This article was published on: 08/22/20 12:00 AM
Low flows could be caused by debris backups, gate malfunctions
By Michael Kohn
Sudden drops in the flow of the Deschutes River have been registered several times this week, causing concern among biologists for the health of fish along the middle stretch of the river.
The Deschutes River flows at a rate of 115 to 125 cubic feet per second this time of year below the North Canal dam near N. Division Street in Bend. Three times this week the flow of the water plummeted to less than 60 cubic feet per second for brief periods of time before returning to normal levels.
Sudden drops in the level of the Deschutes can leave fish and other aquatic creatures stranded in shallow pools or even out of the water. The flow of the Deschutes is regulated by the dam at Wickiup Reservoir, but disruptions in the flow can occur downstream at dams or diversions.
The drop Monday reached as low as 48 cubic feet per second. The Tuesday drop was down to 52 cubic feet per second, and the Wednesday drop was down to 57 cubic feet per second, according to data on the U.S. Bureau of Reclamation website.
The reason for the sudden drop in river flow is being investigated by the irrigation districts that use water from the Deschutes.
Mike Britton, general manager for North Unit Irrigation District, agreed that the most likely possibility for the disruption is the presence of large amounts of aquatic weeds and other floating material in the river that gets caught at diversions.
“When debris (aquatic weeds) build up on diversions, head builds up, and water gets pushed back into the river,” said Britton. “When the diversions are cleaned of the debris, the water gets pulled back into the diversions. River response to these actions is not instantaneous so the river ebbs and flows as the head in the river changes.”
Head is the pressure that forms at a diversion when back-ups occur. Britton said the problem often occurs late in the irrigation season.
Britton said the irrigation districts are working together with the Oregon Water Resources Department to minimize the fluctuations.
Kyle Gorman, region manager for the Oregon Water Resources Department, was advised by the districts that the debris build-up occurs at fish screens at the head of the canals where they connect to the river. When the screens are cleaned, a surge of water enters the canal causing headgate adjustments. Those adjustments create dips in the flow of the river.
“The system is set up to be automated but is not capable of handling quick, large changes,” said Gorman.
Britton has not ruled out other possibilities for the erratic river flows. Fluctuations can occur when PacifiCorp modifies operations at its power plant in Bend, he said. Disruptions can also occur when wave adjustments are made at the whitewater park in Bend.
Shon Rae, deputy managing director for Central Oregon Irrigation District, confirmed that the COID diversion has been affected by aquatic material and the district is working to resolve the matter. Rae added that deliveries to customers have also been curtailed to 80% of normal recently due to low river flows and that number will likely be trimmed to 75% next week.
The Oregon Department of Fish and Wildlife was not immediately able to confirm the impact of the river level drops on fish populations.
“The sudden reduction in flows undoubtedly led to some level of stranding,” said Brett Hodgson, Deschutes District fish biologist. “To what degree, it’s hard to say.”
Hodgson added that the flow reduction also came at a time when fish populations are already stressed from low flows and warm temperatures in the river. Juvenile fish that occupy shallower habitats near the banks are most vulnerable to stranding, he said.
“The impacts are cumulative,” said Hodgson. “A stressed fish faced with rapidly changing environmental conditions will have compromised health and potential mortality. This may not show up right away, rather they may (have) latent impacts. We will attempt to do some sampling early next week and see if we can do at least do a qualitative evaluation.”
Reporter: 541-617-7818,
mkohn@bendbulletin.com