October 3, 2006
Water quality in the Clark Fork River below Milltown dam was monitored daily for turbidity (the “cloudiness” of the water) and weekly for suspended sediment, metals, and arsenic. Upstream sites on the Blackfoot River and on the Clark Fork at Turah were also monitored weekly for metals, arsenic, and sediment.
Starting on September 18th and continuing until the end of the month, the reservoir level was lowered by another foot and a half for a total of almost 11 feet. This drawdown was a continuation of the first drawdown that began June 1st, but was suspended in early July due to mortality of caged fish and high river temperatures.
In early September, turbidity and sediment levels below Milltown Dam remained low, along with very low river flows. Levels of dissolved metals and arsenic were also low during this period and typical of the levels usually seen in the river at this time of year. After the drawdown resumed, turbidity below the dam increased somewhat, but did not exceed 12 NTU, which is the trigger value for daily metals sampling. In the second half of September, turbidity did exceed 6 NTU several times, resulting in turbidity sampling three times per day instead of once per day. Dissolved arsenic increased slightly at both the Turah station (upstream of the dam) and at the monitoring station below the dam, and dissolved zinc increased slightly below the dam. However all arsenic and dissolved metal concentrations were well below the applicable water quality standards and warning limits throughout the month.
Montana Fish, Wildlife & Parks biologists are continuing caged fish studies with the resumption of drawdown, but results are not yet available. If you see one of the cages, please don’t disturb it. This study supplies critical information for monitoring effects of the Milltown project.