Abstract

1
Groundwater Hydrology of the Willamette Basin,
Oregon
Prepared by Brittnie Andrew Western Oregon
University Spring 2008 ES473 Environmental Geology
Abstract 70 of the population in Oregon lives
above the seven major aquifer systems in the
Willamette Valley. The seven primary
hydrostratigraphic units include the High
Cascade, Upper Sedimentary, Willamette Silt,
Middle Sedimentary, Lower Sedimentary, Columbia
River Basalt, and the Basement Confining.
Precipitation recharges these units at an
average rate of 22 in/yr between the high and
lowland areas. The period of greatest rainfall
occurs from November to April and allows for more
water to infiltrate than is withdrawn from the
regional aquifer systems. Summer months with
evaporation and irrigation account for the most
water use. Regional groundwater levels fluctuate
by about 65 ft each year and are projected to
decline with time as population in the valley
increases and recharge varies with changing
climate dynamics.

• Human Help
• The current aquifer system is supplying the
  people within the valley with plenty of the water
  needed to sustain life, and then some. Over time
  based on the fluctuation of the groundwater
  levels more efforts are going to need to be put
  into the task of conserving water. However if
  efforts where made sooner conservation of water
  would be less costly in the future. Basic and
  small actions can be taken to help prevent water
  lose within the system.
• Watering plants in the morning (also helps the
  plants).
• Washing the car in the lawn if needed.
• Collecting rainwater and storing it for future
  use.

• Rainfall Recharge
• The Willamette Valley is a large scale
  watershed. Rainfall on the east side of the Coast
  Range, the west side of the Cascades and within
  the valley provide a system of recharge to the
  underlying water table. Rainfall is greatest in
  winter months. In Salem the annual rainfall is 79
  inches during the months between December and
  March (Conlon et al. 2005). Figure 2
• Figure 3 Shows the water system including
  precipitation for the year 2000 in the central
  portion of the basin. The amount of precipitation
  varies from season to season so understanding the
  exact amount of rainfall recharging the aquifers
  is difficult. Figure 4 shows the water cycle
  system with the influencing factors of ground
  water, precipitation, and percolation.

Figure 3 Water use and budget for Central
Willamette Basin in 2000 (Colon et al. 2005).
Figure 1 shows the general location of the
Willamette value within Oregon as well as sub
basins (OSU, 2008).

• Overview of the Willamette Basin
• The seven primary hydrostratigraphic units
  include the High Cascade, Upper Sedimentary,
  Willamette Silt, Middle Sedimentary, Lower
  Sedimentary, Columbia River Basalt, and the
  Basement Confining all deposited within the
  basin (Figure 1).
• The Willamette Basin is home to 1.9 million
  people, all in need of a continuous water
  supply.
• Rain fall is the primary source of recharge to
  the system.
• Water use, and climate effect the overall amount
  of ground water.
• The future of the basin, water budgeting.

Figure 4 A break down of the Hydrologic Cycle
(S.A. Water, 2008).

• Water Use
• The Willamette Basin groundwater provides water
  to people, farming, and industries by means of
  wells. In 2000 the total amount of water that was
  pumped was about 300,000 acre-ft, of the total,
  81 percent was pumped for irrigation, 14 percent
  for public supply, and 5 percent for industrial
  use (Colon et al., 2005). During the periods of
  little to no recharge, April through September in
  2000, Figure 2, much of the stored water is
  consumed. Evaporation and transpiration are also
  factors in water use. Transpiration, best
  described as tree sweat accounts for the use of
  28in/yr (Colon et al., 2005). This number as well
  varies from year to year, but that is based on
  the depth of saturation and the height of the
  water table. The water table levels fluctuate
  each year for varies reasons.
• The amount of water withdrawn is greater than
  the amount of water imputed. More withdrawn
  during summer months then winter months.
• Warmer climate means higher evaporation rates.
• Building of more wells provides the potential
  for more water to be used.

• References Cited
• Conlon, Terrence D, et al. Ground-Water
  Hydrology of the Willamette Basin, Oregon
  Scientific Investigations Report 20055168
  (2005). 83 pages
• Oregon State University, 2004 Willamette Basin
  Explorer. Dept. home page. June 2004 May 27,
  2008 .
  
• Reed, Christopher W. Town of Longmeadow 2006
  Water Quality Report. Longmeadow Water
  Department. 15 Apr. 2008. 27 May 2008
  ce.htm.
• S.A. Water. The Water Cycle. 8 Feb. 2008. 26
  May 2008 .
• Woodward, Dennis G, Marshall W Gannett, and
  John J Vaccaro. Hydrogeologic Framework of the
  Willamette Lowland Aquifer System (1998).

Figure 2 Willamette River Portland Area (OSU,
2008).