Clark Fork near Galen

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CLARK FORK WATER-QUALITY MONITORING

Home Project Description Retrieve water-quality data Statistical summaries of water-quality data Supplemental sampling in lower Clark Fork basin Links    Reports Contacts

SUPPLEMENTAL SAMPLING IN THE LOWER CLARK FORK BASIN

In 2006, the USGS began a program in cooperation with the USEPA to collect water-quality samples during high flow (March-June) at six sites in the lower Clark Fork basin (fig. 1) from the area near Milltown Reservoir to the confluence with the Flathead River (near Plains). The sampling continued in 2008 after the breaching of Milltown Dam to document the substantial expected erosion of the bottom sediments from the former reservoir area. Supplemental sampling will be conducted again during March-June 2009.

The purpose of the supplemental sampling is to measure trace-element loads transported past various points in the basin to determine the relative contribution from individual sources such as reservoir bottom sediments, tributary basins, and intervening reaches of channel between mainstem sampling sites.  Two major rivers enter the lower Clark Fork in this approximately 120-mi reach: the Bitterroot and Flathead Rivers. The lower Clark Fork basin upstream from Plains drains an area of about 19,960 mi². The six sampling sites (listed below) are USGS streamflow-gaging stations.  Sites 1-3, which bracket the area of Milltown Reservoir, also are part of the upper Clark Fork long-term monitoring network.  Sites 4-6 provide additional spatial resolution on inputs from major hydrologic sources; the combined flow of the Clark Fork at St. Regis (site 5) and Flathead River at Perma (site 6) represents essentially the total flow of the Clark Fork in the reach near Plains.

1. Clark Fork at Turah Bridge, near Bonner  (12334550)
2. Blackfoot River near Bonner (12340000)
3. Clark Fork above Missoula (12340500)
4. Bitterroot River near Missoula (12352500)
5. Clark Fork at St. Regis (12354500)
6. Flathead River at Perma (12388700)

In 2008, a new sampling site was established on the newly constructed bypass channel between Clark Fork at Turah Bridge and Milltown Reservoir (Clark Fork Bypass near Bonner-station 12334570). Data from this site is intended to distinguish between loads derived from the basin upstream from Clark Fork at Turah Bridge and loads derived from previously deposited channel sediments in the reach within and upstream from the former reservoir area. These areas are expected to actively erode as the river adjusts to the steeper gradient after the breaching of Milltown Dam.

Water-quality and streamflow data are used to estimate instantaneous trace-element loads passing each site at the time of sampling.  These data are used to determine the trace-element loads transported during various flow conditions in order to assess the degree of scour or deposition of sediments through the breach of the former reservoir.  The data also are used to determine the percentage of the total load at the downstream end of the reach (below Flathead River) derived from individual upstream sources.  

Milltown Dam (breached in March 2008)

The sampling period of March/April-June was targeted to characterize the rising limb and peak of the annual snowmelt hydrograph, and is the time when transport of metals and suspended sediment is at a maximum. Sampling frequency is about once-weekly during the March-June period. Additional samples may be collected for large runoff peaks or unique conditions associated with reservoir operations. Samples are analyzed for the same constituents (trace elements, hardness, suspended sediment) as in the upper Clark Fork long-term monitoring program. Total nitrogen and total phosphorus also are analyzed to provide supplemental information for nutrient concerns in the lower basin.

The provisional water-quality data and estimated instantaneous loads for sites in the lower Clark Fork basin are transmitted to USEPA as results become available. These data are then transferred to an ftp site administered by USEPA and Envirocon, Inc. (ftp://milltown.envirocon.com) that provides information related to activities at Milltown Reservoir. A report describing the transport of suspended-sediment and trace-element loads through Milltown Reservoir during water years 2004-07 is available at http://pubs.usgs.gov/sir/2008/5080. A similar report is being prepared for water year 2008.