State biologists are blaming a combination of unusual conditions for the deaths of about 3,000 fish last week in a side channel of the Deschutes River upstream of Bend.
Among the dead fish were redband rainbow trout, a species the state lists as sensitive.
Among the conditions the biologists cite are heavy water years in 2011 and 2012. They say that meant the side channel didn't completely dry up this year and there were more fish in it than usual.
Flows in the Deschutes drop in the fall when releases from the Wickiup Reservoir south of Bend are slowed to allow water to be collected for irrigation the next year.
But the biologists for say the reduction this year wasn't as rapid as in previous years.