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Commentaries and editorials

Lower Snake Temperatures

by Rob Masonis
Columbia Basin Bulletin - May 9, 2003

This is my final response to Mr. McKern in the CBB.

At the outset I want to set the record straight: my criticism of Mr. McKern has been directed solely at his interpretation of the scientific evidence regarding the efficacy of removing the four lower Snake River dams - not at his professional qualifications. Mr. McKern should not read more into my comments (e.g., his statement that I do not consider him to be a scientist).

Our debate regarding whether salmon and steelhead mortality caused by the lower Snake dams has been largely eliminated (Mr. McKern contends that it has) has come full circle. He once again cites data showing that salmon survival through the hydro system has improved since the 1970s to support this contention. For the reasons I've presented previously, a more complete analysis of the evidence indicates that the dams are still the primary bottleneck and that transportation of juveniles is not the solution.

Mr. McKern accurately points out that when the lower Snake River was free-flowing it reached instantaneous maximum temperatures that exceeded those in the lower Snake reservoirs, but that is not the primary temperature problem. The primary problem is that the dams maintain elevated water temperatures for several months in the late summer and fall relative to free-flowing conditions.

It is a matter of simple physics. In a free-flowing river, water temperature is largely determined by ambient air temperature because water molecules come into frequent contact with the air. In contrast, reservoirs have a high volume-to-surface ratio, which makes water temperature less responsive to changes in air temperature. Thus, reservoir water temperatures remain elevated long into the fall. (The opposite happens in the spring). Snake River salmon and steelhead did not evolve in these conditions, and high temperatures harm salmon in many ways, including increasing predation and the incidence of disease.

I wish it were true, as Mr. McKern contends, that cold water releases from Dworshak dam create a cool water refuge in the depths of the lower Snake River reservoirs, but it is not. The data show that the relatively shallow reservoirs do not stratify. Nonetheless, Dworshak releases do help reduce water temperatures - particularly in the upper reaches of the lower Snake - but only when those releases occur, which happens only during a small portion of the period in which high temperatures are problematic. The bottom line is that Dworshak releases do not solve the temperature problem in the lower Snake, but dam removal would.

Rob Masonis, Director, Northwest Regional Office, American Rivers
Lower Snake Temperatures
Columbia Basin Bulletin, May 9, 2003

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