Where Irrigation Fits

What we put on our landscapes comes out of the same aquifer that feeds the river.

By Jon Peters, Founder and General Manager, Irrigation Managers

In Part 1 and Part 2, we saw that the dam released more water than in earlier dry years, and one of the things I found very interesting is that in downtown Spokane the river carried more water than in those years. The data shows that the aquifer measured inside its historical range, and a stretch of river still dried up in 2025 and 2026. What appears to have changed is the seepage into the aquifer, which seems to have increased significantly, and nobody has shown why yet. What we can control is how much water we take out of that system in the weeks the river has the least to spare.

Knowing the numbers is what we do. Many of our clients draw their water from this aquifer, and helping them use it wisely is part of our job. That is why the river's numbers matter to us, and why this is the part of the story that lands closest to our work.

What gets pumped, and when

Between 1990 and 2005, USGS documented pumping from the aquifer at an average of about 318 cfs. That is about 230,000 acre-feet a year, or roughly 75 billion gallons, and about a fifth of the water leaving the aquifer. Public water systems took about 64 percent of that, roughly 48 billion gallons. Irrigation outside those systems took 16 percent (about 12 billion gallons), industry 11 percent (about 8 billion), and private wells 9 percent (about 7 billion). Pumping is not flat through the year. It climbs to 600 to 800 cfs in summer, or about 390 to 520 million gallons a day, and the swing is mostly outdoor watering. In figures the City of Spokane presented in 2025, its water production ran 454 gallons per person per day in July, against 130 in January. The weeks the river is lowest are the weeks landscape demand peaks.

Where pumping matters depends on the stretch of river. Upstream, where the river sits well above the water table, the aquifer's level has little say in how much leaks out. Where the water table is close to the riverbed, as in the transition near Barker and Sullivan Road, pumping nearby could change that, which is one of the ideas the seepage study may test. Farther downstream, where the aquifer gives water back, every gallon pumped is a gallon that does not reach the river, and that is where most of the effect on the river's flow shows up. A consultant's model for Spokane's water providers estimates that each cfs pumped at the summer peak lowers late-August flow at the Spokane gauge by 0.35 to 0.65 cfs. The 2015 drought gives a real-world example. Post Falls Dam held steady at 500 cfs, and pumping across the aquifer fell by about 166 cfs after the City of Spokane cut its own. Over the same weeks the river through Spokane rose by about 90 cfs, and the aquifer came up about half a foot. Less water pulled out of the ground meant more water seeping back into the river. Ecology's John Covert presented the finding, and it is a single observation, not a rule.

Total use is not the runaway story

The Spokane Aquifer Joint Board's consultant reports that Washington's irrigation-season use fell from about 450 cfs to about 275 cfs as farmland gave way to housing, with per-person use down about 30 percent from its 1980s peak. The City of Spokane's purchased water has held between 17.8 and 19.9 billion gallons a year from 2020 to 2024. The Bureau of Reclamation's irrigation project on the Prairie (the Post Falls, Hayden Lake and East Greenacres units) has roughly 7,000 irrigable acres today, down from 10,200. On the Idaho side, a 2010 state study counted about 72,000 acre-feet withdrawn a year, about two-thirds of it for irrigating farms, lawns, parks and businesses, and projected that farm irrigation would shrink while total irrigation demand rose with new housing. Spokane County as a whole uses about 235 gallons per person per day, which the Joint Board's atlas puts in the 98th percentile of U.S. counties. Both can be true: total use is below the 1980s peak, and summer landscape watering is still the biggest piece.

The Prairie has changed

Farming on the Prairie has changed, and the change matters most in August. Grass seed fields covered about 12,500 acres of the Rathdrum Prairie in the early 1990s and about 2,400 acres in 2006. A grower told the Spokesman-Review in 2007 that without field burning, seed yields would drop by about 75 percent. In my experience, that and higher costs pushed bluegrass and many of its growers off the Prairie and opened the land to housing. Bluegrass seed is not irrigated in August, because the fields are cut in July and harvested into August. The crops that replaced it, and the houses and parks after them, are watered in August. Total summer irrigation has probably dropped as farm fields disappeared, but August water use may have gone up. I have not found a study that measures it, and it is a question worth answering.

What a site can do

A common thought is that extra water just goes back to the aquifer anyway. Irrigation and septic return puts some back, about 77 cfs against the 318 cfs pumped. Some excess irrigation does seep back to the aquifer, and some runs off through storm drains toward the river, but extra water is not free. It takes electricity to pump, and runoff can carry fertilizers and other contaminants to the river. Nitrates and phosphorus are common ingredients in fertilizer, so when runoff reaches the river they come with it. There they feed algae, and when the algae die, their decay uses up the oxygen in the water. Phosphorus has been a known problem in the Spokane River and Lake Spokane since the 1970s, and Ecology ties it to low oxygen levels. Wastewater plants have cut their phosphorus sharply, and Ecology says the largest remaining sources are nonpoint ones, led by Hangman Creek. Extra water is not harmless. It costs money to pump, and it can cost the river some of its water quality.

In my experience, most irrigators apply at least double the water their plants need, from large sites to homeowners who figure more is better. On any landscape, bringing that back to plant demand is the most dependable reduction available, and it does the most good in July and August, in the same weeks the river runs lowest. Giving up a healthy landscape is not the only answer. The goal is to apply what the plants use and stop applying what they cannot.

That is what the framework we call AIM does: Analyze, Implement, Manage. We start by mapping the area that actually gets water and building a monthly target from what the plants need, plus what a real system has to apply to overcome its own inefficiencies. Then we program the controllers to hit that target and manage it through the season, adjusting before the plants show stress. Without a target, you are guessing.

Where this leaves us

The dam followed its license, and it released more water than in drier years of the past. The aquifer is inside its historical range. A poor winter helped trigger the low release, but it does not explain a river that stayed wet in similar years. Seepage grew, and the reason is still open. When the Spokane Community College study comes out, I will read it and update this series.

Everything above comes down to one number for each site. If you manage a large irrigated site, the question to ask is how much water your site actually needs. This is not a guess or an estimate based on how much water you used last season. It is based on data. If you don't know your number, chances are you are over-irrigating.

At Irrigation Managers we call our approach AIM: Analyze, Implement, Manage. We map the area that is irrigated and use historical weather data to build a monthly target, then make real-time adjustments to match actual weather and manage the site to our full irrigation season target. Give us a call or send a message. There is no obligation, and we are glad to walk you through it. IrrMgmt.com. 1.800.473.7673.

Sources

Flow and well data are my own calculations from USGS records pulled October 7 and 8, 2026. Everything else is linked below.

• USGS Scientific Investigations Report 2007-5041 (Kahle and Bartolino), Table 2, pumpage 1990 to 2005.
• Ralston and Johnson, 2015, Idaho Water Resource Board (idwr.idaho.gov), summer pumping peak; Idaho Water Resource Board, Rathdrum Prairie Aquifer Water Demand Projections, July 2010.
• Spokane Aquifer Joint Board, 2023 SVRP Aquifer Atlas and 2025 MODFlow study (spokaneaquifer.org); City of Spokane presentation to the Spokane River Forum, 2025.
• Spokane Riverkeeper, "How Water Use Affects the Spokane River" (March 31, 2016), reporting an Ecology presentation by John Covert, spokaneriverkeeper.org, for the 2015 pumping example. An advocacy group reporting a secondary source.
• Spokesman-Review, "Dust in the wind," May 10, 2007 (grass seed acreage and the grower's yield estimate).
• Washington Department of Ecology, Spokane River and Lake Spokane Dissolved Oxygen Total Maximum Daily Load (approved by EPA May 2010) and its 10-year effectiveness study (2025), apps.ecology.wa.gov, for phosphorus and low oxygen.
• U.S. Bureau of Reclamation, Rathdrum Prairie Project (usbr.gov/projects, project 387).