Clark Fork near Galen

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CLARK FORK WATER-QUALITY MONITORING

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Project Description - Clark Fork Long-term Monitoring Project

Study area

The Clark Fork originates near the town of Warm Springs in western Montana at the confluence of Silver Bow and Warm Springs Creeks (fig. 1). Along the approximately 150-mi reach of stream from Silver Bow Creek at Butte to the Clark Fork above Missoula, five major tributaries enter: Warm Springs Creek, Little Blackfoot River, Flint Creek, Rock Creek, and Blackfoot River. Water is impounded at two areas along the reach: the Warm Springs Ponds, which are a series of settling ponds near the mouth of Silver Bow Creek; and Milltown Reservoir, which was a small reservoir formed by Milltown Dam at the confluence of the Clark Fork and Blackfoot River. Milltown Dam was breached in March 2008 in preparation for complete removal, thus eliminating Milltown Reservoir after 100 years of existence (1908-2008). The Clark Fork basin upstream from Missoula (referred to as upper Clark Fork basin) drains an area of about 6,000 mi².

Background

Large-scale mining and smelting were prevalent land uses in the upper Clark Fork basin for more than one hundred years, but are now largely discontinued or operate at reduced scales.  Deposits of copper, gold, silver, and lead ores were extensively mined, milled, and smelted in the drainages of Silver Bow and Warm Springs Creeks near Butte and Anaconda from about 1870 to 1980.  Small- to moderate-scale mining also occurred in the basins of most of the major tributaries to the upper Clark Fork.

Berkeley Pit at Butte

Tailings derived from mineral processing commonly contain large quantities of trace elements such as arsenic, cadmium, copper, lead, and zinc, which potentially can be toxic to biota.  Tailings have been eroded, transported, and deposited in stream channels, on flood plains, and in the Warm Springs Ponds and Milltown Reservoir.  The widely dispersed tailings continue to be eroded and transported, especially during high flows.  As the result of the potential toxicity of trace elements to humans and aquatic biota, the USEPA designated areas in the upper Clark Fork basin as Superfund sites.

Exposed tailings on Clark Fork flood plain near Galen	Cross section of tailings deposit on Clark Fork flood plain near Galen

Long-Term Monitoring in the Upper Clark Fork Basin

The objective of long-term monitoring is to systematically characterize the full range of water-quality conditions, track changes over time, and disseminate information to the public.

Collecting a sample by wading

The current (2009) long-term monitoring network in the upper Clark Fork basin (table 1) consists of 19 water-quality stations, 21 streamflow-gaging stations, 4 daily-sediment stations, and 4 turbidity-monitoring stations. The aquatic resources are further characterized by data on trace-element concentrations in biota (aquatic insects) and bed sediment obtained at 13 sites by staff of the USGS National Research Program in Menlo Park, Calif.

The period of record for the various types of data collection in the upper Clark Fork long-term monitoring network is shown in table 1 of the most recent annual Clark Fork data report (Dodge and others, 2008; available online at http://pubs.usgs.gov/of/2008/1318 ). Methods of data collection also are described in that report.

Periodic Water Quality

Water-quality samples for the long-term monitoring program are collected 8 times per year at most sites in the upper Clark Fork basin; biota and bed sediment samples are collected one time per year (table 1). Periodic collection of water samples is distributed throughout the year over a wide range of hydrologic conditions and seasons (fig. 2).

Figure 2. Periodic collection of water samples, 2006 water year.

Water samples are analyzed for concentrations of the trace elements arsenic, cadmium, copper, iron, lead, manganese, and zinc, along with hardness and suspended sediment.  Samples are analyzed for chemical constituents by the USGS National Water Quality Laboratory (NWQL) in Denver, Colo. and for suspended sediment by the USGS sediment laboratory in Helena, Mont.

Daily Records

Daily streamflow records are obtained by continuous-recording gages at most of the sites in the network (table 1).  Real-time streamflow data are transmitted via satellite telemetry and can be viewed at http://waterdata.usgs.gov/mt/nwis .  Daily streamflow data are important because they can be used with the water-quality data to estimate instantaneous and annual loads of constituents.  Historical records of daily streamflow also provide a reference to assess current streamflow conditions relative to long-term averages.

Daily suspended-sediment records are obtained at selected sites (table 1) by collecting suspended-sediment samples at a frequency (about 2 to 14 times per week) that is sufficient to determine daily mean suspended-sediment concentrations.  Suspended-sediment concentrations can vary widely during various runoff, seasonal, and land-use conditions.  Because many trace elements adsorb onto sediment particles, transport rates of trace elements closely correspond to transport rates of sediment.  Therefore, accurate measurements of suspended-sediment loads at key sites in the basin provide a useful index for verifying mathematical estimates of sediment load, which then can be used for estimating annual loads of trace elements (see Lambing, 1991; Lambing, 1998; Smith and others, 1998; and Lambing and Sando, 2008, in the Reports section).

Daily turbidity records are obtained by continuous turbidity monitors operated at selected sites (table 1). Turbidity monitors transmit real-time information via satellite telemetry that can be viewed at http://waterdata.usgs.gov/mt/nwis . Turbidity values are qualitative optical measurements of light scattering that commonly correlate closely to suspended-sediment concentrations. Consequently, turbidity can indicate varying sediment conditions where information is needed rapidly for either management decisions or to trigger sampling activities.

Biota and Bed Sediment

Trace-element concentrations in biota (aquatic insects) and bed sediment indicate the accumulation of trace elements in biological tissue, as well as in the streambed component of the aquatic habitat where tailings can be deposited. Because aquatic insects are not mobile over large distances, the trace-element concentrations in their tissues are largely a function of exposure to trace elements in their immediate aquatic environment. Samples of aquatic insects and bed sediment are collected once-annually at selected sites (table 1) and analyzed for trace elements by staff of the USGS National Research Program in Menlo Park, Calif. The data for trace-element concentrations in biota and bed sediment supplement the water-quality data to aid in assessing the overall condition of aquatic resources in the upper Clark Fork basin. These data are published with the water-quality data in the annual Clark Fork data reports (http://pubs.usgs.gov/of/2008/1318). Additional information on biota and bed sediment sampling can be viewed at http://wwwrcamnl.wr.usgs.gov/tracel .

Sampling for aquatic insects using a kick-net method	Sorting aquatic insects by taxa to prepare samples for tissue analysis

2009 Project Plans and Related Activities

Data-collection activities in 2009 for the Clark Fork long-term monitoring program (table 1) are similar to those conducted last year. A new addition to the monitoring network in the upper Clark Fork basin is a sampling and streamflow-gaging site at Clark Fork above Little Blackfoot River near Garrison (station 12324400). This site was established to provide additional information on conditions in the Deer Lodge valley as remediation proceeds.

Supplemental sampling in the lower Clark Fork basin from near Milltown Reservoir to the confluence with the Flathead River (fig. 1) was conducted in 2006-08 and will be continued in 2009. See the Supplemental Sampling page for more information on these activities.

Maintenance and updates of this Web page will continue in 2009. This Web page will provide access to USGS water-quality and streamflow data, as well as links to other sources of USGS information. Monitoring activities will be described, along with future plans or changes to the current program. Statistical summaries of data for the most recently completed water year, as well as for the period of record, also will be provided to allow comparisons of recent data to a longer-term period. Analysis and interpretations of data will be provided periodically in formal USGS reports (see Reports).

Sampling at Clark Fork at Turah Bridge	Sampling high flow at Clark Fork at Turah Bridge

Sampling at Blackfoot River near Bonner	Suspended-sediment sampling equipment used by observer for daily sediment monitoring.