The Columbia River hydro system hit a seasonal inflection point of the spill year this week. As we flagged in last week's PNW Hydro Flash, Tuesday the 16th marked the transition from 2026 spring spill operations to summer spill rules at the four Lower Columbia (LC) projects — McNary, John Day, The Dalles, and Bonneville — with the Lower Snake dams set to follow this Sunday, the 21st. The switch does the opposite of what we saw back in April at the start of the spring spill period: rather than pulling water off the turbines for juvenile fish passage, a portion of the water being spilled gets handed back to generation. The result showed up right away in the numbers.

Figure 1 | Lower Columbia Hourly Generation Flows and Spill (kcfs) by Project

Figure 1 plots actual hourly generation and spill flows (kcfs) at the four LC dams over the past two weeks. The yellow line indicates where spill would sit under summer rules rather than the spring spill that has been in place leading up to this Tuesday. The blue area riding above that yellow line represents the volume of water that spring spill had been sending over the spillways and that summer rules redirect to the powerhouse. As the regime flipped on the 16th, actual spill at McNary and John Day collapsed down toward the summer line, while The Dalles — which the summer program barely touches — held steady, and Bonneville kept its flat 95 kcfs spill throughout. The largest reallocations, as expected, landed at McNary and John Day.

Figure 2 puts a number on the payoff. Daily average LC generation had been grinding lower through the first half of June, sliding from around 1,780 MW on the 8th into the 1,340–1,460 MW range from the 9th through the 14th as overall system flows ebbed, before ticking up to roughly 1,510 MW on the 15th. Then summer spill arrived: generation jumped to approximately 2,010 MW on the 16th, a gain of right around 500 MW day-over-day. That is squarely in line with the roughly 500 MW bump the LC dams picked up when summer rules kicked in over each of the past two summers.

Figure 2 | Lower Columbia Daily Average Hydro Generation (MW)

The year-over-year picture adds some color. Through the first half of June, 2026 LC output had been running well under both 2024 (~2,100 MW for the month-to-date) and 2025 (~1,880 MW), a reflection of a system that, while still healthy, is past the peak of the spring melt. Tuesday's bump pulled 2026 right back up alongside the mid-June 2024 and 2025 marks near 2,000 MW.

It is worth remembering the bigger backdrop here. Spring spill knocked LC generation from roughly 3.8 GW down to 1.6 GW back in April, so this week's lift, while welcome, is modest by comparison. It also arrives as the broader system softens — the Jan–Jul ESP10 forecast at The Dalles has slipped toward 93.0 MAF and Grand Coulee inflows are down about 21 kcfs since the first of the month. The summer transition will not reverse that decline, but it should slow it. And with summer rules holding through the end of August this year — a month longer than in 2024 and 2025 — the LC dams will carry that incremental generation deeper into the high-demand season.

The next phase of summer spill is slated to kick in at the end of the week as the Lower Snake dams join the Lower Columbia in switching to the less stringent summer spill regime.

For daily coverage of Columbia and Snake river operations, spill rules, and water supply, check out our PNW Hydro content as part of our regional Enterprise Power packages or reach out to us at [email protected].