Abstract

This report presents estimated daily and cumulative loads of suspended sediment and selected trace elements transported during water year 2008 at three streamflow-gaging stations that bracket the Milltown Reservoir project area in the upper Clark Fork basin of western Montana. Milltown Reservoir is a National Priorities List Superfund site where sediments enriched in trace elements from historical mining and ore processing have been deposited since the construction of Milltown Dam in 1907. Milltown Dam was breached on March 28, 2008, as part of Superfund remedial activities to remove the dam and contaminated sediment that had accumulated in Milltown Reservoir. The estimated loads transported through the project area during the periods before and after the breaching of Milltown Dam, and for the entire water year 2008, were used to quantify the net gain or loss (mass balance) of suspended sediment and trace elements within the project area during the transition from a reservoir environment to a free-flowing river. This study was done in cooperation with the U.S. Environmental Protection Agency.

Streamflow during water year 2008 compared to long-term streamflow, as represented by the record for Clark Fork above Missoula (water years 1930-2008), generally was below normal (long-term median) from about October 2007 through April 2008. Sustained runoff started in mid-April, which increased flows to near normal by mid-May. After mid-May, flows sharply increased to above normal, reaching a maximum daily mean streamflow of 16,800 cubic feet per second (ft³/s) on May 21, which essentially equaled the long-term 10th-exceedance percentile for that date. Flows substantially above normal were sustained through June, then decreased through the summer and reached near-normal by August. Annual mean streamflow during water year 2008 (3,040 ft³/s) was 105 percent of the long-term mean annual streamflow (2,900 ft³/s). The annual peak flow (17,500 ft³/s) occurred on May 21 and was 112 percent of the long-term mean annual peak flow (15,600 ft³/s). About 81 percent of the annual flow volume was discharged during the post-breach period.

Daily loads of suspended sediment were estimated directly by using high-frequency sampling of the daily sediment monitoring. Daily loads of unfiltered-recoverable arsenic, cadmium, copper, iron, lead, manganese, and zinc were estimated by using regression equations relating trace-element discharge to either streamflow or suspended-sediment discharge. Regression equations for estimating trace-element discharge in water year 2008 were developed from instantaneous streamflow and concentration data for periodic water-quality samples collected during all or part of water years 2004—08. The equations were applied to records of daily mean streamflow or daily suspended-sediment loads to produce estimated daily trace-element loads.

Variations in daily suspended-sediment and trace-element loads generally coincided with variations in streamflow. Relatively small to moderately large daily net losses from the project area were common during the pre-breach period when low-flow conditions were prevalent. Outflow loads from the project area sharply increased immediately after the breaching of Milltown Dam and during the rising limb and peak flow of the annual hydrograph. Net losses of suspended sediment and trace elements from the project area decreased as streamflow decreased during the summer, eventually becoming small or reaching an approximate net balance between inflow and outflow.

Estimated daily loads of suspended sediment and trace elements for all three stations were summed to determine cumulative inflow and outflow loads for the pre-breach and post-breach periods, as well as for the entire water year 2008. Overall, the mass balance between the combined inflow loads from two upstream source areas (upper Clark Fork and Blackfoot River basins) and the outflow loads at Clark Fork above Missoula indicates net losses of suspended sediment and trace elements from the project area during all periods in water year 2008.

Of the 510,000 tons of suspended-sediment outflow load transported past Clark Fork above Missoula in water year 2008, only 119,000 tons were contributed from the two upstream source areas; thus, 391,000 tons of the outflow load were derived from eroded sediment within the project area. The suspended-sediment load contributed from the project area represented about 77 percent of the outflow load, with the upper Clark Fork basin contributing about 11 percent and the Blackfoot River basin contributing about 12 percent. Most of the large annual net loss of suspended sediment (-391,000 tons) from the project area occurred during the post-breach period (-375,000 tons), which was more than 22 times greater than the load contributed during the pre-breach period; the net loss during the post-breach period represented about 96 percent of the annual net loss for the entire water year 2008.

The large annual net loss of suspended sediment from the project area in water year 2008 resulted in large annual net losses for every trace element. Net losses of trace elements from the project area were about 7 to 20 times greater during the post-breach period compared to the pre-breach period. The largest annual net losses of trace elements from the project area, in percent of the outflow load transported past Clark Fork above Missoula, occurred for cadmium, copper, lead, and zinc—about 82 percent for cadmium (-0.938 tons), 84 percent for copper (-157 tons), 78 percent for lead (-22.5 tons), and 84 percent for zinc (-245 tons). The large annual net losses of trace elements in water year 2008 indicate that much of the outflow trace-element load transported past Clark Fork above Missoula was derived from eroded sediment within the Milltown Reservoir project area, rather than from inflow loads transported from upstream source areas.

Posted May 7, 2009