Evaluation of Lower Umatilla River Channel Modifications Below Three Mile Dam, 1985




channel, channel modification, Three Mile Dam, Umatilla River, adult, salmonids, chinook, salmon, Oncorhynchus tshawytscha, weir


The second year of a study to evaluate passage of adult anadromous salmonids through channel modifications made in the lower Umatilla River below Three Mile Dam was terminated due to inadequate returns of upriver bright fall chinook salmon (Oncorhynchus tshawytscha), Sampling of returning adults was discontinued on November 15, when it became apparent that insufficient numbers of fall chinook salmon were returning to the river to allow the evaluation. Stream flows were monitored through December 31 to document flow conditions present during the period when returns of fall chinook salmon were anticipated. This report summarizes results prior to termination of the study. Arrival of upriver bright fall chinook salmon at the mouth of the Umatilla River was monitored by boat electrofishing below river kilometer (RKm) 1. Attempts to examine passage of salmon through channel modifications were made using marked and radiotagged fish. Forty-eight jack and six adult salmon were marked and released above a weir placed at RKm 2. Five of the adults were also fitted with radiotransmitters. Thirty fall chinook were collected while boat electrofishing at the mouth of the Umatilla River. Fifteen of those collected were captured between October 14 and October 18, after flows had reached 150 cfs. One radiotagged salmon migrated to the dam at flows of 245 cfs. Only one marked jack salmon was recovered at Three Mile Dam.

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