A week ago we wrote up the first leg of the seasonal spill transition as the four Lower Columbia projects flipped from spring to summer fish spill operations on the 16th. This past Sunday, the 21st, the second leg arrived: the four Lower Snake dams — Ice Harbor, Little Goose, Lower Monumental, and Lower Granite — made the same switch, putting the entire lower system fully into summer spill. As our latest PNW Hydro Discussion framed it, the system is now "full into summer," with Grand Coulee's June refill nearly complete and a round of heat set to give way to cooler temperatures by the weekend.

The Lower Snake switch played out differently than the Lower Columbia's, though. Figure 1 plots hourly generation and spill (kcfs) at the four LS projects over the past two weeks, with the yellow line indicating required spill under summer rules. The story here is dominated by flat spill volumes: Lower Monumental holds steady near 17 kcfs and Lower Granite near 18 kcfs across the entire window, so summer rules barely move them. Ice Harbor and Little Goose ease lower as river flows recede, with actual spill stepping down toward the summer line as the new regime takes hold heading into the 24th. Because the Snake's spill volumes are smaller, especially after the early spring runoff and dry water year for the basin, there is simply less water to shift back to the powerhouse than there was on the Columbia.

Figure 1 | Hourly Actual Generation Flows and Spill vs Summer Spill Rules (kcfs)

Figure 2 shows the muted payoff. Daily average Lower Snake generation had been sliding through the back half of June as the spring freshet faded, dropping from roughly 525 MW on the 17th to a June low near 400 MW on the 20th. Summer spill then provided a lift: output climbed back to about 510 MW on the 21st, roughly 640 MW on the 22nd, and held near 625 MW on the 23rd — a gain of a little over 200 MW off the trough. That is a real bump, but a more modest one than the roughly 500 MW the Lower Columbia picked up last week, and it is partly working against the tide of receding flows.

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

The year-over-year picture frames the season. Even with the bump, 2026 Lower Snake output in late June is running below where it sat in both 2024 (near 800–880 MW) and 2025 (above 1,000 MW), a reflection of a freshet that is fading to a lower base on this arm of the system this year.

The bigger backdrop has not changed. With summer rules holding through the end of August — a month longer than in 2024 and 2025 — both the Columbia and Snake will carry this incremental generation deeper into the high-demand season, and Grand Coulee's completed June refill leaves added flexibility to lean on as heat events come and go. The cooler air arriving this weekend takes some of the near-term pressure off the system.

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