Upper Snake River managers are watching the basin’s important east side, where headwater areas so far have seen comparatively less benefit from recent Pacific storms.
Snowpack to date for the Oct. 1 water year is looking good, but total precipitation in the east is trailing the other smaller sub-basins, said Brian Stevens, area water operations manager for the U.S.Bureau of Reclamation’s Upper Snake Field Office in Heyburn, Idaho.
“The eastern side of the Upper Snake basin is by far the largest contributor to runoff volumes in the system, and we still need above-average precipitation for the rest of the winter and spring — into late June — to have potential to fill the system,” he said.
The headwater sub-basins also are important for collecting water that infiltrates the ground and returns to the river over time, Stevens said.
Jackson in Wyoming is the farthest upstream of the Upper Snake’s seven main reservoirs, followed by Palisades and American Falls in Idaho. They also are the biggest.
Jackson and Palisades historically require “a very strong runoff year” to fill, Stevens said. So far, Jackson is 21% full, Palisades is 23% full, and the whole Upper Snake system is 36% full.
A recent string of Pacific storms boosted snowpack and total precipitation in the Upper Snake. But distance and topography reduced the storms’ impact to an extent in the system’s most remote upper reaches.
“The western and southern sides of the basin are looking fairly strong and the eastern side is trailing behind,” Stevens said. “Once we get to June, we would need probably 120% of total precipitation to fill the system.”
Reclamation is “cautious on how good the water year is looking,” he said, “primarily due to scenarios like last year, when we experienced prolonged dry spells.”
