclusions
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Fig. 1 Regional tectonic zone and Cascade mountain range (Aber 2008).
Fig. 3 Thickness of unconsolidated sediment in represented by color scale (USGS, 2008).
Willamette Aquifer Units
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Discussion
This project will focus on two main aspects of the Willamette valley:
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The Willamette lowland aquifer and the five major units it has been
separated into. Each unit will be identified and described in terms of
permeability and composition.
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The second aspect that this project will focus on is the natural vegetation
relating to the Willamette valley and lowland aquifer. Primary vegetation and
the amount of area covered in the Willamette valley will be identified as well.
The five units of the Willamette lowland aquifer system and the many
tributaries of the Willamette river supply the valley with enough water to
support the luscious forests we see today.
Glacial Flood deposits from the Pleistocene period gave birth to the
Willamette aquifer and the surficial unconsolidated deposits in the valley.
The variable porosity and permeability of the Willamette lowland aquifer
system stems from a range of material sizes and is the main source of
Oregon’s groundwater.
Because of the many tributaries from east to west connecting with the
Willamette river, the valley, its people, and it’s natural vegetation should
thrive, assuming the aquifer stays recharged.
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Basemen confining unit- composed primarily of fine grained marine
sedimentary rocks and volcanic rocks of the cascades which posses low
permeability. Marine sandstone, siltstone, clay-stone, and shale are
commonly exposed.
Columbia river basalt aquifer- composed of fine grained basalt flows. The
aquifer has poor vertical permeability but great horizontal permeability. The
high horizontal permeability allows a high rate of water flow to wells.
Willamette confining unit- comprised of fine grained distal alluvial fan and
stream deposits that consist of clay, silt and sand with small gravel beds. The
unit has low permeability.
Willamette aquifer- consists of layers of sand and gravel with interbeds of
sand, silt, and clay. Many layers are often mixed and have a fairly high
permeability.
Willamette silt unit- produced by glacial outburst floods and comprised of
silt. Considered a part of the Willamette confining unit. Has low permeability.
References Cited
Aber S. James, Emporia State University, 2008; map modified from L.Freeman,
URL: http://academic.emporia.edu/aberjame
Jerry F. Franklin and C.T. Dyrness, 1988, Natural Vegetation of Oregon and
Washington: Oregon State University Press
Marshall W. Gannett and Rodney R. Caldwell, 1998, Geologic Framework of
the Willamette Lowland Aquifer System, Oregon and Washington: U.S.
Geological Survey.
USGS, 2008, URL: http://capp.water.usgs.gov/gwa/ch_h/jpeg/H087.jpeg
Westfly, Maps, Willamette river: Internet web resource, URL:
www.westfly.com/or/or_maps/Willamette.gif