U.S. Army Corps of Engineers water managers expect an atmospheric river that’s dumping heavy rain in the Pacific Northwest will help refill some reservoirs this week.
Since the start of refill season for Corps reservoirs, there has been very little rainfall. Reservoirs in the Willamette and Rogue river basins rely on rainfall to refill and generally start filling Feb. 1.
“While every drop of precipitation helps right now, the atmospheric river appears to be impacting areas north of the Willamette Valley more significantly,” said Erik Petersen, operations project manager for the Willamette and Rogue river basins. “We may catch up a little, but we would have preferred this front had expanded further south – it could have helped a lot more. We’re in the middle of refill season for the Willamette and Rogue – we still have time before summer, but we sure need rain.”
The National Weather Service estimates the atmospheric river will last until Tuesday evening, March 1, then continue after a short break.
Year-to-date precipitation across the Willamette was 93% of normal, as of Feb. 28. The Rogue saw worse precipitation at 73%. Most of this rain fell prior to the start of refill season or fell as snow.
The snowpack is 106% of median for the Willamette and 75% of median for the Rogue. Snowmelt helps keep reservoir elevations up in the summer if it lasts and matches outflows – but most years it accounts for a relatively small portion of the system’s storage.
The Corps manages reservoir inflows based on a “rule curve,” or the authorized maximum elevation on a given day to balance flood risk and storage for authorized purposes. Regulators keep both systems’ reservoirs lower in the winter to reduce downstream flooding and refill them in the spring to prepare for recreation and adequate flows for fish.
As of Feb. 28, the Willamette Valley Project’s 13 reservoirs, which include Foster and Green Peter, are currently 9% full, the Corps reported. System-wide reservoir storage is 36% below the rule curve.
View the Willamette River Basin teacup diagram: http://www.nwd-wc.usace.army.mil/nwp/teacup/willamette or /web.crohms.org/nwp/teacup/willamette.
Corps officials encourage the public to visit the Portland District website to check the “teacup diagrams” before heading out to recreate.