The Palouse Basin Aquifer System is a cross-border, layered groundwater system beneath parts of eastern Washington and northern Idaho. Its upper and lower aquifers are only weakly connected; municipal wells in Pullman, Moscow and Palouse draw mainly from the lower aquifer, while most domestic wells are typically in the upper one. Water levels have declined because pumping exceeds limited natural recharge.
Where the Palouse Basin aquifer is
The system lies beneath parts of Whitman County, Washington, and Latah County, Idaho. It supplies communities including Pullman, Moscow and Palouse, as well as Washington State University (WSU) and the University of Idaho (UI). It is a local groundwater system—not a single underground lake and not the same thing as the much larger Columbia Plateau Regional Aquifer System.
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For regional context, the U.S. Geological Survey describes Columbia River Basalt Group units and overlying basin-fill sediments as the primary aquifers of the Columbia Plateau system. That regional description provides geological background; the Palouse Basin Aquifer Committee (PBAC) gives the more local account of this basin’s upper and lower aquifers. USGS: Ground-Water Availability of the Columbia Plateau Regional Aquifer System
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How its layers hold and move groundwater
An aquifer is water-bearing rock or sediment that can store and transmit groundwater. PBAC describes the Palouse Basin as a complex arrangement of basalt and sediment layers containing multiple aquifers, including two major ones: the upper and lower. The formations are not one continuous, uniformly connected reservoir.
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| Feature | Upper aquifer | Lower aquifer |
|---|---|---|
| Connection between the two | PBAC says the connection is generally minor; the difference in water level, or hydraulic head, between them is generally greater than 100 feet. | |
| Typical wells | Most domestic wells are typically completed here. | Municipal wells in Pullman, Moscow and Palouse draw from here. |
These are typical patterns, not a claim that every well is completed in the same formation. PBAC: Aquifer System
Where recharge comes from
Recharge happens when water moves down through soil and geologic layers into water-bearing formations. In the Palouse Basin, geology limits where that movement can replenish the aquifers. PBAC says most recharge occurs at the base of Moscow Mountain, as water moves between sediment layers. The committee continues to study the precise pathways and locations, so that broad finding should not be mistaken for a complete map of recharge.
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PBAC does not establish a single current, basin-wide recharge volume in the cited material. It is therefore more useful to understand the reported imbalance than to attach an unsupported recharge total to it. PBAC: Recharge
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Wells withdraw water from the aquifers. When pumping persistently exceeds natural recharge, the amount stored underground declines and water levels fall. PBAC describes the basin’s situation directly: “More water is pumped out of the aquifer than enters it, and the geology of the basin makes it difficult for new water to replenish the aquifers.” PBAC: Aquifer System
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PBAC currently reports a decline rate of 0.7 feet per year, compared with a rate above 1 foot per year in the 1990s. This is PBAC’s reported rate, not a new calculation in this article. Its overview also reports that per-capita use over the last 30 years fell 30% in Pullman/WSU and 43% in Moscow/UI. Those conservation gains helped slow the decline, but PBAC says use remains above recharge. PBAC: Aquifer System
What the reported pumping total includes
PBAC’s 2025 annual report says Pullman, Moscow, Palouse, WSU and UI together pumped 2.30 billion gallons in 2024. The figure excludes private residential wells, so it is not a total for every user in the basin. PBAC says the 2024 amount was 1.7% below the same operators’ 2.34 billion gallons in 2023 and 19.1% below their 1992 pumping. PBAC: Annual Reports
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How the basin is monitored and managed
PBAC is a cooperative, bi-state committee involving local governments, counties and universities in Washington and Idaho; state water agencies participate as non-voting members. It implements the Groundwater Management Plan enacted in 1992, monitors water levels and annual use, funds research and maintains technical information about the basin. PBAC: About
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Conservation and efficiency improvements reduce demand and have helped slow the decline, but they have not closed the gap between pumping and natural replenishment. PBAC’s research resources include a lecture series by John H. Bush, University of Idaho emeritus faculty, on the basin’s geologic history, groundwater movement and recharge, as well as a technical research archive. PBAC: Research
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What the Clearwater River proposal could do
In spring 2025, the City of Moscow and PBAC began studying a potential Clearwater River diversion as a supplemental supply. It is a proposal under evaluation, not a built, approved or operating replacement for groundwater pumping. PBAC describes two possible approaches:
- Direct municipal use: Treat diverted river water and convey it into municipal systems, reducing the amount of groundwater those systems need to pump.
- Aquifer storage and recovery: Treat the water, inject it into an aquifer for storage, then recover it later.
The options have different timing and water-quality considerations. Clearwater flows are high in winter and spring, while local water use peaks in summer, so direct use may need storage to match supply with demand. For aquifer storage and recovery, PBAC has raised questions about the water quality implications of putting treated surface water into an aquifer. These are issues identified in the committee’s discussion of the options, not settled findings from an independent project review. PBAC: Clearwater River Diversion
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