0 5 10 15 20 25 MILES

Durango

Silverton

Ignacio

Animas River

watershed

study area

SAN JUAN

LA PLATA

Animas

River

watershed

in Colorado

COLORADO

N

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3051

August 2007

Environmental Effects of Historical Mining in the Animas 

River Watershed, Southwestern Colorado

Printed on recycled paper

The Animas River watershed is one 

of many watersheds in the western United 

States where historical mining has left a 

legacy of acid mine drainage that results 

in elevated concentrations of potentially 

toxic trace elements. Many inactive mine 

sites are located on or directly affect 

Federal lands. Cleaning up these Federal

lands and restoring the watersheds 

will require a substantial investment of 

resources and many years of work before 

the aquatic ecosystem can recover. As 

part of a cooperative effort with Federal 

land-management agencies, State natural 

resource agencies, the U.S. Environmental

Protection Agency, and concerned local 

citizens, the U.S. Geological Survey 

(USGS) implemented the Abandoned 

Mine Lands Initiative in 1997. USGS 

scientists have since completed a comprehensive study to understand processes and 

assess sites in the watershed for remediation. These data, coupled with those 

collected from private sites by the Animas 

River Stakeholders Group, State of 

Colorado, Bureau of Land Management, 

and USDA Forest Service, will be used by 

regulatory agencies having the responsibility for watershed cleanup.

Results of this investigation, published in USGS Professional Paper 1651, 

include studies of geology, rock and sediment geochemistry, surface- and groundwater geochemistry, a mine inventory, 

district production and milling history, 

and distribution and health of aquatic 

and riparian biota. An environmental 

risk assessment for trout summarizes the 

biological impact of historical mining on 

aquatic fishery resources. Viable trout 

populations are present in some tributaries 

and parts of the Animas River upstream 

from the confluence with Mineral Creek, 

suggesting some overall improvement of 

aquatic habitat quality. However, little 

to no aquatic life occurs in the upper 

Animas River upstream of Animas Forks, 

the entire length of Cement and Mineral 

Creeks, and several smaller tributaries. 

Historically, parts of Cement and Mineral 

Creeks, the two major tributaries in the 

study area, were always acidic, contained 

high metal loads, and likely did not support any aquatic life other than species of 

algae that can tolerate those acidic conditions (pH<4). Finally, the Professional 

Paper outlines the potential for long-term 

monitoring as a means to document 

ecological recovery following remediation implemented in the past decade by 

Sunnyside Gold, Inc., the Federal and 

State land-management agencies, and 

the Animas River Stakeholders Group. 

A compact disc of data collected during 

the study accompanies the volume.

Setting and Geology

The Animas River watershed study 

area (fig. 1), approximately 145 square 

miles, covers the northern portion of the 

Animas River drainage basin upstream 

of Silverton, Colo.; it also includes an 

extension south to Elk Park. Upstream 

from Silverton, the drainage basins 

of the three main tributaries, Mineral 

Creek, Cement Creek, and Animas River, 

delineate a rugged montane basin, ranging in elevation from 9,305 ft at Silverton 

to more than 13,800 ft in the surrounding 

mountains. Elevations of the three Red 

Mountains (banner photo at top) range 

from 12,219 to 12,747 ft; these three 

peaks form the northwest boundary of 

the study area. The study area is accessible by U.S. Highway 550, from Durango 

on the south or from Ouray on the north.

The study area is underlain by 

Precambrian and Paleozoic rocks that 

were intruded and overlaid by the 

Oligocene Silverton Volcanics in a major 

caldera-forming event. Ring fractures 

control the course of the Animas River 

and Mineral Creek upstream from 

Silverton. Mineralization in the watershed 

was extensive, comprising at least five 

different periods beginning in the Oligocene, 30 million years ago, and ending 

about 5 million years ago. Hydrothermal 

alteration has resulted in widespread 

areas of disseminated pyrite throughout 

the study area, weathering of which has 

resulted in large barren, iron-stained 

areas (banner photo) that release large 

amounts of acid and metals to surface and 

ground water.

The U.S. Geological Survey has completed an extensive environmental study of the 

effects of historical mining on water and sediment quality and aquatic and riparian 

habitat in the Animas River watershed upstream from Silverton, Colorado. Results 

from this study are being used by Federal and State agencies and by the local 

watershed stakeholders group to implement remediation and cleanup activities.

The Watershed Approach provides an effective means to rank and evaluate 

environmental impact of historical mining throughout a region: The mining-related 

sites that, upon evaluation, are shown to cause the greatest impact can become 

the subjects of remediation planning and implementation. This process helps land 

managers evaluate risk and focus remediation efforts on sites that may be the most 

deleterious to water quality and aquatic and riparian habitat.

Figure 1. General location map of study 

area, southwestern Colorado, U.S.A.
The Mining Era

Sometime during the 1700s, 

Spanish explorers discovered placer 

gold in Arrastra Creek, a tributary of the 

upper Animas River. Serious mineral 

exploration began in the early 1870s following discovery of placer gold by the 

Baker reconnaissance scouting team for 

the U.S. Army. Mining began in 1872 

following a treaty with the Ute Tribe 

and continued through 1991—when 

the last remaining site, the Sunnyside 

mine, closed. Mineral deposits include 

copper- and silver-rich breccia-pipe 

deposits associated with subeconomic 

porphyry copper-molybdenum deposits, and base- and precious-metal vein 

deposits containing silver, lead, zinc, and 

gold in hydrothermal vein deposits. The 

most productive mineral deposits were 

vein deposits mined primarily for their 

silver content.

Silver, lead, and gold were the 

primary commodities. Zinc was produced 

in minor quantities prior to World War I, 

but it became a major commodity following the introduction of pre-flotation processing in 1915. Zinc production expanded 

after the Sunnyside Eureka flotation mill 

opened in 1917, until the Great Depression

forced the mine and mill to close. Much 

of the total mineral wealth extracted from 

the gold-bearing vein deposits came from 

the Shenandoah-Dives and Sunnyside 

mines in the upper Animas River basin. 

Total mineral production from the area 

is estimated at 18.1 million short tons 

of ore.

Mining and milling processes in 

the Animas River watershed are typical of those practiced throughout the 

West during the 19th and 20th centuries. 

Beginning in 1872, vein deposits were 

mined underground by small crews who 

selectively mined high-grade ore. Ore 

was hand-sorted and sent to smelters 

by mule pack train and later by wagon. 

Waste rock was discarded in open mine 

stopes or on mine-waste dumps outside 

the portal where the hand sorting was 

done. As more efficient methods of mining and milling were developed and rail 

transport became available, increasingly 

larger amounts of lower grade ore were 

mined and processed. Waste rock was 

disposed of on mine-waste dumps outside 

the mine portal, and mill wastes were 

deposited into the nearest stream course. 

By 1930, before it closed due to falling

metal prices during the Depression, 

the Sunnyside Eureka Mill (fig. 2) was 

processing more than 1,000 short tons of 

ore per day. Mill wastes were disposed of 

on the riparian zone of the Animas River 

and were periodically washed downstream by high spring runoff. Beginning about 1935, mill wastes from the 

Shenandoah-Dives mine were retained 

in tailings ponds at the Mayflower Mill 

site. An estimated 8.6 million short tons 

of mill waste, about 47.5 percent of the 

total ore produced, were discharged 

into surface streams between 1872 and 

about 1935.

Environmental Effects

Historical mining occurred during 

a time when environmental effects of 

mining were poorly understood and of 

little understanding or concern to much 

of society. Effects of mining are noticeable throughout the watershed (fig. 3). 

In many stream reaches, elevated traceelement concentrations are the direct 

result of historical mining exemplified 

by zinc (fig. 4). Other potentially toxic 

trace elements (Al, Cd, Cu, Pb, and Mn) 

in both water and sediment resulting 

from historical mining activity show the 

same general distribution patterns. The 

presence of Paleozoic carbonate rocks as 

well as the carbonate in propylitic hydrothermal alteration in the upper Animas 

River basin resulted in good near-neutral 

water in the Animas River and many 

of its west-flowing tributaries (fig. 5). 

Tracer studies conducted in the watershed 

indicate that the Animas River upstream 

from Cement Creek contributes the largest portion of the manganese. In contrast, 

water from Mineral Creek contributes 

Figure 3. View looking downstream (southwest) from confluence of Cement Creek (right) 

with Animas River (left). Iron colloids precipitate from more acidic surface water in Cement 

Creek (pH ≈ 3.5) upon mixing with more neutral water of Animas River. Some of the iron and 

much of the toxic metal load in Cement Creek are the result of historical mining in the Cement 

Creek basin.

Figure 2. Animas River valley at Eureka. Foundation 

of Sunnyside Eureka Mill is on hill at far left above 

road. Riparian zone downstream from the mill site was 

completely obliterated by discharge and storage of mill 

tailings during the mill’s operation (1917–1930). About 

1 meter of sediment accumulated on the flood plain as 

a result of disposal of mill tailings in the Animas River. 

The site was remediated by Sunnyside Gold, Inc., in 1996. 

Photograph by Kirk Vincent (1999).
Zinc, in parts per million

 EXPLANATION Zinc, in parts per million

 EXPLANATION

Mine or mill

Tailings

107°50'30" 107°45' 107°37'30" 107°30'

37°

58'

37°

52'

30"

37°

45'

Chattanooga Eureka

Animas

Forks

Howardsville

Silverton

Gladstone

Animas River

watershed

study area

Elk Park

A B

Animas River

Elk Creek

M ni eral Creek

> 1,500

750–1,500

375–750

225–375

< 225

0 1 2 MILES

0 1 2 KILOMETERS

> 1,500

750–1,500

375–750

225–375

< 225

Animas River

watershed

study area

107°50'30" 107°45' 107°37'30" 107°30'

37°

58'

37°

52'

30"

37°

45'

0 1 2 MILES

0 1 2 KILOMETERS

Chattanooga Eureka

Animas

Forks

Howardsville

Silverton

Gladstone

Elk Park

Animas

River

South Fork M ni eral Creek

Middle Fork

Arrastra Creek

Cunn ni gham Creek

Elk Creek

the largest portion of the copper load and 

Cement Creek the largest portion of the 

zinc and aluminum loads (fig. 6).

Not all of the poor water quality, 

degraded habitat, and acidic streams 

resulted from mining activities (fig. 4). 

Water chemistry and the colorful landscape in the western part of the watershed resulted from the weathering of 

altered pyritic rock (banner photograph). 

Weathering of pyrite in hydrothermally 

altered areas of the watershed prior 

to mining also results in significantly 

degraded water quality. The presence 

Figure 4. Zinc concentrations in A, pre-mining sediment, and B, modern sediment.

of numerous ferricrete deposits in both 

Cement and Mineral Creeks directly 

associated with extensive hydrothermal 

alteration in the western part of the study 

area is indicative of this process. For 

example, dissolved zinc concentrations 

measured in surface water vary widely 

throughout the watershed (fig. 5) and 

are also directly correlated with areas 

of extensive hydrothermal alteration. 

Releases of cadmium and copper from 

weathering of hydrothermally altered 

rock also limited viable aquatic habitat 

prior to mining.

Ecological impairment and habitat 

degradation are reflected through reduced 

abundance and diversity of aquatic invertebrates, and in some cases, the absence of 

a viable fish community. Native cutthroat 

trout have been largely eliminated from 

study area streams with the exception of 

a few tributaries, and brook trout are now 

the only widespread fish species. Suitable 

winter habitat for trout is limited by the 

availability of deep pools; many of these 

have been filled or shallowed by the large 

sediment loads dumped into the surface 

drainage by historical mills. Biological 

data collected during this study allowed 

the assessment of current stream health 

and established a pre-remediation baseline 

for benthic macroinvertebrate populations and trout. Combining these data with 

hydrothermal alteration mapping, streamsediment geochemical data, and groundand surface-water quality data provided a 

basis for a risk assessment for trout in the 

Animas River watershed (fig. 7). The distribution of fish and benthic macroinvertebrates in the watershed generally reflected 

estimated toxicity risks associated with 

concentrations of aluminum, cadmium, 

copper, and zinc, which occurred at potentially toxic concentrations in acidic stream 

reaches, although risks varied widely in 

neutral waters. Toxicity risk was greatest 

during late winter at base flow, especially 

in stream reaches where seasonal acidification took place.

Figure 5. Distribution of dissolved zinc 

in surface water at low flow (late fall).

SAN MIGUEL CO

HINSDALE CO

SAN JUAN CO

OURAY CO

CONTINENTAL DIVIDE

>1,000

Range of dissolved zinc concentrations, 

in micrograms per liter

>500–1,000

>250–500

>100–250

No data

EXPLANATION

Alteration types

Acid-sulfate (AS)

Propylitic (PROP)

Quartz-sericite-pyrite (QSP)

Vein-related quartz-sericite-pyrite (V-QSP)

Weak sericite-pyrite (WSP)

0

0

1

1

2

2

3

3

4 MILES

4 KILOMETERS 

107°45' 107°37'30"

37°

52'

30"

x

37°

45'

b100

Silverton

Howardsville

Middleton

Chattanooga

Animas

Forks

x Storm

Peak

Ohio x

Peak

Gladstone

 River Animas

Arrastra

Creek

South 

Fork

Creek

Mineral 

Cement Creek

Gulch

Eureka

SUNNYSIDE

MINE

Sunnyside 

Eureka Mill

Mayflower 

Mill

SHENANDOAH-

 DIVES MINE

X

X

X

X
Aluminum

5.72%

42.3%

42.7% 9.32%

Copper

0%

59.6%

30.4% 10%

4.23%

24.1% 40.4%

31.4%

Zinc

Manganese

4.23%

15.1%

31.2%

49.4%

 Animas River Cement Creek Mineral Creek Elk Creek

Mine-Site Remediation

and Monitoring

As of 2004, funding for 20 remediation projects was provided by Echo 

Bay, the parent company of Sunnyside 

Gold, Inc., prior to their release from their 

surface-water discharge permits. Many 

former mine sites operated by Sunnyside 

Gold, Inc., and its predecessors were 

cleaned up; draining mine adits were

plugged; and mill tailings and mine wastes 

were moved to the Mayflower repository. 

Applying the watershed approach, Federal 

land-management agencies have remediated nine sites that contributed large metal 

loads to surface waters. The Animas River 

Stakeholders Group has secured funding, 

largely in the form of NPS 319 grants 

from EPA, to conduct remediation work 

on nine sites on private land. Remediation work is ongoing and will continue for 

many years.

Monitoring is an important tool 

for documenting the effectiveness of 

any remediation effort and assessing the 

recovery of ecological systems. With 

properly collected monitoring data, land 

managers can be more objective about 

the success of projects undertaken and 

remediation goals met, and they can learn 

from application of new remediation and 

reclamation technologies. Monitoring 

data are currently being collected by the 

Animas River Stakeholders Group and 

the Federal land-management agencies.

Figure 7. Zinc toxicity map showing 

categories of risk to aquatic life.

•	 For further information contact: Stan Church, USGS, Denver, Colo. (303) 236-1900 or schurch@usgs.gov, or visit the project 

website: URL: http://amli.usgs.gov/

•	 For the complete report see: Church, S.E., von Guerard, Paul, and Finger, S.E., eds., 2007, Integrated investigations of 

environmental effects of historical mining in the Animas River watershed, San Juan County, Colorado: U.S. Geological Survey 

Professional Paper 1651, 1,096 p. plus CD–ROM. URL: http://pubs.usgs.gov/pp/2007/1651/

•	 This Fact Sheet is available online at: http://pubs.usgs.gov/fs/2007/3051/

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Figure 6. Percent of the total dissolved 

load of Al, Mn, Cu, and Zn calculated 

from tracer data, Animas River, Cement 

Creek, Mineral Creek, and Elk Creek.

Zinc

 EXPLANATION

Stream gauge

No toxicity

Acute toxicity to sensitive taxa

Chronic toxicity to brook trout

Acute toxicity to brook trout

A72

Animas River

watershed 

study area

C O L O R A D O

Durango

Chattanooga Eureka

Animas

Forks

Howardsville

Silverton

Gladstone

0

0 1

1 2 MILES

2 KILOMETERS

107°50'30" 107°45' 107°37'30" 107°30'

 37° 

58'

 37°

52'

30"

 37° 

45'

M34

A72

A68

C48

A53

Middle Fork

South Fork

Cement Creek

Elk Creek

M ni eral Creek

River

Animas

Elk Park