According to Jeff Neal, ODFW fish biologist in John Day, water temperatures in the river rose from 62 degrees on June 26 to a high of 74 degrees the next day. By the first of July the water temperature on the Middle Fork had climbed to over 78 degrees.
“Wild adult chinook can survive temperatures as high at 80 degrees if temperatures rise slowly, but a sudden increase of 12 to 15 degrees is just too much for them,” Neal said. “A similar temperature spike in 2007 killed 118 chinook but this only happens during drought years.”
The fish kill was first discovered by ODFW staff on July 2 when four dead chinook were found near Windlass Creek. A subsequent survey of the 22 mile reach between Galena and Highway 7 found the additional mortalities and 113 chinook that were still alive. The dead fish showed no evidence of having died from a pollution discharge, toxic spill or predation. The number of chinook killed represents approximately 60 percent of the estimated number of chinook in the Middle Fork.
The Middle Fork’s lack of riparian vegetation combined with a wide and shallow channel exposes so much of the river to direct sunlight that water temperatures can rise quickly, Neal said.
“These kinds of temperature spikes will continue to happen until we restore a greater length of the chinook habitat by narrowing the channel, deepening the pools and growing more trees to provide shade,” he added.