There is no doubt that breaching the dam sent a large pulse of sediment into the river this spring, and the pulse continued with the 3-year flood flows we saw this year.  To put this into perspective, these charts compare levels of metals and sediment in river water this spring with past sediment-generating events (red bars).  The samples for these data were collected downstream of the Milltown dam, near the Bandmann bridge, just upstream of East Missoula.  The blue bars represent two different modeling scenarios - predictions of how much sediment would enter the river if the dam failed (modeling done in 1994) and after Stage 2 drawdown of the reservoir (modeling done in 2004 for dam breaching under the original spillway-first scenario).  You will need to download each image in order to read the charts most fully.

Thanks to Missoula County Water Quality District for these charts

Dissolved Arsenic

Arsenic is less harmful to fish than it is to people, and the federal drinking water standard is 10 micrograms per liter (1 ug/L is equivalent to 1 part per billion – ppb).  The dissolved form of arsenic is measured by filtering a water sample from the river in order to exclude sediment.  Arsenic exceeded the drinking water standard on the day following the breach, likely because reservoir sediments were rapidly drained when dam breaching lowered the water level by another 10 feet or so.  After the breach and during peak flow, levels were slightly higher than usual, but below the standard.  Levels were back to average once flows diminished.

Dissolved arsenic concentration

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Dissolved Copper

Unlike arsenic, copper is extremely toxic to fish at levels that are not harmful to people.  Like dissolved arsenic, it’s measured from a filtered water sample in micrograms per liter (ug/L – equivalent to parts per billion).  Dissolved copper never exceeded the one-hour construction standard during the breach, or afterwards during peak flow.

Dissolved copper concentration

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Total Arsenic

Total arsenic in the river is measured by taking an unfiltered water sample and acidifying it to release any weakly-bound arsenic from sediment particles, so it’s a combination of dissolved plus sediment-bound arsenic.  It increases when the river is muddy, in concert typically with total suspended sediments (see below).  There are no standards for total arsenic, but concentrations during peak flow this year were high, and likely deleterious to aquatic life.  Levels were lower, however, than during the 1996 ice scour event.

Total arsenic concentration

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Total Copper
Total copper behaved much like total arsenic – levels were very high during peak flow this year.  Although difficult to prove, these levels may be partially responsible for the stressed condition of trout that stayed in the Milltown reach.  But levels were not as high as the fish-killing 1996 ice scour event.

Total copper concentration

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Total Suspended Sediment (TSS)

Just as the name suggests, this is the total amount of silt, sand, and debris that is carried in suspension in the river.  It’s measured in milligrams of sediment per liter of river water (mg/L), equivalent to parts per million.  Too much sediment in water is tough on fish and other aquatic life.  It physically harms fish gills, and murky water makes it hard for fish to find food.  During the Milltown project, TSS is sampled weekly (although a surrogate - turbidity - is sampled daily).

On March 28^th, the day of the breach, TSS exceeded the daily construction standard. By the following day, TSS was below the standard but still very high.  It was less than what was observed during the ice-jam event of February 1996.  TSS continued to decline during the long runoff period this year – in fact, during peak flow in the river in late May, TSS was substantially less than during peak flow in 1997 (a 10-year event).

Total suspended sediment concentration

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