1120 SW 5th Avenue, Room 1000, Portland, OR 97204
A general description of landscape features and streams is provided for each subwatershed. A table summarizes stream channel length by type (e.g. open channel, ditch, culvert).
Geology & Soils
The dominant geological formation of the region is Columbia River basalt originating in lava flows from the eastern Columbia Basin . Underlying the basalt is the Scappoose formation, sandstone and shale deposited when the region was ancient ocean bottom 22 million years ago. These formations were subjected to vulcanism and tectonic forces, forming the Cascade, Coast, and Tualatin Mountain Ranges. Subsequent weathering and erosion of Columbia River basalt has since shaped these mountains and exposed portions of the Scappoose formation in Portland’s West Hills.
The Columbia River basalt in this area is overlain by the Troutdale formation, a layer of sandstone and gravel eroded from Columbia Basin and Cascade Mountains up to 1500 feet thick (accumulated 10 to 2 million years ago). The Troutdale formation was created as volcanic activity, mudflows, and continuing deformation repeatedly transformed the Willamette, Sandy, Clackamas, and Tualatin rivers into closed drainage basins, forming large lakes. Silt, sand, and gravel were deposited by this process as well as by the Columbia River. East Portland relief features (Mt. Tabor, Rocky Butte, and Kelly Butte) were formed more recently (3 to 0.5 million years ago) by localized volcanic intrusions known as the Boring Volcanoes.
The erosion of the Troutdale formation material by the Willamette, Clackamas, and Columbia rivers exposed these volcanic features further as the rivers migrated extensively across the area.
The most recent significant factor in the region’s geology and soil formation was a series of events known as the Missoula Floods that occurred 6 to 12 thousand years ago. Advancing glacial ice repeatedly dammed the Clarks Fork of the Columbia River, forming an enormous lake. Pressure from the impounded water eventually caused the ice dam to break, releasing up to 600 cubic miles of water that poured out in a matter of days. This cycle of massive floods created erosion patterns on Portland’s east side, most notably Sullivan’s Gulch and the Alameda Ridge. The topography to the west restricted drainage and large lakes, up to 400 feet deep at the site of present-day Portland, formed in the area. The settling gravel, sand, silt, and clay formed the current soils. During this general period a five-foot-thick layer of wind-blown silt (known as Portland Hills Silt) was also deposited on the upper portions of the West Hills. Local rivers have since eroded and migrated through these depositions, forming the current channels and floodplains.
Prior to urban development, the Willamette River was relatively constrained through the plan area but generally had gently sloping banks. The west terrace through Portland had many open streams, lakes, and wetlands connecting to the river. This low terrace flooded regularly and was relatively open before rising steeply into the West Hills. The east terrace sloped gently away from the river and was better drained than the west side, but with numerous streams and significant wetland areas adjacent to the river.
Hydrogeologic Units and Conductivity
Flow of water through a groundwater system is largely dependent on the extent and thickness of relatively permeable water bearing rocks (aquifers) and poorly permeable water bearing rocks (confining units). The aquifers and confining units are called hydrogeologic units.
Eight major hydrogeologic units have been identified in the Portland Basin, a 1,310 square mile area bounded by the Tualatin Mountains to the west, the Lewis River to the north, the foothills of the Cascade Range to the east, and the Clackamas River to the south ( McFarland and Morgan, 1996).
The uppermost hydrogeologic unit is an unconsolidated sedimentary aquifer. The aquifer consists mainly of catastrophic flood deposits that mantle the central part of the basin. The aquifer also includes water-bearing alluvial deposits that occur along smaller streams. Hydrologic conductivity indicates how well fluid flows through a substance. The unconsolidated sedimentary aquifer has the highest median value of hydraulic conductivity, 200 feet per day; but the value ranges from 0.03 to 70,000 feet per day ( McFarland and Morgan, 1996).
The Troutdale gravel aquifer, underlying the unconsolidated sedimentary aquifer, is composed of several geologic formations of poorly to moderately cemented conglomerate and sandy conglomerate, but also includes thick local accumulations of lavas and mantling soil horizon. The Troutdale gravel aquifer ranges from 50 to 250 feet thick. The median hydrologic conductivity for the Troutdale gravel aquifer is 7 feet per day ( McFarland and Morgan, 1996).
Confining unit 1, underlying the Troutdale gravel aquifer, consists of medium to fine grained arkosic sand, silt, and clay, with some vitric sand beds. Confining unit 1 is generally less than 200 feet thick. The median hydrologic conductivity for confining unit 1 is 3 feet per day ( McFarland and Morgan, 1996).
The Troutdale sandstone aquifer, underlying portions of the Troutdale gravel aquifer, consists of coarse vitric sandstone and conglomerate with lenses and beds of fine to medium sand and silt. Generally, the upper two-thirds is composed chiefly of vitric sandstone and the lower one-third is conglomerate. The Troutdale sandstone aquifer is generally from 100 to 200 feet thick, but in southern portions of the basin may reach a thickness of 400 feet. The median hydrologic conductivity for the Troutdale sandstone aquifer is about 14 feet per day ( McFarland and Morgan, 1996).
Confining unit 2, underlying the Troutdale sandstone aquifer, consists of clay and silt with lenses of fine to medium grained basaltic sand. Outcrops of this are limited to southeastern parts of the basin along the Clackamas and Sandy Rivers in Oregon. The median hydrologic conductivity for confining unit 2 is 1 foot per day ( McFarland and Morgan, 1996).
The sand and gravel aquifer present in only portions of the basin, underlying confining unit 2, consists primarily of sandy gravel, silty sand, sand, and clay. The median hydrologic conductivity for the sand and gravel aquifer is about 12 feet per day ( McFarland and Morgan, 1996).
The older rock subsystem underlies and bounds the basin-fill sediments and consists of Miocene and older volcanic and marine sedimentary rocks generally of low permeability. The median hydrologic conductivity for the older rock subsystem is 0.2 feet per day ( McFarland and Morgan, 1996).
Generally, only the first three hydrologic units underlie Portland’s west-side subwatersheds. Under east-side subwatersheds the presence of these seven hydrologic units is varied, though in most cases all seven exist.
Groundwater generally moves from upland areas such as the Tualatin Mountains toward the major discharge points in the basin, such as the Columbia and Willamette Rivers. Upland areas generally have strong downward components of movement and can be classified as “recharge areas” while lowland areas generally have strong upward components of movement and can be classified as “discharge areas.” In the Portland basin, discharge areas are generally limited to narrow zones along the major streams ( McFarland and Morgan, 1996).
Groundwater in the unconsolidated sedimentary aquifer, the uppermost hydrologic unit in most of the Willamette watershed, flows from an altitude of more than 200 feet in the Gresham area toward the Columbia and Willamette Rivers to the west and north ( McFarland and Morgan, 1996).
Perched groundwater, occurring where a zone of saturation is perched on the strata of low permeability above the main water table, is common in the shallowest water-bearing units throughout much of the Portland Basin. These areas have strong downward components of movement or recharge (McFarland and Morgan, 1996). In the Willamette watershed planning area, perched groundwater exists throughout the upland areas in the west-side subwatersheds. Generally, perched groundwater exists above a fragipan layer between 2.5 and 4.5 ft below ground. This shallow perched groundwater can create natural hazards, particularly when exposed by excavation. It can precipitate landslides and cause soil creep, with potentially serious consequences for development. Groundwater is also susceptible to pollution from septic drain field effluent, solid waste leachate, industrial activities, and urban runoff from impervious surfaces that may contain pesticides, herbicides, fertilizers, metals and other contaminants ( City of Portland Bureau of Planning, 1991 and 1992).
Generally, recharge to the aquifer system in the Portland Basin is primarily by infiltration of precipitation. However, within the urbanized portions the two most significant urban sources of recharge are runoff from impervious surfaces to drywells (sumps) and effluent from on-site waste-disposal systems.
McFarland and Morgan (1996) estimated groundwater recharge throughout the Portland Basin. Generally, infiltration in the Tualatin Mountains, the upland portions of most of the west side Willamette subwatersheds, ranged from roughly 20 inches annually in the southern subwatersheds to 30 or more inches annually in the northern subwatersheds (in Forest Park). Infiltration estimates varied greatly for east side subwatersheds. Generally, inner central east-side subwatersheds have infiltration rates of less than 10 inches annually, while outer east subwatersheds may have higher rates closer to 20 inches annually ( McFarland and Morgan, 1996).
No data are available for the Willamette River or other streams in the planning area.
Streams & Water Bodies
BES maintains two sets of flow monitoring data: permanent monitoring data (as recorded by the HYDRA System and maintained by the Data Acquisitions & Management Group) and temporary monitoring data (retrieved from the Bureau’s Janus application and maintained by Field Operations). The permanent monitoring data has creek/river related flow data (one for Balch Creek in McLeay Park from 1985-1996 and another for the Willamette River at Ankeny Pump Station). The temporary monitoring data has flow information for the Balch, Miller, Saltzman, and Stephens Creeks, and the latter three creeks were monitored as part of the Westside Streams Monitoring project.
Data for Balch Creek are available from June 1996-September 2002. Monitoring for that location is no longer active. Data for the Miller, Saltzman, and Stephens Creeks are available from September 2002-December 2002 and from May 2003-July 2003. Monitoring is currently active for these creeks.
For a summary of stream flow monitoring locations Click Here
The section briefly describes the topographic features of each subwatershed.