The major geological factors are:
The volcanic mountains of the Cascades
Huge basalt flows that oozed out over the landscape
The ancient folded mountains of the Coast Range
Many river valleys and lake basins filled with deposits from eroded mountains
The Cascade Mountains
The high Cascade peaks - Mt. Hood, Mt. Jefferson, the Three Sisters, and the rest are volcanoes made up of very porous material. Much of the melting snow and rainwater seeps into the ground and come out at a lower elevation. This is the main source of groundwater for much of central Oregon.
The entire Metolius River comes from the Cascade Mountains' groundwater that emerges as a giant spring. Cascades groundwater also contributes greatly to the flow of the Deschutes River and supplies much of the water for the Klamath Basin. These aquifers are described as volcanic and sedimentary-rock aquifers (USGS).
On the west slope of the Cascades, much of the groundwater discharges into the Willamette and its tributaries. But there is more to the story. Some water that originally fell as snow or rain in the Cascades is making its way slowly but surely to the surface via a less obvious route: through deep layers of fractured rock below the Willamette River, then under the Coast Range and eventually to the Pacific Ocean. This process can take hundreds, if not thousands of years or more!
The Columbia River plateau is built up from many flows of lava that covered large areas. As the lava cooled, the new basalt cracked into the six-sided columns so familiar to anyone who has visited Eastern Oregon.
Imagine more than 150 layers of lava flows, each about 100 feet thick, more or less. Between some of these rock layers are thinner layers of loose volcanic rubble or ash. Water can fill the spaces in the fractured basalt and travel through the loose material between some basalt layers. The USGS Aquifer page describes these basaltic and volcanic-rock aquifers in more detail (also see aquifers in the groundwater basics section of this site).
The Columbia River Basalt Aquifer system is an enormous groundwater resource. Some interesting details:
It covers all or part of ten counties in Oregon, and even a larger area in Washington.
A lot of water is stored underground in all those spaces.
Because the spaces in the fractured basalt are so large, the water flows from wells at tremendous rates.
Groundwater in this region has been used extensively for irrigated agriculture—in fact, plentiful groundwater allows a thriving food packing industry in an area that otherwise would be almost a desert.
The snowmelt water from the Blue Mountains of Eastern Oregon contributes some to the recharge of deep basalt aquifers underlying the Columbia River Plateau. However, overall, recharge is slow because of the low rainfall in the region, and the recharge areas aren’t very large compared to the expanse of the plateau. (See recharge in Groundwater Basics section.)
Historically, withdrawals have been greater than recharge in many areas and restrictions have been placed on groundwater in some parts of this aquifer system.
The Coast Mountains
The Coast Mountains are a bit more complicated. They are made up of folded layers of ancient rocks. Some of these layers transport water fairly well, others not at all. The short story is that water seeps in where it can and moves downward until it hits a layer that stops it. The water eventually emerges as a spring, stream flow, or water in the ocean. If you are drilling a well in the Coast Range you could be lucky and hit a water-filled layer or network of cracks, but don't count on it (Read more about bedrock aquifers in the Groundwater Basics section for more information).
The Siskiyou and Umpqua Mountains of southern Oregon are more solid, however, valleys in the mountainous areas can be important, so keep reading.
The Willamette Valley and other river valleys, including the Rogue, Grand Ronde, and parts of the Snake, make up some of the important aquifers in Oregon.
River valleys are the ultimate partnership between water and rock. Water flows downhill in little trickles into bigger and bigger streams, carrying with it little bits of eroded rock, or during raging floods, gravel and stones.
When the streams join the bigger river, this load is dumped into the river, or if the river is flooding, onto the floodplain. Eons of erosion and flooding result in layer upon layer of loose rock particles piled up on the valley floor. What we end up with is the deep bedrock valley floor tens to hundreds of feet below what we see today as the land surface. USGS describes this as unconsolidated sand and gravel aquifers (also see layered soil aquifers in Groundwater Basics section).
Now here comes the groundwater part of the story: If the layers are coarse sand or gravel, as is often the case, they can store and release large amounts of water. And in addition to having great material for an aquifer, valley floors have a great source of recharge water—runoff from all the surrounding mountains and seepage from the river itself. In fact, if we aren’t careful, pumping wells can actually affect river levels and draw contaminants from the river into otherwise clean groundwater (see induced recharge in Groundwater Basics section).
River valley aquifers can be tricky because not all layers yield plentiful water. Some aquifers are so shallow they are at serious risk of contamination from human activities. Inland, human populations tend to be concentrated in valleys. That makes these aquifers extremely important because the water is in high demand by households, farming, industry, and other uses. All these uses have the potential to contaminate or overuse the precious water resource.
The summary above is not a comprehensive account of the region's geologic history. If you would like more technical information about Oregon's groundwater geology, check out the Groundwater Atlas or groundwater studies by Oregon Water Resources Department, USGS, or many other agencies that work on groundwater protection throughout the state. We highly recommend that you take advantage of the wealth of information available on the USGS Aquifer Basics website. Other additional resources are available to help you learn more about the groundwater geology of your area. Most are available on the web or you can order a printed copy.
Ground Water Atlas of the United States, Segment 7--Idaho, Oregon, Washington (USGS Hydrologic Investigations Atlas 730-H). Very thorough, lots of great diagrams, a bit technical. Check your region:
Region-specific groundwater studies in Oregon:
To order materials or ask for more information about your area contact:
USGS Oregon District office
10615 SE Cherry Blossom Drive
Portland, OR 97216
Oregon District web site