Groundwater in Oregon's Harney Basin: Ancient Water, Falling Levels

As demand for groundwater rose and water levels fell in the Harney Basin of southeastern Oregon, the U.S. Geological Survey and the Oregon Water Resources Department studied how much groundwater the basin has, in a cooperative study from 2016 to 2022. Three findings stand out:

• the lowland groundwater budget is out of balance by about 110,000 acre-feet a year, which has caused substantial falls in water levels;
• streams flowing from the uplands to the lowlands are the main source of recharge to groundwater beneath the lowlands, while precipitation falling on the lowlands adds very little;
• most groundwater pumped from lowland wells is ancient and is not being replenished on any meaningful human timescale.

Figure 1. Location and main features of the Harney Basin, modified from Gingerich and others (2022). Credit: U.S. Geological Survey.

How water moves through the basin

Groundwater in the 5,240-square-mile basin sits in an aquifer system that yields plenty of water in some places and little in others, depending on the rocks and sediments underneath and on how much water enters and leaves. Like a stream, groundwater flows downhill, from recharge areas, where water enters the system, to discharge areas, where it leaves. There is no evidence of substantial groundwater flowing in from neighbouring basins.

The uplands. Recharge is greatest in the uplands, such as Steens Mountain, from precipitation soaking in. But most upland rocks have low permeability — water moves through them with difficulty — so flow paths are short and shallow. More than 80 percent of the water that recharges the upland groundwater system comes back out in nearby streams and springs rather than travelling deep underground; a lesser amount leaves through evapotranspiration by wetland plants. This outflow keeps streams, springs, wetlands and meadows going through dry summers. Some of it runs down as surface water to the lowlands, where it sustains wetlands such as the Malheur National Wildlife Refuge, is diverted for irrigation, or seeps in to recharge the lowland groundwater.

The lowlands. About two-thirds of lowland recharge comes from surface water soaking in through stream channels and flooded ground, both irrigated and natural — mostly during the spring freshet, with smaller amounts all year from permanent streams and flooded wetlands. The other third is deep groundwater flowing in from the uplands. Precipitation falling on the lowlands is not an appreciable source. Water leaves the lowlands naturally mainly through evapotranspiration by deep-rooted dryland plants such as greasewood and salt grass, by wetland plants, and through springs and seeps, with a much smaller amount flowing east underground to the Malheur River Basin through Virginia Valley.

Figure 2. How groundwater is recharged, discharged and moves in the Harney Basin, modified from Garcia and others (2022). Credit: U.S. Geological Survey.

A budget out of balance

Before wells were drilled, the basin's groundwater budget was in balance: what flowed out to streams, springs and native plants roughly matched what came in, and well levels rose and fell around a long-term average with wet and dry years. The upland budget is still little affected by pumping. The lowland budget is not.

In the uplands, pumping removes only a tiny share of the groundwater that leaves the system. In the lowlands, pumping — mainly for irrigated farming — is the largest outflow, about 1.2 times natural discharge. Lowland outflow totals 283,000 acre-feet a year, against 173,000 acre-feet of recharge: a deficit of 110,000 acre-feet a year. Irrigation accounts for 95 percent of all groundwater use in the basin, and the amount pumped for it tripled from 1991 to 2018, from 51,000 to about 150,000 acre-feet a year.

Figure 3. Mean annual upland and lowland groundwater budgets, 1982–2018, modified from Garcia and others (2022). Credit: U.S. Geological Survey.

Figure 4. Irrigated fields in 1991 and 2018, and groundwater pumped for irrigation, 1991–2018, modified from Garcia and others (2022). Credit: U.S. Geological Survey.

Ancient water

Most groundwater beneath the lowlands entered the ground thousands of years ago. Stable isotopes, tritium and carbon-14 in water from wells, springs and streams show that most of it was recharged about 30,000 to 5,000 years ago, when the climate was cooler and wetter. Water recharged since 1953 is mostly confined to a thin, shallow zone under the lowland recharge areas — stream channels, floodplains, flood-irrigated fields and flooded wetlands. Little modern water moves through the lowland system, so most lowland wells draw ancient water, and the areas with the biggest declines get little or no modern recharge at all.

Falling water levels

Groundwater levels normally peak in late winter or early spring — under the weight of ponded floodwater and re-wetted soils, with recharge from winter precipitation and runoff, and as levels recover from irrigation pumping — and fall through summer as plants and pumps draw water out. Where pumping consistently exceeds recharge, levels drop year after year as water is taken from storage. How they drop depends on how many wells there are and how hard they pump, how close they are to recharge areas, and how permeable the pumped rocks and the rocks around them are.

Weaver Spring/Dog Mountain: fast and focused. Pumping has lowered the water table more than 140 feet below pre-development levels, and levels have fallen by up to 8 feet a year since 2016. Irrigation wells here tap highly permeable volcanic rocks and sediments — among the most productive in the lowlands — but little recharge reaches them, and they are hemmed in by much less permeable rocks that cannot refill them fast enough. The ancient water taken from storage is only partly replaced by equally ancient water seeping in. Groundwater once flowed toward Harney Lake, the lowest point; levels here are now nearly 90 feet below the lakebed, so local flow has turned toward the pumping centre instead.

Silver Creek upper floodplain: broad and slow. Pumping has lowered the water table about 10 feet across a wide area since 1980, and wells 200 to 400 feet deep fell by about 0.5 foot a year on average during 2015–19. Here wells draw on a large body of permeable rock beneath less permeable sediments, recharged by Silver Creek seepage (mostly upstream of U.S. Route 20), seasonal flooding, upland inflow and irrigation water — some of it, water chemistry shows, once stored in Chickahominy Reservoir. Levels still fall because pumping and natural discharge exceed average recharge, but the wide permeable area spreads the loss thinly. Continued pumping in the upper and lower Silver Creek floodplains will eventually reduce groundwater discharge downstream, including in Warm Springs Valley, part of the Malheur National Wildlife Refuge.

Across the lowlands, the steepest declines are where pumping is heavy, recharge scant and the surrounding rocks slow to refill. Notable areas include Weaver Spring/Dog Mountain, the Crane area east of Malheur Lake, the U.S. Route 20 corridor between Harney and Buchanan, the Silvies River and Poison Creek floodplains, Virginia Valley and the upper Silver Creek floodplain. Some declines are like a draining bathtub — small yearly changes over large areas, as around Virginia Valley and beneath the Silver Creek floodplain, averaging about 0.5 to 1 foot a year during 2010–18. Others form cone-shaped depressions in the water table: at Weaver Spring/Dog Mountain, Crane and along Route 20 the cones are deeper but narrower, falling up to 8 feet a year, with some more than 140 feet deep.

Sources

Based on "Groundwater Resources of the Harney Basin, Southeastern Oregon," U.S. Geological Survey Fact Sheet 2022–3052 (version 1.1, June 2025), prepared with the Oregon Water Resources Department, published by the U.S. Geological Survey; rewritten in hubnx's own words. Full details are in Gingerich and others (2022a, 2022b) and Garcia and others (2022), USGS reports and data releases, with supporting Oregon Water Resources Department reports.

• The figures come from the fact sheet's PDF: https://pubs.usgs.gov/fs/2022/3052/fs20223052.pdf