Mining History in the Clark Fork Drainage Basin

The Butte mine and the ore smelter at Anaconda, Mont. (fig. 1), once constituted the largest copper-producing complex in the United States, operating from about 1870 to 1982 (table 1; Gammons and others, 2006). Increasing energy costs and decreasing copper prices since the early 1980s reduced the mining industry in Montana, however, leaving the present complex marked by an environmental and urban renaissance in the cities of Butte and Anaconda (Clark Fork Watershed Education Program, 2022).

Mining at the headwaters of the Clark Fork began in 1864 when gold was discovered in streambed deposits, and small-scale placer mining (separating heavily eroded minerals from sand or gravel streambed deposits) began in and near Butte, Mont. (Hoffman, 2001; Clark Fork Watershed Education Program, 2022). Prospectors at this time also found silver in abundance while searching for the lode source of the placer gold (Hoffman, 2001), and the first silver strike, discovered in 1865, marked the beginning of Butte’s successful silver mining episode, which peaked around 1887 (Chadwick, 1982). Many of the silver lodes mined at the time included copper veins (mostly in the form of sulfide minerals), but copper was in low demand because it was not as profitable as gold and silver. By 1893, the Northern Pacific Railroad arrived in Butte, which helped to lower the cost of supplies and provided a cheaper avenue to ship high-grade ore in time for the dawn of the electric age (around 1882) and the need for copper wire (Jenkins and Lorengo, 2002).

While working for a silver mining company in Butte, Marcus Daly discovered the largest copper sulfide deposit in America in 1880 (table 1) and began mining the extensive copper ore (Morris, 1997), which consisted of arsenic, copper, lead, and zinc (Lambing, 1991). By 1884, four large smelters were operating around Butte, and the world’s largest metallurgical plant was under construction in Anaconda, 30 miles west; by 1886, about 10 major ore-processing mill and smelter operations were along Silver Bow Creek (fig. 1; EPA, 2005). In 1902, 17 to 20 percent of all copper production in the United States came from Butte (Jenkins and Lorengo, 2002). To supply hydroelectricity to his sawmills in nearby Bonner, Mont., William A. Clark, one of the three “Copper Kings,” constructed the earth-fill hydroelectric Milltown Dam in 1907 about 7 miles east of Missoula (fig. 1) at the confluence of the Blackfoot River with Clark Fork (Clark Fork Watershed Education Program, 2022). However, when the dam was just months old, a record flood on the Clark Fork washed extensive (about 5.0 million cubic meters, m³) mining sediment contaminated with arsenic, lead, zinc, copper, and other elements downstream, where it settled at the base of the dam; the reservoir accumulated tailings-laden sediment for more than 80 years (Lambing, 1991). The dam was removed in 2008, and the Clark Fork channel was reconstructed in 2010 (EPA, 2021).

During the 1890s, mines around the world, including those in Montana, were consolidated under a single owner, the Anaconda Copper Mining Company (AMC) (Toole, 1950). In the 1920s, the primary milling and smelting facilities near the Clark Fork headwaters were moved to Anaconda (table 1), where most of the copper ore from the Butte area was processed (EPA, 2005, 2010). The small town of Butte became one of the most prosperous cities in the country, called the “Richest Hill on Earth,” and the Anaconda mine was the largest copper-producing mine in the world, producing more than $300 billion worth of metal in its lifetime. By 1917, more than 150 mines were in and near Butte (EPA, 2005). From 1910 through the 1920s, AMC acquired most of the mines in the Butte area. However, after severe financial setbacks during the Great Depression and after World War II and falling copper prices that continued to drop through the 1950s, the Anaconda mine closed in 1947 after producing 94,900 tons of copper (Malone, 2006). Its location later became part of the Berkeley Pit north of Butte in 1955, when AMC mining operations began to transition from underground to open-pit mining of a lower grade, porphyry-copper-style deposit.

Underground mining continued until 1976; however, by the late 1960s, the primary focus was on open-pit mining taking place in the Berkeley Pit (fig. 1, table 1) (Gammons and others, 2006). The Atlantic Richfield Company (ARCO) acquired AMC in 1977 (EPA, 2004); AMC remained a subsidiary and continued to function as AMC (Gammons and others, 2006), and the East Berkeley Pit operated until July 1, 1983, when AMC suspended their entire operations (Gammons and others, 2006). The pumps used to dewater the underground mines and Berkeley Pit operated until April 23, 1982, and ARCO announced they were suspending their Butte operations (except for the East Berkeley Pit) on July 1, 1982 (Duaime and McGrath, 2019). When the underground pumps in the Kelley Mine shut down, the underground mines and the Berkeley Pit began to fill with acidic water (Duaime and McGrath, 2019).

Environmental Legacy of Industrial Mining

After 100 years of mining activity, Silver Bow Creek was a toxic stream with little to no aquatic life. Extensive deposition of mining wastes in the channels and floodplains had adverse effects on water quality and severely affected local aquatic ecosystems by affecting primary and secondary production, nutrient cycling, energy flow, and decomposition (Younger and others, 2004; Knott and others, 2009; Batty and others, 2010). Substantial mine waste deposits collected on the Clark Fork floodplain, all vegetation was gone, and in some places (such as Deer Lodge, Mont.), streamside tailings deposits (locally known as “slickens”) created the appearance of a barren, “slick,” wasteland (Rader and others, 1997).

Early efforts to retain the mine tailings in ponds were often small in scope or unsuccessful. Andrews (1987) estimated that 100 million tons of tailings containing large quantities of heavy metals and acid-producing pyrite were dumped or eroded directly into Silver Bow Creek and in the upper Clark Fork between 1880 and 1982. Mine tailings, commonly 90 percent of the ore separated by milling and flotation, contained concentrations of arsenic, cadmium, copper, lead, and zinc that were 10 to 100 times expected background values (Andrews, 1987). As they traveled downstream, tailings mixed with streambed sediments and were deposited as alluvium that became a secondary source of contamination to the Clark Fork (Moore and Luoma, 1990). The finer-grained deposits were continually resuspended and transported downstream by bed scour, lateral channel cutting, and overland erosion. Soluble salts within the deposits were flushed directly into the streams by surface runoff or leached through the floodplain soils to the alluvial aquifer (Lambing, 1991).

Flooding and Erosion

To further compound the contamination problem, erosion, runoff, and large floods during the 20th century transported and dispersed metal-rich tailings more than 402 km downstream. Eroded tailings mixed with stream sediment and deposited farther downstream in channels, on floodplains, and at the location of the former Milltown Reservoir (fig. 1), an impoundment about 200 km downstream from the mining area near Missoula, Mont., that existed from 1907 to 2008.

The largest flood on record for the Clark Fork and Blackfoot River drainage basins occurred in June 1908, when a warming trend resulted in heavy rains that fell on snow and frozen ground, just months after completion of the Milltown Dam (Smith and others, 1998). Silver Bow Creek, unlike the original creek observed in 1864, was highly constricted, and upstream wetlands were drained to accommodate construction of smelters, so little prevented mass runoff and flooding. On June 6, 1908, the Anaconda Standard reported “entire Montana now paralyzed by destructive floods,” and every bridge in Missoula County (not shown) had washed out (Clark Fork Watershed Education Program, 2022). Vast quantities of toxic mine tailings, enough to cover more than 4 square kilometers (km²) (Andrews, 1987), spread throughout the drainage basin and piled up behind the Milltown Dam.

Based on newspaper reports and other sources, Wheeler (1974) determined that floods with magnitudes between that of the 1908 flood and the smaller floods of 1899 and 1902 occurred in 1887, 1892, and 1894. These floods spread enormous volumes of mine waste concentrated around Butte, Anaconda, and Silver Bow Creek down the Clark Fork, leaving thick deposits (as much as 2 meters in 1908) of metal-rich tailings on the floodplain, particularly between Warm Springs and Garrison (EPA, 2004), and commonly 0.3-meter-thick deposits were left along the Clark Fork in the Deer Lodge valley (Nimick and Moore, 1991). The influx of enormous amounts of sediment to the river system plugged streambeds, causing further flooding in subsequent storms and deposition of contaminants on the surrounding floodplain. By the mid-1980s, nearly 4 km² of barren, tailings-contaminated slicken areas were present in this section of Clark Fork (Moore and Luoma, 1990). These slicken areas subsequently released metals, such as copper, that were remobilized into small pools during rainstorms (fig. 2) (Moore and Luoma, 1990).

Cleanup and Remediation

Initial efforts to decrease the quantity of tailings discharged directly into the river and to mitigate risks of heavy metal exposure to aquatic life and humans included the construction of three settling ponds on Silver Bow Creek (designated near Warm Springs) between 1911 and 1959 to capture tailings eroded from the upper part of the Clark Fork drainage basin (EPA, 2004). During the Warm Springs Pond Remedial Investigation, the EPA estimated that more than 14 million m³ of sediments were contained in the three settling ponds (EPA, 1990), preventing substantial quantities of mining and milling wastes from moving downstream into Clark Fork. Beginning in 1975, lime was added at the inlet of the ponds to induce precipitation of metals and thereby increase settling efficiency (Lambing, 1991). These ponds decreased the amount of contaminated sediments reaching the upper Clark Fork from Silver Bow Creek, but part of the trace element-rich sediments continued to move through the river system (Phillips, 1985). Large-scale mining operations in Butte ceased in 1982, but widespread public concern over the effects of mine tailings throughout the upper Clark Fork Basin persisted. In response, the EPA designated three areas affected by mine tailings from Butte to Milltown Dam as National Priority List. These areas included the Silver Bow Creek/Butte Area site, the Anaconda Smelter Site, and the Milltown Reservoir Sediments site, which were all established in 1982. The entire length of the Clark Fork from the confluence of Warm Springs and Silver Bow Creeks to Milltown Reservoir was incorporated within the Milltown Reservoir site and was designated the “Clark Fork River Operable Unit,” which includes more than 140 river miles.

Federal Superfund remediation activities began in 1983 and have included substantial remediation near Butte and the removal of the Milltown Dam near Missoula in 2008. Many dumps were capped with clean topsoil and revegetated to hold the waste in place, preventing it from eroding back into Silver Bow Creek, seeping into groundwater, and (or) blowing away in high winds (Clark Fork Watershed Education Program, 2022). Further remediation activities included in-place stabilization and neutralization of floodplain tailings, tailings removal, construction of dikes at the inlet to the Warm Springs ponds, construction of berms (low earthen dikes about 30 centimeters, cm, high) along streambanks and around floodplain tailings deposits from Warm Springs Creek to Deer Lodge, and planting vegetation in amended soils to provide stability against erosion (EPA, 2020). Small-scale demonstration projects, which focused on in-situ treatment of soils in the riparian zone and bank stabilization, also have been conducted (Pioneer Technical Services, 2002). These efforts have improved the efficiency of the Warms Springs treatment ponds, ongoing cleanup of Silver Bow Creek, and other activities completed in the Butte area.

Remediation in Clark Fork was implemented to mitigate some of the more acute sources of contamination (Pioneer Technical Services, 2002; EPA, 2004), and, to date (2023), all activities have been spatially restricted to the upper 45 km of the river. In consultation with the EPA and the Montana Department of Environmental Quality, the ARCO prepared major parts of the final Clark Fork Operational Unit Remedial Investigation and Feasibility Study, completed several in-situ demonstration projects and streambank stabilization projects, and conducted a Time Critical Removal Action at Eastside Road in Deer Lodge (EPA, 2004). To reduce the tailings deposited into the river, large berms were constructed between 1989 and 1990, and, in December 2004, the EPA issued a record of decision that included removal of Milltown Dam (EPA, 2004). The dam was breached on March 28, 2008, to facilitate removal and excavation of the contaminated sediment.

Previous USGS Water-Quality Monitoring and Research

The data presented in this report were collected as part of an ongoing USGS long-term monitoring program that includes assessments of metals in surface stream water, fine-grained bed sediment, and aquatic macroinvertebrates and that was designed to help identify metal trends relative to naturally varying hydrologic conditions and implementation of remediation efforts (USGS, 2023). Since 1985, monitoring data have shown elevated concentrations of cadmium, copper, lead, and zinc within the upper reach of Clark Fork and decreasing concentrations with increasing distance downstream (Hornberger and others, 1997, 2009). Constituent-based trends also corresponded to bioassessment monitoring studies, where species diversity was lower in parts of the upper Clark Fork (McGuire, 2007).

To monitor water-quality changes related to remediation activities, the USGS began collecting data in the upper Clark Fork Basin in 1985 at six sites above Milltown Reservoir (fig. 1) (Lambing, 1991). The data were collected in cooperation with the State of Montana, the Montana Power Company, and the EPA. The site upstream from Missoula was added to the network in 1986, and the site near Galen was added in 1988. Another 14 sites were added between Silver Bow Creek at Opportunity and in the mainstem downstream from Missoula in 1993 (fig. 1; not all sites are shown) (Lambing and others, 1995). The long-term data collected from mainstem sites and major tributaries enabled identification of the primary source areas contributing sediment and metals to the Clark Fork and tracking of changes over time as remedial treatments were implemented. Concentrations and mean annual loads of suspended sediment and trace elements at network sites are published in annual reports (for example, Clark and others, 2020, 2021) and associated data are in the USGS National Water Information System (NWIS) database (USGS, 2023).

To evaluate the effectiveness of mine-waste remediation at the Clark Fork Superfund complex, Hornberger and others (2009) evaluated concentrations of copper and cadmium in fine-grained bed sediment and tissue of the Hydropsyche spp. (caddisfly; Pictet, 1834), from 1986 to 2006 collected along 200 km of Clark Fork between Silver Bow Creek and the former Milltown Dam. Concentrations of copper and cadmium in fine-grained bed sediment and caddisflies were lower in the upper reach at sites closer to remediation. A significant positive relation between metal bioaccumulation and stream discharge in the middle reach indicated a flow-induced redistribution of contaminants throughout the river. Correlations between stations in sediment concentration trends showed longitudinal decreases from upstream to downstream.

More recently, Sando and others (2014) analyzed water-quality data and characterized flow-adjusted trends in mining-related contaminants for 22 sampling sites distributed across the Clark Fork River Operable Unit for water years 1996–2010. Results of this study indicated moderate to large decreases in flow-adjusted concentrations and loads of copper and other metallic elements and suspended sediment in Silver Bow Creek upstream from Warm Springs Creek. However, in the Clark Fork reach from Galen to Deer Lodge, mobilization of copper and suspended sediment from floodplain tailings and streambanks was a large source of contamination. The Clark Fork reaches downstream from Deer Lodge were smaller sources of metallic elements. Also, during 1996–2010, small temporal changes were observed in arsenic loads and flow-adjusted concentrations in the Silver Bow Creek and Clark Fork reaches downstream from the Clark Fork at Opportunity.

Description of the Study Area

The upper Clark Fork Basin is in west-central Montana within the northern Rocky Mountains physiographic province (not shown), which is characterized by rugged mountains and intermontane valleys (Fenneman, 1946). The Clark Fork valley is about 145 km long and ranges in elevation from about 4,800 feet near Galen, Mont., to about 975 meters near Missoula, Mont. The north-trending valley is flanked on the east by mountains along the Continental Divide and on the west by the Flint Creek Range.

Sites included in the current study (fig. 1, table 2) encompass the upper Clark Fork Basin upstream from the Clark Fork above Missoula and are on the mainstem channels of Silver Bow Creek and Clark Fork. Originating near Warm Springs, Mont., the 780-km Clark Fork of the Columbia River Basin originates at the confluence of Silver Bow, Willow, and Warm Springs Creeks and drains an extensive region of the northern Rocky Mountains in western Montana and northern Idaho. Across Montana, the river flows northwest and empties into Lake Pend Oreille in the Idaho Panhandle. It is the largest river by volume in Montana (Clark Fork Watershed Education Program, 2022). The Clark Fork above Missoula drains a watershed of 15,400 km². From Butte to near Missoula (148 river miles), six major tributaries enter the Clark Fork: Blacktail Creek, Warm Springs Creek, Little Blackfoot River, Flint Creek, Rock Creek, and Blackfoot River. Smaller perennial and intermittent tributaries drain the surrounding mountains and terraces on both sides of the Clark Fork valley.

In its upper 32 km, the Clark Fork is known as Silver Bow Creek, which originates in the mountains north of Butte from the confluence of Basin and Blacktail Creeks. The upper reaches of Silver Bow Creek in and near Butte contain numerous mine shafts, pits, mills, smelters, and tailings piles and ponds (fig. 3). Downstream from Butte, Silver Bow Creek flows west about 16 km and north about 16 km to its confluence with Warm Springs Creek, marking the start of Clark Fork. Between Butte and the confluence with Warm Springs Creek, large areas of the intervening basin were affected by production and dispersion of waste materials (rock, water, and smelter emissions) primarily from milling and smelting activities of AMC. About 8 km upstream from the confluence of Silver Bow Creek and Warm Springs Creek, Silver Bow Creek enters the Warm Springs ponds constructed during 1908–59 (CDM Smith, 2005) to retain and treat contaminated sediment. Upstream from the Warm Springs ponds, Silver Bow Creek at Opportunity represents the outflow of the Silver Bow Creek Basin above substantial retention and diversion structures. Passing east of Anaconda, Clark Fork flows north for about 43 river miles past the towns of Galen, Deer Lodge, and Garrison. (fig. 1).

In the reach above Garrison, the Clark Fork is a highly meandering river, but the river downstream becomes narrow and confined by highway and railroad embankments. A broad valley extends from the Warm Springs ponds to Garrison and is bordered by high terraces reaching several hundred meters above the river (Lambing, 1998). Near Garrison is the confluence with the Little Blackfoot River, and downstream 8 km east of Missoula is the confluence with the Blackfoot River.

Between Deer Lodge and Garrison, the extent of floodplain tailings (slickens) along Clark Fork is like that in the valley upstream from Deer Lodge (Smith and others, 1998). Although slicken areas are rare in this area, cutbank soils are enriched with metals, and metal concentrations in bed sediments are highly variable (Axtmann and Luoma, 1991).

The taxonomic richness of sensitive invertebrate species is higher here than in the upstream reach. For example, species of caddisfly (order Trichoptera) and stonefly (order Plecoptera) that are absent in the upper reach of the river first appear below the confluence of the Little Blackfoot, indicating a change or improvement in habitat quality, decreased trace element concentrations, and recruitment of organisms from a relatively unaffected source (McGuire, 2007).

Between Garrison and Drummond where the Clark Fork valley narrows, floodplain tailings (slickens) are less extensive than in the Deer Lodge valley (not shown), and channel meandering decreases (Lambing, 1998; Smith and others, 1998). Downstream from Drummond, the Clark Fork valley is narrow (less than 1.6 km wide), and meandering decreases further in association with the narrow valley and presence of highway and railroad embankments (Lambing, 1998).

Just beyond the confluence with the Little Blackfoot River near Garrison, the Clark Fork valley turns abruptly northwest across western Montana for about 77 river miles to the former location of Milltown Reservoir near Bonner. This river segment is more channelized, although it remains single-threaded and sinuous (Smith and others, 1998). The downstream reach of the Clark Fork from the Rock Creek confluence to below Missoula shows the lowest trace element concentrations in fine-grained bed sediments and biota because of dilution from the Rock Creek, Blackfoot River, and Bitterroot River drainages (Axtmann and others, 1997), although concentrations in this area are higher than in these tributaries (Axtmann and Luoma, 1991). From Turah Bridge, the Clark Fork flows through the area of the former Milltown Reservoir.

The primary surface-water uses in the 15,539-km² upper Clark Fork Basin include agricultural irrigation, stock watering, small-scale industry (Cannon and Johnson, 2004), cold-water trout fishing (Morey and others, 2002), and recreational activities.

Purpose and Scope

Following a previous synthesis of monitoring data by Hornberger and others (1997) for 1985 to 1995, the current study is a synthesis of monitoring data collected from 1996 to 2016 to describe the spatial distributions and temporal changes in copper, arsenic, cadmium, and zinc concentrations in surface water, fine-grained bed sediment, and the tissue of a widely distributed benthic insect (macroinvertebrate) genus, Hydropsyche, a netspinning caddisfly. This study also characterizes the co-occurrences of these spatial and temporal concentration changes as they relate to remediation activities and evaluates changes in tissue concentrations as a response to metal concentrations in water and bed sediment. As with Hornberger and others (1997), data were selected herein (USGS, 2023) to provide independent indicators of metal concentrations in the river and changes in metal enrichment over time. However, the current study differs from the previous work by qualitatively comparing the spatial and temporal patterns in the data rather than by calculating trends, which was done in Sando and Vecchia (2016).

Sampling sites in Clark Fork were included in this study to identify patterns of contaminant transport and accumulation at key locations. The key findings describe the variability in different indicators of metal exposure. However, unlike Hornberger and others (1997), which compared pre- and post-remediation conditions, all data included herein were collected after initiation of remediation in the upper reach of the basin. Metal concentrations in bed sediment and insect tissue were consistently low at tributary sites; accordingly, except for the Blackfoot River, tributary sites were not included in our analyses. Data collected from the Blackfoot River near Bonner (USGS streamgage 12340000; USGS, 2023) were used to represent baseline conditions for comparisons with mining-effected sites in the Clark Fork mainstem.

The goal of the current study is to describe the spatial and temporal variations of arsenic, copper, cadmium, and zinc in surface-water samples (dissolved and total recoverable fractions), fine-grained bed sediment samples, and aquatic macroinvertebrate (insect) tissue concentrations from 1996 to 2016. Discussion of metal bioaccumulation is restricted to concentrations measured in caddisflies, a net-spinning, omnivorous, filter-feeding insect that is metal tolerant and ubiquitous in western streams (Morse and others, 2019).

Analytical and Statistical Methods

Like many mining-affected sites, certain trace elements characterize the contamination in Clark Fork sediments, including copper, cadmium, arsenic, and zinc. An EPA ecological risk assessment of the lower Clark Fork (EPA, 1999) concluded that copper was most likely to cause adverse ecological effects, whereas bioaccumulated cadmium and zinc were not highly correlated with changes in the community composition (Luoma and others, 2010). Arsenic concentrations can be elevated in the Clark Fork through remediation activities that alter river pH resulting in increased solubility (Hornberger and others, 2009), but ecological effects of arsenic contamination are more localized than for copper because of the more restricted occurrence of arsenic (Luoma and others, 2010). Therefore, although water samples were analyzed for concentrations of arsenic, cadmium, copper, iron, lead, manganese, zinc, and other constituents, the data presented here include only copper, arsenic, zinc, and cadmium, and focus is placed more on copper in the results and discussion. Cadmium concentrations were near or below the detection limits in all samples, so cadmium results are summarized here but not included in all analyses or in the final discussion. Also, concentrations of all metals were highest in Silver Bow Creek, upstream from the Warm Springs ponds, and greatly reduced immediately below the outfall. So, the data collected from the Silver Bow Creek monitoring sites (OP and PO; table 2; fig. 1) were analyzed separately from data collected in the Clark Fork mainstem. Data collected from the Blackfoot River near Bonner (USGS streamgage 12340000; USGS, 2023) were used to represent baseline conditions for comparisons with mining-affected sites in the Clark Fork mainstem.

Statistical Analysis

Long-term trends in constituent transport that represent changes in supply or delivery can be difficult to distinguish from the natural variability caused by differences in runoff. Streamflow (or volumetric discharge) has large and complex, site-specific effects on seasonal and interannual variations in sediment transport and metal availability. In most Montana and Wyoming streams during low-flow conditions, concentrations of dissolved major ion constituents adjusted for streamflow are less variable and lower than unadjusted concentrations. Conversely, during high-streamflow conditions, flow-adjusted concentrations are less variable and higher than unadjusted concentrations (Sando and others, 2014). So, although streamflow conditions were an important consideration for understanding spatiotemporal changes in constituent concentrations, all the surface-water data presented here and included in the statistical analyses were adjusted to describe variations in constituent concentrations independently from the effects of season, weather, or climate. To do this, the flow-related and seasonal variability in the water-quality dataset were determined in R–QWTREND based on the 5-day mean daily discharge (in cubic feet per second), and the dataset was adjusted from the model output (Vecchia and Nustad, 2020). R–QWTREND compares constituent concentrations with concurrent and lagged flow (delay between when rainfall occurs and when the discharge of the river increases) at multiple time scales and uses the periodic functions of sine and cosine to model seasonal variations.

Streamflow, a function of water volume, water velocity, area, and time (Turnipseed and Sauer, 2010), has a substantial effect on water quality, sediment transport, and the habitats of aquatic organisms. Streamflow is important for determining water quality and in interpreting water-quality data. So, although water-quality data were corrected to minimize variations caused by streamflow, streamflow data were included in the analysis because it was important to understand the overall variation in Clark Fork streamflow and the effect of streamflow on the spatial and temporal variations in contaminant concentrations. The Clark Fork above Missoula (USGS streamgage 12340500; fig. 1) often is referenced in studies of Clark Fork hydrology because it was established in 1930 and is the most representative of long-term conditions. Therefore, we used streamflow data from this streamgage to represent basin-wide hydrologic characteristics during the study period (fig. 4).

Pearson product-moment correlation analysis, a measure of the strength of a linear association between variables, was used to compare trace element concentrations between the environmental compartments sampled (surface water, fine-grained bed sediment, and insect tissue), after arranging the data temporally (annual mean values of all sites combined and using April to August yearly mean for surface water) and spatially (mean or grand mean value [mean of yearly means] of all years at each site).

To compare spatiotemporal trace element concentration changes between the three environmental compartments that vary in concentration range and measured units (volumetric versus gravimetric), the concentrations in water, sediment, and tissue were range-standardized by creating a common measurement scale in values from 0 to 1 for comparison (Legendre and Legendre, 1983).

Silver Bow Creek—Temporal Variations in Copper, Arsenic, Cadmium, and Zinc Concentrations

The highest concentrations of metals within the Clark Fork superfund complex have historically occurred above the Warm Springs ponds at Silver Bow Creek at Opportunity (fig. 1, OP; USGS streamgage 12340500). EPA freshwater criteria for metals are expressed in terms of the dissolved metal in the water column, so comparisons of results to the aquatic life criteria were made only with water sample concentrations measured in the dissolved fractions. During the 20-year period between 1996 and 2016, the grand annual mean (mean of April to August yearly means) copper concentration in the dissolved fraction of water samples collected from OP was 31.1 plus or minus (±) 15.4 µg/L (one standard deviation; table 3, fig. 5), which is above the acute and chronic aquatic life criteria of 24.3 µg/L. April to August mean dissolved copper concentrations at this site were highest in 1997 (59.6 µg/L; data are not available for 1996), then declined to 9.25 µg/L by 2016 (table 3; fig. 5B). Dissolved copper concentrations were much lower at the Silver Bow Creek at Warm Springs (site PO; USGS streamgage 12323750) about 7 river miles downstream. For the PO site, the grand annual mean (mean of April to August yearly means) dissolved copper concentration between 1996 and 2016 (4.65±1.42 µg/L) was below acute and chronic aquatic life criteria and lower than the grand annual mean concentration in the Clark Fork mainstem water samples (7.42 ± 2.30 µg/L, all sites combined: USGS streamgages 12323800, 12324200, 12324680, 12331800, 12334550, and 12340500) over the same period.

The grand annual mean (mean of April to August yearly means) dissolved zinc concentration also was much higher in OP (173 ± 139 µg/L) than at PO (4.89 ± 2.42 µg/L) or in the Clark Fork mainstem sites (24.6 ± 18.1 µg/L) and exceeded the aquatic life criteria of 197 µg/L (acute and chronic) for zinc (table 3). As with copper, the maximum April to August yearly mean dissolved zinc concentration was observed in 1997 (470 µg/L; no data in 1996; fig. 5D) at OP, and April to August yearly mean concentrations fell at this site and remained between 58.8 and 43.0 µg/L in more recent years, 2013–16. In contrast, April to August yearly mean dissolved arsenic concentrations were highest in surface-water samples from PO (20.8 ± 3.13 µg/L) compared with OP (10.0 ± 2.45 µg/L) or the Clark Fork mainstem sites (10.4 ± 1.10 µg/L; table 3). The 20-year peak in April to August yearly mean dissolved arsenic occurred at PO in 2001 (26.6 µg/L; fig. 5A), whereas the arsenic maximum April to August yearly mean at OP was 14.4 µg/L in 2004 (fig. 5A). April to August yearly mean dissolved cadmium concentrations were below 1.0 µg/L in most years at both OP and PO (fig. 5C). April to August yearly mean dissolved cadmium concentrations remained below the chronic criterion of 1.26 µg/L in all years at PO and in all measured years at OP except for 1997 and 1998, when the April to August yearly mean concentrations were 1.65 µg/L and 1.41 µg/L, respectively.