Response of the Upper Animas River 

Downstream from Eureka to Discharge 

of Mill Tailings

By Kirk R. Vincent and John G. Elliott

Chapter E22 of

Integrated Investigations of Environmental Effects of Historical 

Mining in the Animas River Watershed, San Juan County, Colorado

Edited by Stanley E. Church, Paul von Guerard, and Susan E. Finger

Professional Paper 1651

U.S. Department of the Interior

U.S. Geological Survey
Contents

Abstract .......................................................................................................................................................893

Introduction and Background ..................................................................................................................893

Purpose and Scope ..........................................................................................................................895

Acknowledgments ............................................................................................................................895

Geographic Setting ...........................................................................................................................895

Geologic History ................................................................................................................................895

Hydrology ...........................................................................................................................................897

Flow and Sediment Transport Theory ............................................................................................897

Present Condition of Channels and Flood Plains ..................................................................................900

Spatial Variation in Geometry .........................................................................................................900

Channel Conveyance .......................................................................................................................906

Bed Sediment Mobility .....................................................................................................................908

Implications of Channel Patterns ............................................................................................................909

Sources of Sediment Supplied to the River ...........................................................................................912

History of Ore Milling........................................................................................................................912

Other Sources of Sediment .............................................................................................................914

Photographic Evidence of Change and Recovery ................................................................................914

Early Oblique Photographs ..............................................................................................................914

Aerial Photographs 1945 to 1997 ....................................................................................................917

Methods.....................................................................................................................................917

Results .......................................................................................................................................921

Changes Inferred from Flood-Plain Stratigraphy .................................................................................922

Stratigraphy and Sedimentology ....................................................................................................924

Constraints on Ages of Sediment Deposition ..............................................................................925

Stratigraphic Interpretations ..........................................................................................................928

Sediment Chemistry...................................................................................................................................931

Geochemical Methods and Results ...............................................................................................931

Geochemical Conclusions ...............................................................................................................931

Current Location of Tailings .....................................................................................................................934

Summary and Conclusions .......................................................................................................................936

References Cited........................................................................................................................................938

Plate

[Plate is on accompanying CD-ROM]

 6. Stratigraphy of late Holocene channel and flood-plain deposits exposed in a trench 

excavated across the Animas River valley floor 1.4 kilometers downstream of the 

Eureka townsite, San Juan County, Colorado
Figures

 1. Location map of upper Animas River study site ..................................................................894

 2. Panoramic photographs of Animas River valley near Eureka, one taken ca 1904 

and a repeat photograph taken in 2001 ..................................................................................896

 3. Longitudinal profile of upper Animas River valley ...............................................................901

 4. Aerial photographs of study reach, taken in 1951 ...............................................................902

 5–7. Graphs showing:

 5. Threshold relation between valley slope and bankfull discharge for braided 

 and meandering streams ..................................................................................................910

 6. Pattern of boundary shear stress for a hypothetical single-thread channel 

 conveying a flow with 2-year recurrence interval .......................................................911

 7. Combined ore processing rates for the succession of mills in the town of 

 Eureka and in Eureka Gulch .............................................................................................912

 8. Etching of photograph taken of Eureka in 1877 ...................................................................915

 9–12. Photographs showing:

 9. Eureka, and reaches A and B of Animas River, in 1906 and 1999 ..............................918

 10. Trench section, cemented gravel, and ground-water table below level 

 of stream .............................................................................................................................923

 11. Fresh exposure of flood-plain sediment containing fine-grained tailings, 

 near the trench site ...........................................................................................................926

 12. Willow bush that was repeatedly flood trained and covered by sediment .............927

 13. Diagram showing summary of dated stratigraphy of the Animas River valley

flood plain near the trench site ..............................................................................................930

 14. Graphs showing concentration of vanadium and zinc in flood-plain sediment .............932

Tables

 1. Annual instantaneous peak discharge (m3/s) at streamflow gauging 

station 09357500 Animas River near Howardsville ..............................................................898

 2. Streamflow characteristics at USGS streamflow gauging station 09357500 

Animas River at Howardsville, 1936 through 1982 ...............................................................899

 3. Dimensions and other physical characteristics of reaches of the upper 

Animas River ..............................................................................................................................906

 4. Streambed sediment characteristics of the upper Animas River near Eureka ..............907

 5. Statistics from the digital processing and rectification of upper Animas River 

valley aerial photographs ........................................................................................................920

 6. Changes in geomorphic and wetland characteristics in reaches of the upper 

Animas River, from 1945 to 1997 .............................................................................................921

 7. Data for radiocarbon samples collected near the trench, downstream of 

the Eureka townsite ..................................................................................................................929

 8. Concentration of selected elements in deposits of coarse-grained and finegrained mill tailings ..................................................................................................................933

 9. Spatial extent of surficial fine-grained tailings as interpreted from aerial 

photographs, by reach and year ............................................................................................934

 10. Estimates of volumes and weights of tailings remaining in our study reach .................936

Abstract

The Animas River watershed study area in the San Juan 

Mountains of Colorado was the site of extensive mining and 

milling during the late 19th and early 20th centuries. We focus 

on the reach of the Animas River between Howardsville and 

Eureka, which receives water and sediment from the headwaters area of the upper Animas River. Using geologic mapping, stratigraphic and sedimentological studies of flood-plain 

sediment, geochronology, historical records, oblique and aerial 

photographs, sediment transport calculations, and geochemical analysis of sediment, we conclude the following. Prior to 

mining, the river valley near the eventual town and mill site 

at Eureka was composed of shallow braided, gravel bedded 

channels, and multi-thread channels with well-defined banks 

downstream. The multi-thread channels were located within a 

silty flood plain consisting of willow thickets and intermittent 

and localized beaver ponds. A radical change in the stream 

and flood-plain environment started shortly after A.D. 1900 

and concluded downstream of Eureka with aggradation of 

channels and burial of older sediment with sheets of gravel. 

This was caused by milling, not mining or other anthropogenic 

activities. Mills in and near Eureka supplied huge quantities of 

tailings to the river, at rates 50 to 4,700 times greater than the 

natural rate of production of sediment from hillslopes prior to 

mining. The tailings were introduced at a location where the 

valley profile is strongly concave, where the river is inherently prone to braiding. The introduction of sand-sized tailings 

altered the bed-load sediment transport regime, and the result 

was aggradation and stream braiding.

Flood-plain sediment deposited prior to mining has 

naturally high zinc concentrations close to 1,000 ppm, but 

the introduction of tailings resulted in an increase of zinc 

concentrations to more than 4,000 ppm on average and locally 

to as much as 10,000 ppm. The presence of tailings in stream 

sediment also increased the concentration of cadmium, 

copper, lead, and manganese, above the concentrations in 

prehistorical sediment. Using vanadium as a lithologic tracer 

for sediment derived from hillslope erosion, we estimate 

that the fine fraction of streambed and flood-plain sediment 

deposited after A.D. 1900 contains, in general, two-thirds 

tailings and one-third natural sediment. Of the original tailings 

produced, between 70 and 80 percent has been flushed from 

the study reach by streamflow, and on the order of 10 percent 

was mechanically reclaimed and placed in a repository. Of 

the tailings that remain in the study reach, on the order of 

10 percent exists at the surface as easily identifiable tailings 

beds, but the majority is in the subsurface dispersed within 

gravel deposits.

Introduction and Background

The portion of the Animas River valley between the 

abandoned mining towns of Eureka and Howardsville (fig. 1) 

is important ecologically and in terms of human activities, 

because it constitutes about 20 percent of the broad and flat 

ground available in the otherwise rugged San Juan County, 

Colo. Yet the sediment in this 7-km long reach contains high 

concentrations of metal, and near Eureka the river is braided 

and flows across a gravel flood plain devoid of vegetation. As 

such, that reach is unique in the region. Late 19th and early 

20th century hard-rock mining in the watershed (Jones, this 

volume, Chapter C) created a variety of disturbances. The 

milling of ore, in particular, resulted in the introduction of 

large volumes of fine-grained tailings into the fluvial system.

Recent interest in reclamation of inactive mines and river 

restoration has necessitated identification of contemporary 

and prehistorical fluvial processes active in the upper Animas 

River valley, and that is the subject of this chapter. With a 

better understanding of these processes, strategies designed to 

address channel instability and contaminant mitigation may 

have a greater probability of success.

The written record for the area and mining in the area 

both began in 1871. For that reason the terms prehistorical and premining are synonymous as used herein, and they 

specifically refer to before A.D. 1871.

Chapter E22

Response of the Upper Animas River Downstream 

from Eureka to Discharge of Mill Tailings

By Kirk R. Vincent and John G. Elliott
Dome Mountain

Middleton

Eureka

Howardsville

Tower Mountain

Reach A

Reach B

Reach C

Reach D

Reach E

Galena Mountain

Porcupine Gulch

Otto Gulch

M aggie Gulch

Cataract Gulch

Hematite Gulch

Brendel Gulch

Animas River

Cunningham Creek

Minnie Gulch

Edith Gulch

1 - Trench Site

2

09357500

09357500

3

3

Eureka Gulch

A

Niagara Gulch

nimas River

37°52'30"

107°37'30" 107°35'

37°50'

0 1 2 MILES

0 1 2 KILOMETERS

Denver

Study area

COLORADO

Colorado Springs

Pueblo

Grand

Junction

Valley floor

Alluvial fan

Lakes

Streams

Intermittent streams

Braided streams

Channel survey and number

Streamflow-gauging station

 and number

EXPLANATION

Figure 1. Location of Animas River study site between the former mining towns of Eureka and Howardsville, upstream of 

Silverton. Valley floor is composed of sediment deposited by the Animas River and tributary debris fans. Letters A through E 

denote reaches discussed in text.

894 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 895

Purpose and Scope

The purpose of this study is to define the late Holocene 

geomorphic history of the upper Animas River between the 

abandoned mining towns of Eureka and Howardsville (fig. 1) 

and identify the important fluvial and anthropogenic processes 

that resulted in the current condition of the channel and flood 

plain.

• We present the current physical condition of channels 

and flood plains, and the contemporary hydrology and 

sediment transport regime, for comparison with past 

conditions and potential future conditions.

• We discuss the history of mining and milling in the 

headwaters area because of the large impact the release 

of mill tailings has had on the physical and chemical 

nature of the river channels and flood plain.

• We document how the channels and flood plain have 

changed over the past 3,000 years, based on the stratigraphy exposed in a trench excavated across the valley 

floor (pl. 6).

• We document where mill tailings currently reside in 

and on the flood plain at the trench site, and estimate 

that for the rest of the reach.

• We compare the historical supply rate of mill tailings 

with prehistorical supply rates of sediment in order to 

understand the cause of historical channel change.

• We compare the chemistry of prehistorical and historical sediment in order to understand how the release of 

mill tailings changed the concentration of metals in 

stream sediment.

• We document the recovery of the channels and 

flood plain over the past 50 years, and changes in 

the locations of tailings, using a time series of aerial 

photographs.

• Lastly we discuss constraints on future efforts to 

mitigate contamination caused by the presence of mill 

tailings, and constraints on future efforts to reconfigure 

the stream channels in the study area.

This chapter also serves as an example for future work 

in other areas. We demonstrate the consequences of the 

introduction of mill tailings into the environment that should 

be expected in other mining areas, and how to identify those 

consequences. We also demonstrate how the results of numerous geomorphological methods can be integrated into an 

understanding of the configuration of stream channels as they 

were prior to disturbance, and an understanding of the constraints on future channel configuration. At the national level, 

projects aimed at “restoring” disturbed channels have typically 

proceeded without that basic understanding.

Acknowledgments

The U.S. Bureau of Land Management funded the 

trenching and other aspects of our work; we acknowledge 

Stephanie O’Dell and Rob Robinson for that funding. The 

trench (pl. 6) was excavated using a trackhoe operated by 

Bill Simon. The mineralogy of coarse-grained tailings observed 

in the trench was corroborated by Steve Fearn. Mike Scott 

interpreted the growth rings of the willow shown in figure 12. 

Stevan Gyetvai rectified and registered the aerial photographs 

from GPS data, and computed channel lengths and valley 

floor and wetland areas with GIS techniques. Jonathan Evans 

assisted with channel surveys and streambed-sediment measurements. Rob Robinson, and in particular J. Dungan Smith 

provided many constructive comments on the manuscript.

Geographic Setting

This chapter documents the nature and history of landforms along 7 km of the upper Animas River between the 

abandoned mining towns of Eureka and Howardsville (fig. 1). 

The Animas River originates in subalpine valleys of the 

San Juan Mountains of San Juan County, Colo., flows southwest through the study reach (fig. 2), ultimately passes the 

towns of Silverton and Durango, and joins the San Juan River 

at Farmington, N. Mex. In the study area, the valley floor is at 

altitude of 2,970 m, but the surrounding hillslopes extend up to 

4,111 m. Those hillslopes are dominated by exposed bedrock 

and tundra vegetation above the timberline (at about 3,600 m), 

and are dominated by Engelmann spruce below the timberline. 

In the valley bottom, spruce forest and open grasslands dominate alluvial fans. Stream terraces and flood plains either are 

devoid of vegetation (fig. 2) or support thickets of bog birch 

and willow, sedge wetlands, and spruce forests.

Geologic History

The bedrock geology, and the mineralization that 

made the area famous, are discussed by Yager and Bove 

(this volume, Chapter E1 and pl. 1) and Bove and others (this 

volume, Chapter E3). For our chapter the salient aspect of the 

bedrock geology is that it is composed of erosion-resistant 

volcanic rock. Thus talus fields are common, hillslope soils 

are thin and fragmental, and alluvial deposits are coarse and 

contain little silt and clay. The latest Quaternary geologic 

history of the region has been presented by Blair and others 

(2002) and Vincent and others (this volume, Chapter E16), 

and we add to that knowledge base. Atwood and Mather 

(1932) developed the original maps showing the extent of 

latest Pleistocene glaciation in the San Juan region. Alpine 

glaciers occupied the Animas River valley, and the subsequent action of streams is superimposed on that glacial 

terrain. Few if any surficial deposits predate the last glaciation. Although numerous rock glaciers are scattered through 

the San Juan Mountains, there is no evidence of glaciation at 
Figure 2. Panoramic photographs of Animas River valley near Eureka; mouth of Eureka Gulch visible right of center and Howardsville just out of sight down 

valley on extreme left. In center of photographs, view looks west. Upper photograph (A) was taken by an unknown photographer in or before 1904 and was published by the Silverton Standard (1904). It is provided here courtesy of William Jones. Note buildings of Sunnyside Mill # 2 (AMLI site # 165) and town of Eureka right of center and Silverton Northern train in center foreground. Lower photographic mosaic (B) was taken in 2001 by Kirk Vincent. Our reach B dominates the photograph, and lower portion of reach A is visible on far right. Dashed line and T, location of trench (pl. 6).

A

B

T

896 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 897

any time during the Holocene. Whether by glacial erosion or 

stream erosion, bedrock became exposed in the valley bottom 

upstream of Eureka forming a waterfall. The exposed bedrock 

controls the base level for the river upstream and causes a 

steep, concave slope in the valley below—at the head of the 

study area. During the Holocene, tributary streams have shed 

sediment into the valley, forming alluvial fans, which the 

Animas River has not been able to remove. In our study area 

(fig. 1), tributary streams enter the valley from the northwest 

and southeast in opposition to one another, and the largest 

fans occur at the mouths of Minnie and Maggie Gulches and 

Cunningham Creek. As a result the Animas River flood plain 

consists of three wide reaches, with intervening reaches where 

the river (with little or no flood plain) passes between pairs 

of encroaching fans (fig. 1). Perhaps only a fraction of each 

fan is active at present. The wide flood-plain reaches have 

been aggrading slowly for at least the last 3,000 years. That 

and other aspects of the late Holocene geomorphic history are 

developed herein.

Hydrology

Precipitation in the area is predominantly snowfall, 

but rain is delivered in the summer by thunderstorms and 

by Pacific tropical storms in the fall. The Silverton weather 

station records mean annual temperature of 1.7°C and mean 

annual precipitation of 61 cm (National Weather Service 

data), but precipitation is about twice that in the surrounding 

mountains (von Guerard and others, this volume, Chapter B). 

The upper Animas River is a snowmelt-dominated stream 

with annual peak discharge usually occurring from late May 

through June (table 1), but intense rainfall in the late summer 

has produced large floods (Pruess, 1996). Streamflow data 

(table 1) are available for the gauging station on the Animas 

River at Howardsville (09357500), which was operated by the 

USGS from 1936 to 1982 and by the Colorado State Engineer 

since 1983. The drainage area upstream from the gauge is 

145 km2

. Annual peak discharges vary widely in magnitude 

and reflect the water content of the preceding winter snowpack 

and the spring weather patterns.

Annual floods recorded at station 09357500 have ranged 

from 12.9 m3

/s in 1977 to 56.1 m3/s in 1949 (table 1). Other 

large floods were observed in the upper Animas River valley, prior to establishment of the stream gauge, specifically 

in 1927, 1921, and 1911. A streamflow gauging station on 

the Animas River at Durango, Colo., about 80 km downstream 

from Howardsville, recorded a peak discharge of 570 m3/s 

in 1927. A regression between the annual peak discharge 

at the Howardsville gauge and the peak discharge at the 

Durango gauge resulted in a relatively poor coefficient of 

determination (R2

 = 0.69); however, the data from the gauge 

at Durango suggest that the 1927 flood at Howardsville may 

have been larger than any recorded since 1936 (the year that 

the USGS began gathering streamflow data at Howardsville). 

Oblique photographs in the San Juan County, Colo., Court 

House collection show two Howardsville bridges washed 

out by flooding in 1921. A tropical storm on October 5, 1911 

(Pruess, 1996) resulted in extreme flooding and damage in the 

watershed (Durango Evening Herald, 1911). The discharge 

of the 1911 flood was probably extreme in the study reach, 

because the site (Gladstone) where the most intense rainfall 

was recorded (21 cm of rainfall in a 24-hour period) is located 

8 km northwest of Howardsville. USGS historical background 

observations indicate that the 1911 flood at Howardsville 

was the “greatest flood since at least 1885.” Although the 

discharges for these historical floods are not known, the 

events are important in that they occurred during or just after 

the period of the largest introduction of sand-sized mill tailings 

to the fluvial system in the reach of concern in this chapter.

Hydrologic characteristics of the upper Animas River 

valley were determined using data for the Howardsville stream 

gauge (table 1) from 1936 through 1982, the period for which 

the gauge was operated by the USGS. Flood frequency and the 

duration of daily mean discharges were calculated by standard 

USGS procedures (U.S. Interagency Advisory Committee on 

Water Data, 1982). A flood recurrence interval is a statistical 

expression based on a long flood series. At the Howardsville 

gauge, an annual peak discharge of 56.8 m3

/s is thought to 

be equaled or exceeded every 100 years, and the discharge 

of 41.3 m3

/s has a recurrence interval of 10 years (table 1). 

Floods with recurrence interval greater than 10 years occurred 

eight times since the stream gauge was established: in 1938, 

1949, 1952, 1953, 1957, 1973, 1985, and 1995. Only one 

flood (1949) with recurrence interval greater than 50 years 

occurred since 1936. The annual peak flow with 2-year recurrence interval has magnitude of 27.7 m3

/s. This discharge, as 

an instantaneous annual flood peak, was equaled or exceeded 

in 32 years from 1936 to 1998 (table 1). As a daily mean discharge (table 2), 27.7 m3

/s was equaled or exceeded approximately 0.14 percent of the time on average, or on less than one 

day per year.

Flow and Sediment Transport Theory

In this chapter we make several discharge and sediment 

transport calculations in order to explore why the Animas 

River is locally braided, and in order to understand the general 

magnitude of flow events required to mobilize the streambed 

and certain tailings deposits. To be concise we present in this 

section all of the transport equations used in the chapter. The 

flow equations presented are appropriate for turbulent water 

flows that are steady and horizontally uniform, which can be 

approximated in a complicated geometric situation by reach 

averaging. Where appropriate we note the value of constants 

in parentheses.

We calculate the discharge conveyed by select channels 

using the following relationships. By definition the volumetric 

rate of flow, discharge (Q), equals the cross-sectional area (A) 
times the mean velocity (U) of the flow. In a steady, horizontally uniform flow the reach-averaged mean flow velocity can 

be calculated using

 U = (u*/k) [ln(h/z

o

) – 0.74] (1)

where

u* is the shear velocity (defi ned in equation 2),

k is von Karman’s constant (0.408),

h is the reach-averaged fl ow depth,

and

z

o is a parameter related to the reach-averaged 

channel roughness.

Because the channels in question are gravel bedded, we 

assume that z

o

 is equivalent to 0.1* D84 (Whiting and Dietrich, 

1989) where D84 is the 84th percentile of the bed-sediment 

size distribution. Owing to bed and channel irregularities 

z

o

 might be as much as three times this value. The equation assumes a quasi-logarithmic velocity profile (Wiberg 

and Smith, 1991). In our application we ignore bank effects 

because the channels are wide compared to their depths. For 

that reason we estimated the reach-averaged cross-sectional 

shape to be that of the surveyed cross sections, and then we 

subdivided the cross sections into short width-increments of 

uniform depth. Discharge was calculated for each increment 

Table 1. Annual instantaneous peak discharges (m3/s) at streamflow gauging station 09357500 Animas River near Howardsville.

[Discharges were measured in ft3/s and were converted using m3/s = ft3/s * 0.028317 rounded to the nearest whole number. Noted in parentheses are floods 

known from anecdotal evidence, and the position in the time series of aerial photographs used by this study]

Year Month Discharge Year Month Discharge

1885 (unknown) (large flood) 1964 May 24

1965 June 33

1911 Oct. (large flood) 1966 May 20

1967 May 27

1921 (unknown) (flood) 1968 June 35

1969 May 21

1927 June (large flood) 1970 Sept. 33

1971 June 23

1936 May 24 1972 June 23

1937 May 25 1973 June 43

1938 June 48 1973 Sept. (photograph)

1939 June 19 1974 May 18

1940 May 23 1975 July 38

1941 June 31 1976 June 23

1942 June 30 1977 June 13

1943 June 20 1978 June 34

1944 June 32 1979 June 32

1945 June 27 1980 June 34

1945 Oct. (photograph) 1981 June 28

1946 June 30 1982 June 27

1947 June 32 1983 June 34

1948 June 34 1984 May 31

1949 June 56 1985 June 49

1950 June 22 1986 June 36

1951 June 24 1987 June 23

1951 Sept. (photograph) 1987 Sept. (photograph)

1952 June 43 1988 June 22

1953 June 46 1989 May 16

1954 May 15 1990 June 27

1955 June 30 1991 June 24

1956 May 26 1992 June 16

1957 June 48 1993 June 28

1958 June 32 1994 June 37

1959 June 24 1995 July 43

1960 June 34 1996 May 31

1960 August (photograph) 1997 June 41

1961 May 23 1997 October (photograph)

1962 June 23 1998 June 23

1963 May 18

898 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 899

using equation 1 and the appropriate increment-area, and 

then summed to obtain the total discharge. For rivers with the 

morphology of the Animas River in the upper part of our study 

area, this procedure is accurate (J. Dungan Smith, oral commun., 2004). In equation 1 the shear velocity is

u* = (ghS)1/2 (2)

where

 g is the acceleration due to gravity (9.8 m/s2),

 h is again the reach-averaged flow depth,

and

 S is the reach-averaged slope of the riverbed.

Technically S is the sine of the slope (slope in degrees), but we 

substituted the tangent (rise over run) into the equation. In this 

application where the slopes are close to 1/2°, that substitution 

results in an insignificant error (0.005 percent).

Sedimentary particles on a riverbed begin to move when 

the local boundary shear stress (τb

), approximated here by the 

reach-averaged value, exceeds a critical value (τc

) that depends 

on sediment and fluid parameters. In a steady, horizontally 

uniform flow the boundary shear stress is

τb

 = ρghS (3)

where

ρ is the fl uid density (1,000 kg/m3

).

The critical shear stress for initiation of motion was first 

evaluated by Shields (1936) and now forms the basis for sediment transport calculations. For its application see Henderson 

(1966), Graf (1971), or Middleton and Southard (1984). In 

general, the critical shear stress can be calculated by estimating what is known as the critical Shields’ stress or Shields’ 

parameter (τ∗

)

c

 and using

τc = (τ∗)

c

 [(ρs – ρ)gD] (4)

where

ρs is the sediment density (assumed to be 

2,700 kg/ m3)

and

D is the particle size of interest.

Although the Shields method can be applied to individual 

grains in a poorly sorted sediment (Wiberg and Smith, 1987), 

it is usually used to calculate the initial motion of the entire 

mixture and for this purpose the median diameter (D50) of the 

sediment distribution is used (Wilcock, 1992, p. 297).

The crux of the problem is in evaluating the Shields 

parameter (Elliott and Hammack, 2000; Komar, 1987; Wiberg 

and Smith, 1987; Andrews, 1983; Parker and others, 1982; 

Neill, 1968). The Shields parameter depends on the geometric 

properties of the sediment grains (including shape and angularity), on the pockets within which the sediment particles rest 

(which depend on the shape, angularity, and sorting of the bed 

material), on any cohesive forces among sediment grains, and 

on the particle Reynolds number. For the flows of interest here 

and materials of pebble and larger size, however, the dependence of the Shields parameter on cohesive forces and on the 

particle Reynolds number disappears, but the dependence on 

the geometric properties of the grains and the bed becomes 

very strong. For elliptical-shaped coarse sediment, significant 

motion occurs when the Shields parameter is approximately 

0.06 (Wilcock and McArdell, 1993; Wiberg and Smith, 1987). 

For smoother, more spherical clasts, this value is smaller. 

In addition, the original Shields relation was generated for 

significant motion. The first motion of pebbles and cobbles 

usually occurs at a substantially lower value of the Shields 

parameter because the first clasts to move are those resting in 

favorable pockets in the bed (Lisle and others, 1993; Andrews 

and Smith, 1992; Milhous, 1982). As a compromise when 

Shields’ approach is used for first motion of coarse materials, 

this asymptotic value is usually reduced to 0.03 or less. We use 

0.03 as the value of the Shields parameter in our calculations 

of initiation of first motion, and we use 0.06 as the value of the 

Shields parameter in our calculations of significant motion.

We discuss sites where the bed material D50 is 39 mm and 

110 mm (table 4), in the sections titled “Channel Conveyance” 

and “Bed Sediment Mobility.” Using equation 4 and the 

assumptions made herein, we calculated the critical shear 

stress for initiation of motion of those materials to be 19.5 

Table 2. Streamflow characteristics at USGS streamflow 

gauging station 09357500 Animas River at Howardsville, 1936 

through 1982.

[Recurrence interval in years equals the reciprocal of exceedence probability 

(streamflow duration in percentage of time specific discharge is equaled or 

exceeded)]

Peak flow frequency Daily discharge

Recurrence

interval

(years)

Discharge

(m3

/s)

Equaled or 

exceeded

(percent time)

Discharge

(m3/s)

1.05 16.5 95 0.371

1.11 18.5 90 0.413

1.25 21.3 85 0.444

2.00 27.7 80 0.475

5 36.0 75 0.507

10 41.3 70 0.538

25 47.7 65 0.591

50 52.3 60 0.654

100 56.8 55 0.741

200 61.3 50 0.869

500 67.2 45 1.01

40 1.24

35 1.58

30 2.08

25 2.87

20 4.05

15 6.06

10 8.98

5 13.5
and 55 N/m2

, respectively. The shear stress for significant 

motion of those materials is 39 and 110 N/m2 (Newtons per 

square meter), respectively.

The bed load transport of gravel can be calculated using 

the equation of Meyer-Peter and Müller (1948), which in 

essence is an expression of the boundary shear stress in excess 

of the critical shear stress. Again, for its application see 

Henderson (1966), Graf (1971), or Middleton and Southard 

(1984). Just as critical shear stress is scaled by the Shields 

parameter (equation 4), the relation uses what is called the 

Shields stress

 (τ*

)b = τb / [(ρs – ρ)gD] (5)

which is a function of boundary shear stress (equation 3). 

For the sediment-volume transport rate per unit channel width 

(Qs), equation 5 can be formulated as

Qs = (8[(τ*

)

b

 – (τ∗)

c

 ]1.5) * ( [ρ′ gD3]0.5) (6)

where the sediment/fluid density contrast is expressed as the 

fraction ρ′ = (ρs

 – ρ)/ρ.

We use equation 6 to calculate sediment transport for 

several types of channels present in the landscape, in the 

section titled, “Bed Sediment Mobility.” We also calculate 

sediment transport for a hypothetical single-thread channel, 

and estimate the magnitude of sediment deposition by comparing changes in Qs moving in the downstream direction, 

in the latter half of the section titled, “Implications of Channel 

Patterns.”

Present Condition of Channels 

and Flood Plains

We start our analysis by discussing the general planimetric and large-scale geomorphic characteristics of the upper 

Animas River and the flood plain as it was in the latter half 

of the 20th century. We utilized topographic maps, aerial photographs taken between 1945 and 1997, and onsite measurements made in 1998. The discussion is illustrated using (1) a 

longitudinal profile of the valley (fig. 3) constructed using the 

1955 USGS 1:24,000 quadrangle maps, which have contour 

interval of approximately 13 m (40 feet); (2) aerial photographs taken in 1951 (fig. 4); and (3) data derived from those 

sources (table 3).

Elsewhere in this chapter we discuss changes that 

occurred during the first half of the 20th century using floodplain stratigraphy (pl. 6), and discuss less dramatic changes 

that occurred during the latter half of the 20th century using 

the sequence of aerial photographs.

Spatial Variation in Geometry

The geometry of the channels and flood plain of our study 

area is spatially variable (fig. 1) and for that reason is discussed 

in terms of reaches (fig. 3, table 3, fig. 4), lettered A through E. 

A major spatial pattern is the variability in the width of the valley floor (figs. 1 and 3). By valley floor we mean the deposits 

of the Animas River including the active streambeds, the flood 

plain, and any low terraces present. The repetitive widening 

and narrowing of the valley floor is the result of constrictions 

imposed by tributary debris fans and talus cones. The major 

constrictions are imposed by fans shed from Minnie Gulch, 

Maggie Gulch, Cataract Gulch, and Cunningham Creek, in conjunction with the fans or cones shed from unnamed watersheds 

directly opposed to the named watersheds (fig. 1).

A second major pattern is the systematic change in valley 

gradient. The longitudinal profile of the study reach is fairly 

smooth and is concave-up (fig. 3), with gradients decreasing 

down valley from 0.029 m/m in the reach just upstream of the 

Eureka townsite to 0.008 m/m in the reach at Howardsville. 

Upstream of the study reach, the river emerges from a steep 

bedrock canyon, and downstream of Howardsville the river 

enters another constricted bedrock canyon. Thus, outside of 

the alluvial study reach the river is bedrock controlled. Inside 

our study reach the stream has formed itself within its own 

alluvium (not bedrock controlled), and the stream gradient 

is not influenced in a major way by the presence of the debris 

fans. By that we mean there are no inflections in the profile 

coincident with the debris fan constrictions (fig. 3). In narrower valleys in the region, such as Cement Creek (Vincent 

and others, this volume), debris fan constrictions have forced 

the main stream to aggrade upstream of them. This resulted 

in a segmented longitudinal profile, with the main stream 

being steep and the streambed being coarse grained on the 

downstream sides of tributary debris fans. Although the valley 

gradient was not obviously influenced, the fan constrictions 

have two consequences. First, the width of any braiding (past 

or present) was restricted by the narrow space available at the 

fan constrictions. Second, flow in the wide reaches during 

major floods must converge as it enters the narrow reaches. 

This is a partial explanation of the channel patterns as they 

were in 1998, and in 1951 (fig. 4). The channel was braided 

with low banks in reaches A and B near Eureka, was confined 

to a single- or double-thread channel adjacent to the Minnie 

Gulch and Maggie Gulch debris fans, and was multi-threaded 

with relatively prominent banks downstream from both fans. 

The reaches are now discussed individually.

What we refer to as reach A (figs. 3 and 4A) heads at 

the base of a waterfall in a constricted canyon and extends 

downstream approximately 0.7 km, past the Eureka townsite, 

to the Eureka Gulch confluence. The valley floor in reach A 

is relatively narrow (mean width 170 m) and steep (mean valley slope 0.025 m/m), and the longitudinal profile is strongly 

concave-up (fig. 3). Elsewhere herein we present evidence 

900 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 901 Figure 3. Longitudinal profile of upper Animas River study reach, showing the gradient, locations of lettered reaches and the trench (pl. 6), and the spatial variation in width of the valley floor.

3,0503,0002,9502,9002,850

0 1 2 3 4 5 6 7 8 9 10 11

VALLEY DISTANCE DOWNSTREAM, IN KILOMETERS

CONTOUR ELEVATION, IN METERS

4003753503253002752502252001751501251007550250

VALLEY WIDTH, IN METERS

Longitudinal valley profileReach boundarySurveyed cross sectionsWidth of valley bottom

Eureka Gulch

Minnie Gulch

Maggie Gulch

Cunningham Creek

Brendel Gulch

Reach

A

Reach B

Reach

C

Reach D

Trench

Reach

E

Eureka

Howardsville

Cataract Gulch

EXPLANATION
Figure 4 (above and following pages). Aerial photographs taken in 1951 of upper Animas River valley. A, reach A upstream 

of Eureka; B, reach B from Eureka Gulch to north end of Minnie Gulch alluvial fan; C, reach C including constrictions caused 

by Minnie Gulch and Maggie Gulch fans; D, reach D from Maggie Gulch alluvial fan to Cataract Gulch alluvial fan, and upper 

portion of reach E.

0 0.25 0.5 KILOMETERS

0 0.25 MILES

Niagara Gulch

Reach boundary

Reach

boundary

Eureka townsite

Eureka Gulch

Flood-plain boundary

 

107°34'12" 107°33'36"

37°52'48"

Animas River

REACH A

REACH B

A

902 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 903

Eureka

Reach boundary

Cross section 2

(between cross bars)

Cross section 1

and trench site

Eureka Gulch

Tailings

107°34'12"

37°52'48"

37°52'12"

Reach

boundary

REACH A

REAC

H B

REACH B

REACH C

E

D

TI H

G

U

L

C

H

FAN

MINNIE GULCH

FAN

0 0.25 0.5 KILOMETERS

0 0.25 MILES

B
Reach boundary

Cross section 2

(between cross bars)

Reach boundary 

Cross section 3

Middleton

Otto Gulch

Flood-plain

boundary

0 0.25 0.5 KILOMETERS

0 0.25 MILES

107°34'48" 107°34'12"

37°51'36"

REACH B

REACH C

MINNIE

GULCH

FAN

Minnie

Gulch

Maggie

Gulch

REACH C

REACH D

MAGGIE GULCH FAN

C

904 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 905

Reach

boundary

Reach

boundary

Flood-plain

boundary

107°35'24" 107°34'48"

37°51'00"

37°50'24"

0 0.25 0.5 KILOMETERS

0 0.25 MILES

REACH C

REACH D

REACH D

REACH E

PORCUPINE

GULCH FAN

CATARACT

GULCH

FAN

D
that the Animas River has been, and has the geomorphic 

tendency to be, fully braided in reach A. The river appears as 

a single-thread channel on the photograph of figure 4A, however, and that probably is the result of anthropogenic activities. 

In the field we observed evidence that the streambed had been 

recently rearranged by heavy equipment upstream of the State 

Road 110 bridge at the Eureka townsite. That road is visible 

as a white stripe in figure 4. In order to protect the integrity of 

the road and bridge (and perhaps the townsite), maintenance 

crews occasionally rearrange the streambed in order to keep all 

streamflow passing under the bridge (Rob Robinson, Bureau 

of Land Management, written commun., 2004). The repetitive nature of that maintenance hints at the sediment transport 

dynamics of the Animas River in reach A.

Reach B (see photographs of fig. 2) extends from the 

Eureka Gulch confluence to the north (upstream) end of the 

extensive Minnie Gulch alluvial fan (fig. 4B). The reach is 

characterized by a valley floor several hundred meters wide 

(mean width 290 m) with mean valley slope of 0.016 m/m. 

The longitudinal profile is concave-up, but the decrease in 

gradient moving downstream is not as pronounced as it is in 

reach A. In 1951 (fig. 4B), a large fraction of the valley floor 

was covered by beds of tailings, visible as irregularly shaped 

white patches on the photograph. The owners of the Sunnyside 

mine (AMLI site # 116) physically removed most of those 

tailings beds before the photograph of figure 2B was taken in 

2001. Thus at present, the valley floor consists of an expanse 

of gravel almost entirely devoid of vegetation (fig. 2B), a 

portion of which is occupied by the braided channels of the 

Animas River. The eastern margin of the valley floor is vegetated, and has been protected from the river by the earthen 

causeway supporting the roadbed of State Road 110. The fact 

that the roadbed is elevated above the valley floor in reach B 

suggests that the channels have tended to migrate rapidly or 

frequently, and (or) that floodwaters have frequently spread 

out broadly from the shallow channels. Throughout this chapter we demonstrate that the Animas River in reach B experienced major changes during the first half of the 20th century, 

and less dramatic changes during the second half of the 

20th century. The trench (pl. 6) discussed later was located 

at the downstream end of this reach (fig. 4B).

Reach C extends from the north end of the Minnie 

Gulch fan to the south end of the Maggie Gulch fan. The 

valley floor in reach C was severely constricted by two pairs 

of opposing alluvial fans (figs. 1 and 4C) and the valley-floor 

width ranged from 15 to 240 m with a mean of 87 m. The 

valley slope is fairly linear in profile with mean of 0.011 m/m 

(fig. 3). The river in reach C is a single thread where constricted laterally by the debris fans and multiple-threaded 

where the valley is wide between the fan-pairs.

Reach D extends from the south end of the Maggie 

Gulch fan to approximately Cataract Gulch. This reach 

had a mean valley-floor width comparable with reach B 

(280 m) and a mean valley slope of 0.009 m/m. The river is 

multiple-threaded in reach D; however, the channel position 

has been relatively stable historically. Similar to reach B, the 

active channels in reach D occupy roughly half of the valley 

floor. Unlike reach B, the remainder of the valley floor is a 

low-lying vegetated flood plain locally including wetlands 

(fig. 4D).

Reach E extended from Cataract Gulch to the 

Cunningham Creek confluence at Howardsville (fig. 3). No 

geomorphic measurements were made in this reach because 

of the presence of an active tailings pond on the valley floor 

and an earlier mechanical realignment of the main channel.

Channel Conveyance

In this section discharge calculations are made for 

channels at three sites in order for us to understand the 

general magnitude of flow events required to mobilize 

the streambed and certain tailings deposits. The sites were 

selected to be representative of the range in modern channel 

morphology. The trench site (cross section 1) was selected 

to represent a braided reach, cross section 2 represents sites 

where the river is multi-threaded, and cross section 3 is a 

site where the valley floor was constricted by alluvial fans. 

The site locations are labeled using the cross section numbers in figures 1, 3, 4B, and 4C. Cross sections 1 and 2 were 

located at the downstream end of reach B. Cross section 3 

was located at the upstream end of reach C, where the valley 

floor is constricted by the Minnie Gulch alluvial fan on the 

east and by a talus fan shed from an unnamed subbasin on 

the west (fig. 1).

Field measurements were made during the low-flow 

conditions of September 1998. We surveyed channel geometry 

and gradient, using a theodolite with electronic distance meter. 

Water-surface slope was measured for each cross section over 

a downstream length of approximately 180 to 195 m, which 

had vertical fall of from 1.5 to 1.8 meters. The low-flow 

water-surface slope was relatively consistent through the three 

surveyed sections: 0.0093 m/m at cross section 1, 0.0090 m/m 

at cross section 2, and 0.0084 m/m at cross section 3. The 

mean slope for the 700-m long encompassing reach was 

0.0097 m/m. In our calculations we use the local slopes 

previously mentioned.

Table 3. Dimensions and other physical characteristics 

of reaches of the upper Animas River.

[Locations of the reaches are indicated in figures 1, 3, and 4. For reach B, 

0.2931 km2 was used for wetland and vegetation calculation, for reasons 

discussed in the text]

Stream 

reach letter-- A B C D

Reach length, km 1.000 1.475 1.587 1.730

Valley floor area, km2 0.1245 0.4374 0.1751 0.4644

Drainage area, km2 67.83 71.25 100.63 108.55

Valley gradient 

(mean), m/m.

0.025 0.016 0.011 0.009

Valley floor width 

(mean), km.

0.170 0.290 0.087 0.280

906 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 907

Elsewhere in this chapter we present and use slope data 

derived from topographic contour maps (fig. 3; table 3). For 

the sake of comparison with the reach with surveyed slope of 

0.0097 m/m, the mean slope computed using the three nearest 

contour lines was 0.0121 m/m.

We determined bed-sediment size characteristics 

using the “pebble-count” method of Wolman (1954), which 

involves measurement of sediment-particle intermediate 

(b-axis) diameters. The results are listed in table 4. Measurements of sediment on the channel banks below the approximate 

bankfull elevation were made at regularly spaced intervals 

along the channel at the surveyed cross sections. The measurements excluded tributary-derived debris-flow sediment exposed 

in cut banks at cross section 3. Sediment in the upper Animas 

River ranged from pebbles to large cobbles. Minor amounts 

of coarse sand and small boulders also were found among these 

sediments, although medium sand and finer grained sediment 

were almost nonexistent in the streambed surface. The median 

particle size was 39 mm (medium-sized pebbles) at cross section 2, and 110 mm (small cobbles) at cross section 3 (table 4). 

The sediment size characteristics at cross section 1 are similar 

to those at cross section 2.

Cross section 1 coincided with the edge of the trench 

excavated by this project and is thus depicted as the ground 

surface on the stratigraphic log (pl. 6). We direct the reader 

to specific channels using the horizontal distances labeled on 

the trench log. The portion of the valley floor east of the State 

Road, east of 287 m, does not receive streamflow from the 

Animas River and thus is not of concern here. Of the remaining valley floor surface, about 40 percent of the width consists 

of tailings beds or gravel where the clasts have dark coatings 

composed of manganese and iron compounds. As these materials are visible in the earliest aerial photographs, we conclude 

that they were not substantially modified by streamflow during 

the latter half of the 20th century. The remainder of the valley 

floor has been modified by streamflow during recent decades. 

The active channels consist of one main channel at 155 m, 

which has one prominent bank 0.9 m high and a point bar, 

and two wide zones containing numerous shallow channels 

with indistinct banks and intervening low gravel bars. The two 

zones of shallow braided channels are located between 107 

and 161 m (including the main channel), and between 191 and 

286 m. Two discharge calculations were made using the channel cross-sectional geometry shown on plate 6, equation 1, 

slope of 0.0093 m/m, and D84 of 9 cm. As mentioned in the 

section on theory, the channel cross sections were subdivided 

into short increments of relative uniform depth, and the discharge for each increment calculated using equation 1 and the 

appropriate increment-area; then the incremental discharges 

were summed to obtain the total discharge.

We estimated the minimum discharge needed to inundate 

the tailings beds along the trench transect by assigning the 

water surface to be at the lowest level of those beds, at 7.38 m 

on the elevation scale of plate 6. This means that we assumed 

the water surface was horizontal (everywhere at the same 

elevation) across the width of all zones of flooding. The cumulative top-width of flow was 124 m. The result was a discharge 

of 42 m3

/s, which is a rare discharge according to the measurements made at the Howardsville stream gauge (tables 1 and 2). 

As an annual instantaneous peak at that gauge, this discharge 

has a recurrence interval of about 10 years (table 2).

That result is used where we discuss the fate of the tailings that remain in our study area. The discharges determined 

in the remainder of this section are used in the two immediately following sections of this chapter.

We calculated the approximate bankfull discharge at 

cross section 1 by modeling the flow needed to overtop the 

gravel bars adjacent to the channels. For this calculation of a 

moderate flow, it would be inappropriate to assume that the 

water surface was everywhere at the same elevation. Note that 

during low-flow conditions the water surface in each channel 

was at a unique elevation (pl. 6), which is typical of braided 

streams. For that reason we modeled the flow needed to overtop the gravel bars adjacent to each channel that conveyed low 

flow at the time the trench was excavated in August of 1988 

(pl. 6). The result is four flow-width zones between 106 and 

113 m, 147 and 160 m, 195 and 236 m, and 268 and 281 m on 

plate 6. The cumulative top-width of flow was 69 m, and the 

result was a discharge of 17 m3

/s.

Cross section 2 is located 217 m downstream from cross 

section 1, and along that distance channels begin to converge 

as they enter the valley floor constriction caused by the two 

fans (figs. 1 and 4B). At cross section 2, both the channels 

and flood plain are distinctly different from those upstream 

in reach B. The channels have transitioned from being braided 

(multiple shallow channels with largely indistinct banks) to 

what we call multi-threaded: more than one channel but most 

having steep and relatively high banks. At cross section 2, 

the valley floor is about 240 m wide, and about 60 percent of 

that width consists of flood plain vegetated by willows and 

Table 4. Streambed sediment characteristics of the upper 

Animas River near Eureka.

[The critical shear stress for initiation of motion of the median sediment 

size was calculated with equation 4 and the given Shields parameter (τ*

)c]

Location

Cross section (fig. 1) 2 3

Downstream distance km 2.77 2.98

Stream reach B C

Sediment size characteristics

1st percentile size mm 6 13

16th percentile size mm 21 53

Median sediment size mm 39 110

84th percentile size mm 91 196

99th percentile size mm 145 311

Mean size mm 51 125

Standard deviation mm 37 73

Geometric mean mm 44 101

Standard deviation 2.1 1.9

Critical shear stress

Using (τ*

)

c

 = 0.030 N/m2 19.5 55
grasses. During low-flow conditions four channels conveyed 

water. One channel was small, and a second was a beaver pond 

at the time of the field measurements. The other two channels 

had steep banks about 0.6 m high. One channel was about 

16.5 m wide and contained a mid-channel bar, and the other 

channel was 12.5 m wide and did not contain a bar.

We calculated the approximate bankfull discharge at 

cross section 2 by modeling the flow needed to overtop the 

banks of the two main channels. The consequence of ignoring 

the small channel and the beaver pond cannot be evaluated 

with available data, but the result is likely an underestimate. 

The discharge calculations were made using the surveyed 

cross-sectional geometry, a slope of 0.009 m/m, D84 of 9 cm, 

and equation 1. The cumulative top-width of flow was 27 m, 

and the result was a discharge of almost 16.6 m3

/s—essentially 

the same as the 17 m3

/s bankfull discharge calculated for cross 

section 1.

Cross section 3 was located 255 m downstream from 

cross section 2, and along that distance the fan constrictions 

cause all channels to converge into one (figs. 1 and 4C). No 

flood plain was present at cross section 3, and the channel is 

shaped like a parabola with high banks exposing fan sediment. 

As there was no clear evidence of bankfull stage at cross 

section 3, we solved the flow equation for the stage required 

for the channel to pass a discharge of 17 m3

/s. We used the 

surveyed cross-sectional geometry, slope of 0.0084 m/m, D84

of 20 cm, and equation 1. The result is that a flow top-width 

of 11 m, maximum depth of 1.14 m, and mean depth of 0.75 m 

will convey 17 m3

/s.

As flow moves from cross sections 1 to 2 to 3, the channel geometry and hydraulics change. The channels transition 

from being braided (multiple shallow channels with largely 

indistinct banks), to what we call multi-threaded (more 

than one channel but most having steep and relatively high 

banks), to a fully confined single channel. For the modeled 

discharge of 17 m3

/s, the calculations indicate the following changes moving downstream. Flow width diminishes by 

about 85 percent, decreasing from 69 to 27 to 11 m. Mean 

flow depth increases by more than a factor of 3, increasing from 0.22 to 0.41 to 0.75 m. Mean flow velocity nearly 

doubles, increasing from 1.1 to 1.5 to almost 2 m/s.

Bed Sediment Mobility

We next evaluated the mobility of the bed sediment at 

the three cross sections, using equation 4 and the assumptions 

contained in the section on theory. Using the results for the 

modeled discharge of 17 m3

/s, and the measured sediment 

characteristics (table 4), we calculated whether the local shear 

stress on the bed exceeded the critical shear stress for initiation of motion. By local shear stress we mean that within each 

width increment of the cross sections.

For a discharge of 17 m3/s at cross section 1, initial motion 

of the bed would occur over cumulative width of 26 m or about 

22 percent of the total width of the bed. For that discharge at 

cross section 2, initial motion of the bed would occur over 

cumulative width of 25 m or slightly more than 90 percent of 

the total width of the bed. For that discharge at cross section 3, 

initial motion of the bed would occur over cumulative width of 

almost 7 m or about 62 percent of the total width of the bed.

The volumetric rate of bed load transport was calculated 

at the three cross sections using equation 6 and the assumptions contained in the section on theory. A model discharge 

of 17 m3/s was used. The resulting bed load transport for 

cross section 1 was 22 m3/hr, and for cross section 2 it was 

15 m3

/hr. The bed load transport calculations involve a variety 

of assumptions, and the results are highly sensitive to slope. 

In the calculation for cross section 2, for example, if a slope 

of 0.0095 m/m had been used (instead of 0.0090 m/m) the 

resulting transport rate would have been 22 m3

/hr. Thus, 

given the uncertainties in the calculations, we consider the 

bed load transport rates at the upper two cross sections to be 

indistinguishable.

From the calculations for cross section 3, using the 

properties of the cobble gravel observed there during lowflow conditions, we learned that the Shields stress on the 

bed (equation 5) did not exceed 0.05. Thus the cobble gravel 

would not experience significant motion, rather some slow rate 

of marginal transport. Calculation of the marginal sediment 

transport at this location is not necessary to achieve the goals 

of this project. From the calculations for the upstream cross 

sections, it seems clear that considerable pebble gravel would 

be delivered to cross section 3 during bankfull flows. Assuming the entire bed was covered by pebble gravel, the calculated 

transport rate was 78 m3

/hr, but that is far in excess of the calculated supply of pebble gravel from upstream. We repeated 

the calculation assuming 20 m3/hr of pebble gravel was 

transported in the thalweg (the lowest portion of the channel 

bed) of cross section 3, and learned the width of the zone of 

pebble gravel transport would be on the order of 1 m, or about 

9 percent of the bed width. We suspect that the cobble gravel 

is something of a lag deposit, moving during major floods but 

only occasionally (slowly) during normal high flows. During 

normal high flows, however, the calculations show that all 

pebble gravel supplied from upstream would be transported 

through the confined reach.

It is possible to conclude the following, considering 

the limitations of the analysis. The bed material of the study 

area can be mobilized by bankfull flows, and pebble gravel 

is particularly mobile. As we discuss in the next section, 

bankfull flow may have a recurrence interval on the order of 

2 years; thus the bed material is mobilized frequently. At the 

downstream end of reach B, the Animas River transitions from 

being fully braided, to multi-threaded, to a single confined 

channel. Through that transition the changes in flow width, 

depth, and velocity are substantial. Yet, it appears that during bankfull flows the bed load sediment transport rate may 

be uniform (not changing in the downstream direction) and 

thus neither substantial net-aggradation nor net-erosion of the 

bed is expected. That reach at least, may be in geomorphic 

908 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 909

equilibrium, no net-changes in bed level during channel forming flows, even though the channel geometries change moving 

downstream in substantial ways.

The reach encompassing the three cross sections has a 

relatively uniform slope, in comparison to the profile of our 

7-km long study area as a whole (fig. 3). We develop the sediment transport implications of the convex longitudinal profile 

in the next section.

Implications of Channel Patterns

One objective of this study was to identify the range 

of channel characteristics likely to be in equilibrium with 

the existing streamflow regime, valley morphology, and the 

caliber and supply rate of sediment in the upper Animas River 

valley. We asked whether the stream channels could have been 

meandering in the past or could be in the future. We evaluated 

this in two ways, first by comparing data for the Animas River 

to empirical relations for other rivers.

All other factors being equal, valleys tend to have 

braided channels as the gradient increases beyond some 

threshold condition. Leopold and Wolman (1957) demonstrated that channel gradient and bankfull discharge could be 

used to define a threshold condition between channels with 

a meandering versus braided pattern. Leopold and Wolman 

(1957) used a broad definition of braiding, including both 

fully braided streams such as those in our area located above 

the trench site and multi-threaded channels such as those 

located downstream of the trench site (figs. 2 and 4). They 

also avoided sites where channels were laterally confined, 

such as at our cross section 3. The Leopold and Wolman 

(1957) threshold is illustrated in figure 5, where sites plotting 

above the line had braided channel patterns whereas those 

plotting below the threshold had meandering patterns. Subsequent work has shown that the tendency to braid is influenced 

by sediment supply rate (Schumm and Khan, 1972) and that 

the threshold is influenced by the grain size of bed and bank 

sediment (Osterkamp, 1978; Begin, 1981). Streams with high 

sediment supply tend to be braided, and sand-bedded streams 

braid at lower gradients than do gravel-bedded streams. There 

are other nuances involved with the threshold of braiding; 

nonetheless, the Leopold and Wolman (1957) relation remains 

a rough statistical discriminator between meandering and 

braided patterns where the bed sediment is relatively fine 

grained gravel (Carson, 1984), such as the bed of the Animas 

River in our study reach.

In order to place data for the Animas River in figure 5, 

we had to estimate the magnitude of bankfull flow moving 

down the study reach. Hydrologists have long recognized that 

flood discharge (in perennial streams) increases moving downvalley as tributaries enter and thus increase drainage area. This 

phenomenon must be considered here because numerous tributaries join the Animas River within the 7-km long study reach 

(fig. 1). Leopold and Miller (1956) studied streams draining 

mountains near Santa Fe, N. Mex., and related bankfull discharge (QBf) to drainage area (Ad

) using a power function

QBf = a(Ad

)

b

 (7)

Conceptually, if precipitation were uniform throughout a 

watershed having uniform infiltration properties, runoff 

would be directly proportional to watershed area. In other 

words the exponent of equation 7 would be b = 1. This is generally not the case for flood events, however, because runoff is 

not generated from all portions of large watersheds (whereas 

it might be for small watersheds). For example, the exponent 

typically ranges from b = 0.7 to b = 0.9 (Dunne and Leopold, 

1978). In the Leopold and Miller (1956) study, the exponent 

was b = 0.8, QBf had a recurrence interval of 2.3 years, and 

the coefficient was a = 0.52 for metric units of discharge 

(m3/s) and drainage area (km2). An alternative scaling relation was developed from measurements of low flow. Kimball 

and others (this volume, Chapter E9) made synoptic discharge 

measurements at numerous locations in the upper Animas 

River study area on August 14, 1998. The low-flow discharges 

ranged from 0.357 m3

/s above Eureka Gulch to 3.56 m3/s at 

the Howardsville gauge (fig. 1). Casting these data in the form 

of equation 7 results in an exponent of b = 1.8.

We started by assuming that bankfull discharge for the 

study area has a recurrence interval of 2 years, and ultimately 

tested that assumption. As shown in table 2, the flow with 

a 2-year recurrence interval (Q2) at the Howardsville gauge 

has a magnitude of 27.7 m3/s. The sites where low flow was 

measured were plotted on maps, and the drainage area upstream 

of each site was determined from the topographic maps. We 

estimated Q2 at each site by scaling (decreasing) 27.7 m3/s using 

equation 7 and exponents of b = 0.8 and b = 1.8. The results 

are plotted in figure 5, with the sites labeled using the letter of 

the reach in which they were located. The two methods yielded 

nearly identical estimates for Q2

 above Cunningham Creek, the 

first tributary upstream of the gauge, but they vary by a factor 

of about three in reach A above the Eureka townsite (fig. 5). 

Because the exponent 1.8 is greater than 1, use of that exponent 

likely results in unrealistically low estimates of Q2

 for sites 

upstream of the gauge.

We tested our assumptions by using the calculated bankfull discharge near the trench site (fig. 1). As discussed in the 

section titled, “Channel Conveyance,” a bankfull discharge of 

17 m3

/s was estimated by calculating the flow needed to overtop the active bars at the trench site and at cross section 2. The 

combined assumptions that bankfull discharge was equivalent 

to Q2

, and that an exponent of 0.8 in equation 7 was appropriate for the study area, result in a discharge of 15 m3

/s that 

closely approximates the bankfull discharge determined using 

field measurements. Use of the exponent of 1.8 results in a 

discharge of 7.5 m3

/s, a low value, as expected; but the important issue at hand is that we not grossly overestimate bankfull 

discharge and erroneously place the results into the braided 

field of figure 5.
Data for all study reaches plot above the threshold line in 

figure 5, within the field for braided streams. Data for reaches 

A and B plot the highest on the diagram. In conclusion, the 

Animas River in the study area was likely never a classical, 

single-thread meandering stream, and this is certainly true for 

the reaches near Eureka. This simple comparison with data for 

other streams suggests that the Animas River has the propensity to be multi-threaded, or fully braided where not confined 

by alluvial fans or dense brush on the flood plain. We next 

addressed the channel pattern issue by exploring the sediment 

transport dynamics.

Meandering streams persist in the landscape because 

sediment transport is uniform along the length of the stream, 

over time scales of individual floods. Stream braiding is 

caused by localized sediment deposition resulting in shoaling 

and branching of the flow. Thus, in order for a single-thread 

channel to persist, local, even temporary bed aggradation 

cannot be large relative to the channel depth during a flood. 

Otherwise the channel may widen, adding sediment and 

promoting shoaling, or excess flow will pass overbank likely 

creating additional channels. Spatially uniform sediment transport is accommodated by adjustments in channel geometry, in 

response to changes in independent variables. For example, 

at the downstream end of reach B, the narrowing valley floor 

forces channels to converge, and the character of the channels 

to change dramatically, yet our calculations indicated that sediment transport may be uniform during bankfull flows. A major 

independent variable (over time scales of individual floods) 

Figure 5. Threshold relation between valley slope and bankfull discharge 

for braided and meandering streams, modified from Leopold and Wolman (1957). 

The 2-year discharge for the Animas River was used as a proxy of bankfull 

discharge.

0.001

0.010

0.100

1.000

1 10 100

ESTIMATED 2-YEAR FLOOD, 

IN CUBIC METERS PER SECOND 

Discharge estimated with Leopold and Miller (1956)

Discharge extrapolated from measured low flow (1998)

Braided/meandering threshold line

Braided channels

Meandering channels

A

A Reach designation

A

A

A

A A

B B

B B

C C

D D

E

C C

VALLEY SLOPE, IN METERS PER METER

910 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 911

for the Animas River is the 72 percent decrease in valley 

slope from 0.029 to 0.008 m/m within our study area (fig. 3). 

From a sediment transport perspective that decreasing slope is 

counterbalanced, at least in part, by the discharge increasing 

downstream. Here we pose a thought question: in our study 

area is it possible for bed load transport to be uniform within a 

single-thread channel?

Consider a hypothetical single-thread channel running 

the length of our study area, which has imposed upon it the 

existing valley gradient, general bed material, and flow regime. 

By flow regime we mean how discharge increases downstream. 

We utilized the discharge with 2-year recurrence interval 

(27.7 m3

/s at the stream gauge) scaled as a function of drainage area raised to the 0.8 power, calculated previously. That 

discharge pattern is presented, and the valley profile is reproduced, in figure 6. For simplicity, the hypothetical channel 

is rectangular in cross section and its width increases downstream as a function of discharge raised to the 0.5 power. It 

has long been recognized for meandering streams that bankfull 

width scales as a function of bankfull discharge, and a typical exponent is 0.5 (Dunne and Leopold, 1978, p. 639). We 

arbitrarily assigned the width to be 8 m at the head of reach A, 

and used that scaling relation to obtain the widths downstream. 

The width was 11.3 m at the downstream end of reach D. We 

ignored any supply of sediment from tributaries, and as it turns 

out that assumption was not detrimental to this calculation. We 

assumed the streambed to be uniformly composed of pebble 

gravel with D50 of 4 cm and D84 of 9 cm. Calculations were 

made for subreaches, each bounded by the contour lines from 

which slope was calculated. As such there are two subreaches 

for both reach A and reach B, shown by the breaks in slope in 

figure 6.

Flow depths were calculated using equation 1, and the 

discharge and width specified for each subreach using the 

approach discussed in the preceding paragraph. The depth 

nearly doubled, increasing from 0.41 m near the head of 

reach A to 0.74 m at the downstream end of reach D. The 

boundary shear stresses for the hypothetical flow, calculated 

using equation 3, decreased by more than half through the 

study area. The calculated shear stress values for each subreach are shown in figure 6, along with a smoothed profile 

emphasizing that most of the decrease occurred within reaches 

A and B. Uniform sediment transport in our hypothetical 

channel requires uniform boundary shear stress, and this was 

not achieved. If sediment were in transport at the upper end of 

reach A, then sediment must deposit as boundary shear stress 

decreases downstream. The necessity for sediment deposition 

would be exacerbated if tributary streams also delivered sediment to the trunk stream.

In order to demonstrate the general magnitude of sediment deposition, we calculated hypothetical bed load sediment transport rates using equation 6. Sediment transport 

rates decreased by 86 percent, from 316 m3

/hr near the head 

of reach A to 43 m3/hr at the downstream end of reach D. Most 

of that decrease occurred within reaches A and B, and we considered changes in sediment transport in reaches C and D to be 

essentially not detectable. An estimate of deposition rate was 

made by subtracting the transport rate in a subreach from that 

y = 98.088x-0.3063

R2 = 0.9659

2,900

2,950

3,000

3,050

CONTOUR ELEVATION, IN METERS

0

20

40

60

80

100

120

Reach A Reach B

Reach C

Reach D

Discharge (RI = 2-year)

valley 

topographic

profile

smoothed

shear stress

profile

calculated

shear stress

DISCHARGE (m /s ) AND SHEAR STRESS (N/m )

012345678

VALLEY DISTANCE DOWNSTREAM, IN KILOMETERS

2 2

Figure 6. Pattern of boundary shear stress for a hypothetical single-thread channel conveying a flow with 2-year 

recurrence interval (RI).
in the subreach just upstream. Technically the results are for 

the solid-volume of particles, so a porosity of 30 percent was 

used to convert particle-volume to bulk-volume of the deposit. 

The deposit was assumed to spread out uniformly over the 

width and length of a subreach. The results were a deposition 

rate of about 4 cm/hr in the lower half of reach A, and less 

than 1 cm/hr in the lower half of reach B. If the hypothetical 

flow were sustained, the channel in the lower half of reach A 

would be full of gravel in about 10 hours, and the channel in 

the lower half of reach B would be full in about 2 days. Recall 

that as a daily mean discharge, the flow with 2-year recurrence interval was equaled or exceeded at the stream gauge 

approximately 0.14 percent of the time on average (table 2), 

or 12 hours per year. When channels fill with gravel, flow is 

forced around the deposit, and a braiding pattern is the inevitable result.

The results of the calculation for this hypothetical channel allowed us to conclude the following. A meandering 

stream (self-sustaining, single-thread, gradually widening 

channel) cannot persist in reaches A and B. Aggradation is 

inevitable, because of the influence the rapidly decreasing 

valley slope has on sediment transport. Perhaps an engineered 

channel resembling a meandering stream could be designed to 

accomplish uniform sediment transport. The width of such a 

stream would need to gradually narrow, and the banks would 

have to be hardened, and very high, to prevent the river from 

widening or escaping onto the previous braid plain. Natural 

processes would not create such a channel. Natural processes 

did create a channel network with spatially varying geometry 

that did accommodate uniform bed load transport. In subsequent sections we use a variety of evidence to demonstrate the 

following. In prehistorical times reach A was broad, shallow 

and braided. Through reach B the Animas River transitioned 

into being multi-threaded; each thread was relatively narrow, 

with meter-high banks composed of cohesive flood-plain silt 

strengthened and protected by a dense thicket of willows. That 

bed load transport was uniform is evidenced by the fact that 

during the thousand years before A.D. 1900 the channels did 

not aggrade appreciably, as will be demonstrated.

Sources of Sediment Supplied 

to the River

In order to understand the origin and ages of metal-laden 

sediment in our study reach, and to help identify the causes 

of channel change, we evaluated the various sources and rates 

of supply of sediment to the river. This involves an understanding of the history of mining and other land-use practices, 

in particular the milling of ore (fig. 7). In this section we 

present various measurements as they were reported, in feet, 

miles, and tons. Presumably “tons” means U.S. short tons 

(2,000 pounds).

History of Ore Milling

Jones (this volume) has discussed the history of mining in 

the whole region. The history presented herein focuses on the 

mills in the headwaters area of the upper Animas River that 

produced significant tailings, which might have impacted the 

river between Eureka and Howardsville. We adopt the names 

and site numbers (given in parentheses) of Church, Mast, 

and others (this volume, Chapter E5, fig. 5). The SunnysideThompson Mill (site # 113) was located in upper Eureka 

1860 1880 1900 1920 1940 1960 1980

0

200

400

600

800

1,000

1,200

CALENDAR YEAR

?

TONS PROCESSED PER DAY

Figure 7. Combined ore processing rates for the succession of mills in the town of 

Eureka and in Eureka Gulch. These rates (from Bird, 1986; Marshall, 1996; and Sloan and 

Skowronski, 1975) were quoted in “tons,” which presumably means U.S. short tons. (To 

convert short tons to metric tons, multiply by 0.907; ?, production rate uncertain.)

912 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 913

Gulch about 5.5 km from the Eureka townsite and close to the 

Sunnyside mine on the shores of Lake Emma. The Sunnyside 

Mill # 1 (site # 158), sometimes called the Midway Mill, was 

located about 3 km up Eureka Gulch from its confluence with 

the Animas River. The Sunnyside Mill # 2 (site # 165) and 

the Sunnyside Eureka Mill (site # 164) were both located in 

Eureka. There were also various mines and mills (sites # 24, 

# 26, and # 27) near the Animas Forks townsite, which is on 

the Animas River about 7 km upstream of Eureka.

The first prospectors explored the area in 1860; however, 

the Civil War brought those expeditions to a halt (Bird, 1986). 

In 1871 expeditions resumed and the prospectors learned that 

placer mining would not be viable, but that money could be 

made mining hard rock. Mining did not begin in earnest until 

after the September 13, 1873, Brunot Treaty, which ceded land 

from the Ute Indian tribe. Through the 1870s mining produced 

valuable metals, but the quantity of ore (hand sorted and 

“high-graded” by miners and shipped from the area on mules 

and burros for processing) was relatively small. Mines around 

Animas Forks, for example, “were worked in the 1870’s and 

produced 800 tons of ore” (Marshall, 1996, p. 140). Compared 

to later production, 800 tons in a decade is paltry and none of 

that ore was milled on site. The Sunnyside mine ultimately 

became famous as the biggest gold mine in Colorado history, 

but in 1879 its “workings consisted of one shaft 10 feet deep 

and one drift 30 feet in length” (Bird, 1986, p. 16).

During the 1880s and 1890s mining (and eventually 

milling) occurred at a limited scale, and was intermittently 

disrupted by mechanical failures, natural disasters, and economic setbacks. In 1888 the first mill (a “ten-stamp” mill) 

was constructed close to the Sunnyside mine and located 

“just below Lake Emma” (Bird, 1986, p. 29), meaning that 

the sand-sized tailings were discharged into the Eureka Gulch 

drainage. Late in 1889 the Sunnyside Mill # 1 began operation next to the creek in Eureka Gulch and processed 15 tons 

of ore per day. This early mill would have been much like that 

shown in figure 11 of Jones (this volume). Stamps were added 

to that mill in 1897, increasing capacity to 50 tons per day. 

After the Silverton Northern Railroad tracks reached Eureka 

in 1896 (Sloan and Skowronski, 1975), the first mill in Eureka 

(Sunnyside Mill # 2) was constructed in 1899 and began 

production in 1900 (Marshall, 1996). It was designed for a 

capacity of 125 tons per day; in figure 2 it is visible as a dark 

rectangle on the right side of the mouth of Eureka Gulch. It 

was similar to that shown in figure 12 of Jones (this volume).

These stamp mills were inefficient. A “stamp” was a 

piston device used to crush ore to the size of sand, and economical minerals were then concentrated using various means, 

generally mechanical, for railroad shipment to smelters. The 

remaining sandy material was discharged from a mill’s tailrace into the Animas River or its tributaries, and these tailings 

contained metals. In 1899 “60 percent of the values ended up 

in the tailings that were carelessly dumped into Eureka Creek” 

(Bird, 1986, p. 34). These “values” were unrecovered gold and 

silver, but the tailings also included pyrite, galena, pyroxmangite, and some sphalerite hosting other metals such as lead 

and zinc. Around 1900, the Sunnyside Mill # 2 recovered less 

than 50 percent of the metals contained in lead-zinc ores being 

exploited at that time, having been designed to recover gold 

(Bird, 1986, p. 68). The stamp mills were thus inefficient at 

recovering minerals of economic value at the time, and the 

tailings released then contained elements such as lead and zinc 

that are of concern now.

The stamp mills were generally in full production from 

1900 to 1918. Late in 1904 the Silverton Northern Railroad 

began service to the town of Animas Forks and the new Gold 

Prince Mill (site # 27) located there. The Gold Prince Mill had 

a reported capacity of 500 tons per day (Marshall, 1996), but 

production from the mill (and others in the area) was probably small. It was dismantled in 1910, and railroad service to 

Animas Forks was abandoned in 1916 (Sloan and Skowronski, 

1975). Uniquely, the Sunnyside mine remained profitable, 

because its ores were rich in gold. In 1910 it consisted of 

“ten miles of underground workings” and the Sunnyside 

Mill # 2 was processing 3,500 tons of ore per month (Bird, 

1986, p. 114). In 1912 an electrostatic zinc recovery plant 

was added to the Sunnyside Mill # 2 (presumably reducing 

zinc concentrations in the sand-sized mill tailings), and that 

and rising zinc prices probably saved that mine (unlike the 

others) during World War I.

The year 1918 marked the beginning of a new era. The 

new Sunnyside Eureka Mill was completed; it is shown in 

figure 19 of Jones (this volume). This mill was capable of processing as much as 600 tons of ore per day using a revolutionary selective flotation process that increased mill efficiency 

and profitability (Bird, 1986). Ore crushing techniques were 

also significantly improved at this time, resulting in finegrained tailings composed of silt-sized and finer (<300 mesh) 

particles. The old stamp mills were abandoned. Full-scale production was hampered, however, because of various disasters, 

until the end of 1919; but then the Sunnyside mine was forced 

to close late in 1920 because metal prices plunged. In the fall 

of 1921 the mine reopened and the boom years began. By 

1923 the mine was producing 16,000 tons of ore per month, 

20,000 tons per month by 1925, and broke all records by producing 1,100 tons of ore per day by 1929 (Bird, 1986). After 

the October 1929 stock market crash, however, the Sunnyside 

mine struggled and then closed in September 1930. The mine 

was reopened again in 1937, faltered, and closed again in 

August 1939. At the time of closing, a total of 2,500,000 tons 

of ore had been mined and milled, producing $50,000,000 in 

metals—and Eureka became a ghost town (Bird, 1986).

Figure 7 illustrates the trend in the quantities of ore 

milled by the Sunnyside mine. About 80–90 percent of the 

processed ore was released as tailings (King and Allsman, 

1950). Using the 80 percent value and the ore processing rates 

in figure 7, mill tailings production rates were calculated and 

converted to metric tons. Most coarse-grained stamp mill 
tailings were produced between 1900 and 1918, at rates in 

excess of 35,000 t (metric tons) per year. Most fine-grained 

tailings were produced after 1921 and before 1930, at rates 

between 150,000 and 330,000 t per year.

Other Sources of Sediment

The rates of sediment supply from other sources are estimated for comparison with that from milling. The calculations 

herein include the watershed area of Eureka Gulch, so we 

are discussing the delivery rate to the top of reach B. Known 

rates of hillslope sediment yield span 6 orders of magnitude, 

from 0.004 to 500 t/hectare/year. These two numbers are for 

undisturbed primeval forests and for intensely disturbed croplands and construction sites, respectively (Dunne and Leopold, 

1978). The prehistorical sediment production rates for the 

upper Animas River setting were probably at the low end 

of that spectrum, around 0.01 or perhaps 0.1 t/hectare/year, 

resulting in total yield from the 73.6 km2

 watershed of perhaps 

74 or 740 t per year.

Logging probably did not increase sediment yields significantly, because only 12 percent of the watershed is forested 

and only a fraction of the original forest was logged. The area 

was grazed, and grazing can increase sediment yields by perhaps 10 times (Dunne and Leopold, 1978), but only about half 

of the headwaters area of the upper Animas River basin is soil 

mantled and thus susceptible to increased erosion by grazing, 

which by nature of the high altitude is seasonal. Grazing (and 

to a lesser extent logging) may have increased the watershed 

sediment yield by hundreds to a few thousand metric tons per 

year.

Erosion of mine-waste dumps also supplied sediment to 

streams. In the Animas River basin upstream of the trench site, 

there are 24 mine shafts, 209 mine portals, and 679 prospect 

pits, based on the mining symbols on the USGS topographic 

maps. Each of these excavations has a pile of waste rock, and 

each waste dump contains about 200 t of material covering 

an area of about 50 m2

, on average (J.T. Nash, oral commun., 

1999). Together the waste dumps cover an area of approximately 4.5 hectares. An erosion rate of 100 t/hectare/year is 

typical for disturbed land in general (Dunne and Leopold, 

1978). If this is appropriate for mine dumps, collectively the 

dumps in the basin lost 450 metric tons of sediment per year, 

but only a fraction of the coarse sediment mobilized from 

dumps reached streams because most mine sites are located 

high on hillslopes several kilometers from streams.

In conclusion, prehistorical sediment production rates 

for the watershed upstream of reach B may have been on the 

order of 74–740 t per year. Human activities, such as logging 

and mining (but excluding ore milling), may have increased 

sediment production to a few thousand metric tons per year, 

or less. Ore milling in the early part of the 20th century, in 

contrast, resulted in tailings being supplied directly to the fluvial system at rates varying from 35,000 to 330,000 t per year.

Photographic Evidence of Change 

and Recovery

Matched pairs of photographs taken over various intervals of time have been used as a scientific tool for a century 

or more (Rogers and others, 1984), and have been particularly useful for documenting changes in the physical landscape (Malde, 1973) and changes in vegetation communities 

(Hastings and Turner, 1965). Oblique photographs allow 

qualitative evaluation, at least, of changes occurring during 

periods as long as 150 years. Aerial photographs allow quantitative evaluation, using modern computational techniques, 

of changes over periods generally less than 60 years. We have 

used both types of photographs to help understand historical 

changes that have taken place in the study reach.

Early Oblique Photographs

We inspected historical photographs in order to understand the character of the Animas River during the early days 

of mining, before milling and other human activities became 

intense. For that reason attention was paid toward authentication of photograph dates.

The earliest picture of the study reach that we were able 

to find is a print based on a photograph taken in 1877 (fig. 8) 

and shows reach A and the town of Eureka (fig. 4A), which 

was established in 1876. The title on the print gives the 1877 

date and states that it was “photographed by P.A. Felt.” The 

picture was clearly reproduced using a printing press, and 

therefore the accuracy of the image depends in part on the 

method used to transfer the photograph to the printer’s plates 

(Ostroff, 1981). Certain lithographic plates were made directly 

from negatives, similar to the way photographic prints are 

made, and the result was a highly accurate image (although 

an artist could make changes like adding clouds). Other plates 

were wood engravings made by either tracing directly or copying by eye from a photograph, and the results were variable 

in accuracy (Ostroff, 1981). Based on the high contrast and 

absence of gray-tone range, we believe the image in figure 8 

was printed from a wood engraving. We compared the cliffs 

and strips of trees in center-left background to photographs 

(see fig. 2) and found they were rendered accurately. For that 

reason, we believe figure 8 accurately depicts the Animas 

River as it was in 1877. Felt took the photograph 1 year after 

Eureka was established and 6 years after miners arrived in 

the area. Before 1877 it is likely the miners had only two 

impacts of potential significance to this discussion. One was 

the cutting of trees for firewood and building material, and 

indeed what appear to be tree stumps are visible in center 

foreground of figure 8. No tree stumps are visible adjacent to 

the channel, however, so apparently the Animas River in reach 

A was not previously lined with riparian trees. The second 

impact was the use of beaver as a food source, and their likely 

914 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 915 Figure 8. Reproduction of photograph taken by P.A. Felt of Eureka in 1877. Denver Public Library Western History Collection No. X-11438, used with permission. The view is north, up the Animas River, and shows reach A.
extermination. Beaver promote the growth of woody shrubs, 

which in turn promote the stability of channel banks and flood 

plains (Kean and Smith, 2004; Smith, 2004). We cannot evaluate the efficacy of beaver in our study area, because they were 

likely eradicated prior to this earliest depiction of the area. 

Nonetheless, thickets of willow are not apparent adjacent to 

the channel in figure 8. In conclusion, the channel of reach A 

was wide, sinuous, had low banks, and was apparently braided 

in 1877, if not before. The apparently braided nature of reach 

A is consistent with our findings in the section on implications 

of channel patterns, and other evidence present in the next 

paragraphs.

Another early photograph of Eureka (Bird, 1986, his 

fig. 41), attributed by Bird to have been taken in 1880, shows 

the channel in the upper portion of reach B as wide and 

braided. We were unable to locate the original print, and thus 

cannot confirm the date or describe the detailed character of 

the channel, but the tracks of the Silverton Northern Railroad 

are not evident, so that photograph was likely taken before 

1896. The next photograph discussed (C-Eureka, 92.181.35) 

was found at the Colorado Historical Society archive in an 

album of photographs taken by the “Candee and McNutt families in 1883 and 1884.” The photograph is of people standing 

on the edge of the channel just downstream of Eureka. The 

streambed was composed of small-diameter cobbles, had vertical banks less than 1 m high, and may have been on the order 

of 30 m wide. The flood plain was composed of fine-grained 

sediment (not gravel), and was vegetated by scattered clumps 

of willow (about 2 m high and wide) on an otherwise barren 

plain that appears to have been heavily grazed. No tree stumps 

were visible. Currently in that viewscape of the upper portion 

of reach B, there are no willows, the channel is fully braided 

and no meter-high banks are present (fig. 2B). Evidently the 

upper portion of reach B aggraded since the late 1800s, burying the channel banks observed in the 1883–84 photograph.

In 1906 George L. Beam took the photograph reproduced 

as figure 9A, from the railroad grade looking downstream 

(south) at Eureka, and at our reaches A and B of the Animas 

River. Although Sloan and Skowronski (1975, p. 256) indicated that the photograph was taken by Beam in 1911, Thode 

(1989, p. 86) stated it was taken by Beam in 1906. We assume 

the 1906 date is correct because Thode is the acknowledged 

authority on George Beam and his photographs, which Thode 

resurrected from long-forgotten Denver and Rio Grande 

Railroad Company files. In any case, a photograph taken 

earlier from the identical viewpoint shows the same braided 

nature of the channels in reach A, except the channels are in 

slightly different locations. That photograph (labeled “Eureka” 

in the Silverton Northern folder of the Colorado Railroad 

Museum) must have been taken earlier because of the absence 

of the large white building that is prominent in the middle of 

figure 9A. Sloan and Skowronski (1975) credited the Colorado 

Historical Society as the source of the photograph (fig. 9A), 

but we have not been able to locate the original in that 

archive nor in the Jackson Thode collection of Beam photographs at the Colorado Railroad Museum. We reproduced 

the photograph with permission from Sundance Publications 

(D.A. McCoy, written commun., 2004). For spatial reference, 

the oval-shaped patch of spruce on the hillslope in the center 

of the photograph, above the large white building in Eureka, 

is visible in the center of the left half of figure 2. The canyon 

of Minnie Gulch meets the Animas River valley off the center 

of the left margin, and the patch of spruce on the valley floor 

in the background is on the Minnie Gulch debris fan. In the 

1999 repeat photograph (fig. 9B) the toe of the Minnie Gulch 

fan is located at the apex of the dogleg in the road, visible as 

a light stripe at center left. Our boundary between reaches B 

and C crosses that bend in the road (fig. 4B). This pair of 

photographs is important for three reasons. First, the lower 

portion of reach A was clearly braided in 1906. By 1999 the 

channel was much less braided, and appears to have incised 1 

or 2 meters. In addition onsite inspection revealed the tracks 

of heavy machinery on the streambed in 1999. Notice in 

figure 9B the elevated berm of gravel on the left bank adjacent to the townsite that serves to orient the channel under the 

bridge. The braided pattern (fig. 9A) and the need for maintenance to protect the bridge and townsite (fig. 9B) are consistent with our findings in the section on implications of channel 

patterns. Second, in 1906, in the upper portion of reach B, just 

downstream of Eureka, the channel was wide and apparently 

braided or at least multi-threaded. Third, in the lower portion 

of reach B, approaching Minnie Gulch, wide areas of densely 

vegetated flood plain existed, unlike today. These observations 

from the 1906 photograph help interpret figure 2A, because of 

the tonal quality and perspective of the ca 1904 print discussed 

next.

The earliest photograph that shows a large extent of 

our study reach is reproduced as figure 2A. A print of this 

photograph is on file with the Colorado Historical Society 

(C-Eureka, F7005) but is not dated on the back. We believe 

that the photograph was taken in 1904, or shortly before, 

for two reasons. It was published in 1904 by the Silverton 

Standard (1904). The railroad grade leading upstream to 

Animas Forks is clearly visible in figure 2A, which is significant because the Silverton Northern Railroad began service to 

the town of Animas Forks late in 1904 (Sloan and Skowronski, 

1975), and thus the photograph could not have been taken too 

many years before 1904. This photograph was repeated in 

2001 (fig. 2B) from approximately the same site.

The following changes in the landscape between 1904 

and 2001 are evident by visual comparison of the prints in 

figure 2. Vegetation on bedrock hillslopes and talus slopes 

changed very little, and certain individual spruce trees can be 

identified in both prints. The present road is located along the 

same general course as was the railroad, and these features 

tended to protect the east (foreground) side of the valley 

floor from erosion by the Animas River. Within that eastern 

fringe of the valley floor, the willow bushes and scattered 

spruce trees are only slightly less dense in the 2001 photograph (fig. 2B). In 1904 a number of large deciduous trees, 

presumably cottonwood, were present near the railroad. A 

few cottonwoods are present now. The fringe of shrubs just 

916 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 917

Figure 9 (following two pages). Photographs showing reaches 

A and B of the Animas River, taken from the Silverton Northern 

Railroad grade looking downstream at Eureka. A, Photograph taken 

in 1906 by George L. Beam, reproduced courtesy of Sundance 

Publications. B, Repeat photography by Kirk Vincent in 1999, from 

approximately the same location as A.

west of the railroad in 1904 is essentially absent now. The 

band of dense (dark) shrubs and spruce along the west edge of 

the valley in 1904 is entirely absent now. The areas of dense 

shrubs might have been low terraces, or flood plain that had 

not been reworked in some number of centuries. The nature 

of the channel and flood plain varied along the central axis of 

the valley in 1904, but care must be taken in the interpretation 

(Rogers and others, 1984) because of the presence of areas in

sunlight and areas of shadow in figure 2A. Just downstream of 

the Eureka bridge (and in the foreground of the large building 

with the white roof) the channel is clearly wide and multithreaded and apparently had prominent banks. In the center 

of the photograph the river channel or channels are obscured, 

but the flood plain is occupied by sparse and small bushes 

that are absent now. The density and sizes of willows increase 

downstream in the 1904 photograph, and those plants are also 

absent now. At the downstream end of reach B, in the vicinity 

of our trench labeled T (fig. 2B), the 1904 print shows sunlight 

apparently reflecting off a multi-threaded stream, but the zone 

of channels is narrow and is bounded by flood plain covered 

by dense shrubs. At present, that area consists of a wide zone 

of shallow braided channels, and the flood plain is devoid 

of shrubs. The stratigraphy of the trench (pl. 6) supports the 

interpretation of multi-thread channels with prominent banks 

at that location early in the 20th century, and before, as will 

be discussed later.

In conclusion, reach A was braided before mining 

and milling became intense (figs. 8 and 9A), and was likely 

braided in prehistorical times. In the early days of mining, the 

Animas River in reach B consisted of multi-threaded channels, 

and the valley bottom consisted of areas of sparse willows and 

areas of dense vegetation. Except for the areas protected by 

the elevated roadbed, that entire reach is now fully braided and 

devoid of vegetation. The changes documented using oblique 

historical photographs largely occurred during the period of 

production of sand-sized tailings (see section on flood-plain 

stratigraphy), but other changes occurred after 1945, as discussed next.

Aerial Photographs 1945 to 1997

Methods

Aerial photographs of the upper Animas River valley, 

figure 4 for example, reveal the channel condition and position at several discrete moments in time. Examination of sets 

of rectified photographs allows quantification of changes in 

channel position and vegetation during the periods between 

the photograph dates (Gurnell, 1997), and is most accurately 

done in a Geographical Information System (GIS). Aerial 

photographs of the upper Animas River valley taken in 1945, 

1951 (fig. 4), 1960, 1973, 1987, and 1997 were selected for 

that type of geomorphic analysis. Most of the photographs 

were taken in August or September when seasonal streamflow 

had receded to low levels, and the timing of the photographs 

with respect to the flood series is shown in table 1.

The upper Animas River aerial photographs were enlarged 

to 18×18 inch prints having scales of between approximately 

1:6,300 and 1:14,000. The enlarged photographs were digitally 

scanned and these images were electronically processed to 

remove optical distortion and photographic parallax common to many aerial photographs. These aerial photographs 

were registered and rectified before geomorphic measurements and analysis were conducted. Digital image processing 

and geometric correction are described in greater detail by 

Jensen (1996). Image digital processing, rectification, and 

registration were done with ArcView Spatial Analyst software 

(Environmental Systems Research Institute, 1996).

Raw, unprocessed photographs can be registered and 

rectified to a digital orthophoto quadrangle (DOQ) map 

(Elliott and Gyetvai, 1999), or they can be registered and 

rectified to points of known latitude and longitude. Because 

DOQ maps of the upper Animas River valley did not exist at 

the time of this analysis, two 1997 aerial photographs were 

rectified to 10 and 13 reference points on or near the upper 

Animas River flood plain using a Precision Lightweight 

Global-Positioning System Receiver (PLGR). Reference 

points were features identifiable both on the ground and in 

the 1997 aerial photographs, for example, railroad intersections, building foundations, road culverts, and earthen berms. 

Calculated PLGR latitude and longitude coordinates in the 

upper Animas River valley had horizontal errors ranging 

from 5 to 13 meters.

The rectified 1997 photographs became the standardized images to which the other aerial photograph sets were 

registered and rectified. Features visible in all photographs 

were used as rectification control points. These points were 

at elevations similar to the flood-plain elevation, thereby 

reducing the potential for rectification error caused by relief 

displacement (Gurnell, 1997). The unprocessed, earlier aerial 

photographs were rectified to the scale and coordinates of the 

1997 standardized photos with second order polynomial affine 

transformation functions fitted to the (17 to 39) control points 

using a least-squares criterion (Jensen, 1996). The goodness of 

fit between the rectified and the 1997 standardized photo was 

evaluated quantitatively with root-mean-square error (RMS) 

statistic calculated by the ArcView Spatial Analyst extension. 

The RMS error describes the deviation between the controlpoint locations on the rectified photograph and the controlpoint coordinates calculated by the transformation function 

for the Easting and Northing directions. Table 5 presents 

summary statistics from the photograph rectification procedure. The RMS errors (0.83 to 2.50 m) were small (Gurnell, 

1997). Consequently, we have a high degree of confidence that 

the apparent channel and flood-plain changes observed during 
A

918 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 919

B
Table 5. Statistics from the digital processing and rectification of upper Animas River valley aerial photographs.

[The 1997 photographs were rectified to reference point coordinates determined in the field with a Precision Lightweight Global Positioning System Receiver. 

Earlier photographic images were rectified to the standardized 1997 image with a second-order polynomial affine transformation function. The term “upper 

reach” covers our reaches A, B, and C. “Lower reach” covers our reach D]

Photograph date

and reach

Number

of control

points

Root mean

square error,

Easting

(meters)

Root mean

square error,

Northing

(meters)

Chi square,

Easting

Chi square,

Northing

1945 Upper reach 17 1.34 1.44 30.53 35.43

1945 Lower reach 19 1.88 1.64 66.90 51.06

1951 Upper reach 23 0.83 1.09 15.98 27.48

1951 Lower reach 19 1.60 1.49 48.68 42.00

1960 Upper reach 24 1.12 1.10 30.21 29.06

1960 Lower reach 31 1.30 1.40 52.48 60.87

1973 Upper reach 27 1.57 1.29 66.13 44.92

1973 Lower reach 39 1.13 1.13 49.77 49.79

1987 Upper reach 29 1.37 1.29 54.26 47.90

1987 Lower reach 24 1.50 1.00 53.78 23.82

1997 Upper reach 13 1.99 1.47 51.65 156.64

1997 Lower reach 10 2.50 2.00 62.44 40.09

the five photo-defined periods represent true changes on the 

land surface. After rectification, channel locations from all 

6 years were digitized and plotted on a single map. Changes 

in the stream channel position and other characteristics

were measured with the ArcView Spatial Analyst extension 

(Environmental Systems Research Institute, 1996).

We started the analysis by mapping the extent of 

Animas River deposits. We refer to these areas as the valley 

floor because they include the active streambeds, the flood 

plain, and any low terraces present. Operationally the boundary of the valley floor was defined as the foot of hillslopes or 

the toes of alluvial fans or talus cones. The intention of the 

valley floor map unit was to depict areas that have been or 

could be inundated during floods on the Animas River.

One measurement typically obtained from aerial photographs is sinuosity. Channel sinuosity is defined as the ratio 

of stream channel length to valley length for a specified valley 

segment or reach. Sinuosity is most commonly used as a 

descriptor of a meandering, single-thread channel; however, it 

also can be used to describe a property of a braided or multithreaded channel (Richards, 1982, p. 10). In this study, we 

measured the normalized stream length (Lct/Lr

), which is the 

total of all conveying channel segment lengths (Lct) visible in 

an aerial photograph divided by valley length of the reach (Lr

). 

A value of 1 would represent a single and perfectly straight 

channel. Larger values reflect the combined effects of the sinuosity of individual threads and, to a larger degree in this case, 

the total number of threads of flow. Values of Lct/Lr

 were calculated for reaches A, B, C, and D for each aerial photograph 

year (table 6), and help document recent channel adjustments 

in the upper Animas River valley.

Channel density describes the relative abundance of 

channel segments per area of the valley floor. In this study, 

channel density is defined as the total length of conveying 

channels in a reach divided by the reach valley-floor area. 

Channel density was calculated for reaches A, B, C, and D for 

each aerial photograph year (table 6). A high value for channel 

density can occur because numerous channel threads occupy a 

specific area of the valley floor, as with a braided channel, or 

a high value can occur if there is a single channel in a laterally confined reach with a small valley-floor area. The trends 

in channel density match the trends in the normalized stream 

length (Lct/Lr

), as discussed later in this section.

Riparian vegetation promotes streambank and flood-plain 

stability by increasing flow drag and thus decreasing boundary shear stress, by increasing bank sediment mass cohesiveness, and by inducing deposition of sediment carried in the 

streamflow (Thorne, 1990; Smith and Griffin, 2002; Smith, 

2004). Vegetation cover and wetland areas also have important 

ecological functions. Valley-floor areas classified herein as 

either vegetated or wetland were delineated on the rectified 

aerial photographs from all six sets of aerial photographs, and 

the results are presented in table 6. These areas on the 1987 

aerial photographs were field checked in 1998. Vegetated areas 

were covered with grasses and woody species including willow, cottonwood, and spruce, and generally had at least thin, 

rudimentary soils at the surface. Wetland areas were covered 

with willows, grasses, and sedges, or were flood-plain areas 

where water was impounded predominantly by beaver dams 

and manmade impoundments. Areas of bare fluvial sediment 

and actively conveying channels were excluded from either 

classification. The area of reach B evaluated for vegetation and 

wetland analysis was 67 percent of the reach B area evaluated 

920 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 921

for channel parameter due to a large, late-day shadow that 

obscured part of the valley floor in the 1973 aerial photograph. 

That same unobscured portion of valley-floor area was used 

to compute wetland and vegetated areas in the other aerial 

photographs (table 6). The numbers of ponds evident in the 

photographs were counted. The ponds visible in the 1997 photographs were verified in the field in 1998 to be beaver ponds. 

The Forest Queen wetland in reach B has an area of open 

water, but it was not obvious in the aerial photographs.

Results

During the time period represented by the aerial photograph series, the nature and position of channels and the 

character of the flood plain changed within individual reaches 

(table 6). The least change occurred in areas where the valley 

floor was narrow, such as at fan constrictions.

There were both spatial and temporal variations in the 

normalized stream length within reaches. Reaches A and C 

consistently had the lowest Lct/Lr

, and the positions of channels 

were the most stable. Streamflow usually was contained in a 

single- or double-thread channel in reaches A and C in most 

years for which aerial photographs were analyzed. These 

reaches had the narrowest valley floors and consequently 

offered the least opportunity for multiple channels to develop 

and move. The normalized stream length values for these 

reaches, typically1.5–2.0, are controlled by the sinuosity of 

the channels. As discussed in several sections of this chapter, 

however, reach A has the inherent tendency to be braided, but 

a single channel was maintained through use of heavy equipment. Reaches B and D had the largest Lct/Lr values, typically 

2.7–4, reflecting the presence of multiple channels in the 

reaches with the widest valley floors.

Reaches A, C, and D experienced the same pattern of 

changes in Lct/Lr

 through time (table 6). In general, normalized stream length values increased during the period between 

years 1945 and 1973, and then decreased after that. This 

change may be a response (creation of threads of flow) to 

the large floods that occurred in 1949, 1952, 1953, and 1957 

followed by natural recovery (abandonment of certain channels). Each of these floods (table 1) had recurrence interval 

Table 6. Changes in geomorphic and wetland characteristics in reaches of the upper Animas River, from 1945 to 1997.

[Channel density, Lcr/reach area; normalized stream length, Lcr/reach length; nc, not calculated]

Year of 

photo

Total channel

length (Lcr)

(km)

Channel 

density 

(km/km2

)

Normalized

stream length

(km/km)

Wetland or

vegetated area

(km2)

Reach

wetland or

vegetated area

(percent)

Number

of beaver

ponds

Reach A—”The Falls” to Eureka townsite

1945 1.502 12.07 1.50 nc nc 0

1951 1.384 11.12 1.38 nc nc 0

1960 2.222 17.85 2.22 nc nc 0

1973 2.519 20.24 2.52 nc nc 0

1987 1.579 12.68 1.58 nc nc 0

1997 1.443 11.60 1.44 nc nc 0

Reach B—Eureka townsite to Minnie Gulch fan

1945 2.875 6.57 1.95 0.0214 7.30 0

1951 4.860 11.11 3.29 .0107 3.64 0

1960 4.001 9.15 2.71 .0110 3.74 0

1973 4.292 9.81 2.91 .0125 4.27 0

1987 4.643 10.61 3.15 .0095 3.25 0

1997 6.532 14.93 4.43 .0084 2.86 0

Reach C—Minnie Gulch fan to Maggie Gulch fan

1945 2.498 14.27 1.57 nc nc 0

1951 2.311 13.20 1.46 nc nc 0

1960 3.185 18.19 2.01 nc nc 0

1973 3.214 18.36 2.03 nc nc 0

1987 2.333 13.33 1.47 nc nc 1

1997 2.657 15.18 1.67 nc nc 1

Reach D—Maggie Gulch fan to Cataract Gulch fan

1945 5.232 11.27 3.02 0.2738 58.95 4

1951 6.266 13.49 3.62 .2399 51.65 5

1960 6.617 14.25 3.83 .2446 52.66 5

1973 6.883 14.82 3.98 .2245 48.34 5

1987 4.896 10.54 2.83 .2315 49.85 14

1997 3.948 8.50 2.28 .2900 62.44 17
greater than 10 years (table 2) and generated sufficient boundary shear stresses to erode banks and transport large volumes 

of sediment as bed load.

Reach B had a contrasting trend: Lct/Lr

 generally 

increased through the period of record—but in a complicated 

way. Normalized stream length was higher in 1951 than it 

was in 1945 or 1960, and it increased dramatically between 

1987 and 1997 (table 6). The trend for this reach is unique, 

and it is difficult to explain in terms of a response to large 

floods (table 1). The actions of heavy machinery, however, 

are evident in the reach. For example, five manmade berms 

oriented perpendicular to the valley axis were conspicuous 

in the earliest aerial photographs of reach B (fig. 4B). They 

did not impound the river during the period of record. The 

purpose of these berms was evidently to trap tailings on their 

upstream sides, and they deflected streamflow eastward in 

the earlier aerial photographs. With time, each of these berms 

was breached at one or more locations and their east ends 

were removed, increasing the area of the valley floor that the 

river channels could occupy. Remnants of the berms were 

about 4 m high in 1998, and thus the Animas River did not 

cause the new breaches. Lastly, operators of the Sunnyside 

mine removed tailings from reach B in 1997 (Rob Robinson, 

Bureau of Land Management, written commun., 2004), which 

likely explains the unnatural ground-surface texture evident 

in figure 2B. These mechanical actions may have increased 

stream branching, and thus normalized stream length.

Data for the vegetated and wetland areas illustrate 

important differences between reaches B and D (table 6). 

Approximately 50–60 percent of the valley floor in reach D 

was vegetated or wetland during the period covered by the 

aerial photographs. In reach B, however, only 3–7 percent of 

the valley floor hosted vegetation or wetlands during the same 

period. The trend in vegetation and wetland area for reach D 

is almost the inverse of the trend in normalized stream length 

for that reach. When vegetation or wetland areas were lost in 

reach D, presumably due to lateral erosion accompanying the 

large floods in 1949, 1952, 1953, and 1957, both the length of 

channels and the portion of the valley floor occupied by active 

channels increased. Following those large floods, the vegetated 

and wetland area of reach D increased from 48 to 62 percent, 

and the normalized stream length decreased from 4.0 to 

2.3 km/km, between 1973 and 1997. Part of that decrease 

resulted from certain channels having been turned into ponds 

by the action of beaver. The large flood in 1985 apparently had 

little adverse effect on the vegetated and wetland area calculated from the 1987 aerial photograph.

A dramatic increase in the number and size of beaver 

ponds in reach D was observed between the 1973 and 1987 

photographs (table 6), perhaps partially reducing the flooderosion susceptibility in this reach. These changes were not 

the result of introduction of beaver, as they are indigenous. 

For example, the growth rings of a spruce tree rooted in a 

relict beaver dam near Gladstone (Vincent and others, this 

volume, pl. 5) indicate that the tree sprouted in 1818; thus, 

beaver were present in the area prior to mining. The increase 

in beaver habitat since the 1970s might, in part at least, be the 

result of reduced hunting pressure (according to a long-time 

resident who did not give his name). No beaver ponds were 

evident in reaches A and B during the study period, and the 

near absence of a food source may be a partial explanation. 

One pond appeared in the 1987 photograph of reach C.

Vegetation and wetland areas in reach B have been nearly 

nonexistent since at least 1945, in contrast to the vegetation 

cover evident in 1904 and 1906 photographs in figures 2A and 

9A. There has been no recent recovery in reach B vegetation 

since 1987, in contrast to reach D. Almost no trees and shrubs 

were present along the banks of the upper Animas River in 

reach B when on-site measurements were made in 1998 or in 

2001 when the photograph of figure 2B was taken. The lack of 

vegetation recovery in reach B could be explained in several 

ways. The vast expanse of gravel may simply be an unsuitable 

substrate for rapid colonization of willow in this high-altitude 

setting. The floods of 1949, 1952, 1953, and 1957 (fig. 2) may 

have removed some emergent vegetation in reach B, and the 

actions of heavy machinery may have done the same. Large 

areas of tailings beds were evident in the photographs, and 

these beds may have inhibited the growth of young willows in 

those locations by either physical or chemical processes. The 

extent of those tailings beds decreased through time, however, 

as discussed in a later section. For example, 40 percent of the 

light-colored tailings visible in the 1951 photograph (fig. 4B) 

had been removed, presumably by the action of floods, by the 

time the 1960 photographs were taken. Yet tailings remain in 

the interstices of the gravel, as demonstrated in a later section. 

The presence of tailings in the gravel may have contributed to 

the absence of willow recruitment by some chemical processes, although that possibility has not been tested directly.

Changes Inferred from Flood-Plain 

Stratigraphy

We excavated a trench (pl. 6) across the alluvial valley 

bottom in order to understand from a stratigraphic point of 

view the following: the prehistorical nature of the Animas 

River, the historical changes in that system, and the chemistry 

of fine-grained sediment deposited in the distant and recent 

past. The location of the trench was at the downstream end of 

reach B, as indicated in figures 1, 2, 3, and 4B.

The trench was excavated using a trackhoe in September 

of 1998. For practical reasons the trench was excavated and 

studied in “skip sections” (fig. 10), such that short (10–30 m 

long) sections of the transect were excavated, studied, and 

backfilled. Then the intervening sections were excavated to 

complete the stratigraphic log of plate 6. One reason that the 

trenching was feasible is that the water table of the shallow 

alluvial aquifer was on average 1.5 m below the streambed at 

the time of excavation (fig. 10). An electric pump connected 

922 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 923 Figure 10. Photograph of a section of the stratigraphic trench (near 156 m on pl. 6), showing the ground-water table 1.6 m below the level of that thread of the stream in late September 1998. View is looking east down the trench transect, with Colorado State road 110 visible in the background. The trench was located at the downstream end of reach B, as indicated in figures 1 through 4.
to a portable generator was used to temporarily lower the 

water table in trench sections, which allowed us a short time 

to view and sample the deeper sediment. Our depiction of the 

stratigraphy beneath the water table on plate 6 is therefore less 

reliable than that above the water table, which was available 

for inspection for many days.

Figure 10 is an appropriate vehicle for starting the 

discussion of the valley bottom stratigraphy for several reasons. 

The ground-water table was below the level of (and thus disconnected from) the stream during that period of late-season baseflow conditions. In late July 1998 the water table was observed 

to be about 1 m below the ground surface in a test pit (at horizontal distance 187 m, pl. 6), and 2 months later it had dropped 

to about 2.2 m below the ground surface at that location when 

the trench was excavated. The water in the stream was not 

infiltrating, and thus not recharging the shallow ground water in 

a significant way, because the streambed was completely coated 

and sealed by an algal mat. The algal mat evidently formed 

during the summer because the streambed had no shade. The 

mat was also fragile. After the trackhoe rolled over a channel 

(shown at 275 m on plate 6) one time, all of the stream water in 

the channel abruptly drained into the subsurface. Understanding 

the connection or disconnection of surface water and shallow 

ground water is essential to tracer dilution studies such as that 

of Kimball and others (this volume). They did not detect a gain 

in discharge within the braided reach upstream of the trench 

during their August 1998 tracer dilution study, which is consistent with our field observation of the surface water/ground 

water disconnection.

The foreground of figure 10 also illustrates the weak 

stratification typical of the gravel deposits, and that the 

pebbles and cobbles were subangular to subrounded. That 

rounding of the edges of clasts was the result of abrasion by 

natural stream processes through time. We mention this to 

indicate that the origin of these clasts was not the result of 

blasting of bedrock or milling of ore. In the middle left of the 

photograph, a vertical bank of dark gravel with an orange layer 

was exposed, illustrating the discoloration and cementation 

of gravel by iron (orange) and iron/manganese (dark) secondary salts that were common in the stratigraphic section. Much 

of that cementation must have occurred in the past century 

or less. In the background were several large relict willow 

bushes. These appear similar to the clumps of willow observed 

in the 1883–84 photograph, and in the 1904 photograph of 

figure 2A.

Stratigraphy and Sedimentology

The trench was excavated perpendicular to streamflow 

and extended nearly the full width of the alluvial valley bottom. 

The only areas not excavated were the two main channels, 

Colorado State road 110, and short sections of flood plain at 

the extreme east and west sides of the valley. The trench face 

was 297 m long and exposed 607 m2

 of sediment in the trench 

wall (pl. 6). Four sedimentary facies were present, and their 

spatial extents are discussed next in percents of the total area 

of the exposure. Those sediments consisted of (1) sandy gravel 

deposits (83 percent), (2) dark-brown silt beds (12.6 percent) 

containing minor amounts of peat, (3) lenses and tabular beds 

of sand-sized tailings (1.2 percent), and (4) tabular beds of 

fine-grained tailings (3.2 percent). The gravel facies represents sediment transported as bed load and deposited on the 

streambed, and the brown silt beds represent sediment carried 

in suspension and deposited on the flood plain. These deposits 

are discussed in terms of their stratigraphic position, low to 

high elevation in the section, and thus in terms of general age 

from oldest to youngest.

The base of the section was dominated by sandy gravel 

deposits. These gravels locally contained (1) discontinuous 

parallel bedding usually inclined less than 5°, (2) medium, 

curved, parallel, trough crossbedding, (3) sixteen lenticular (two were tabular) beds of brown sandy silt between 5 

and 60 cm thick and less than 3 m in length, and (4) tabular 

or irregular-shaped beds of silty to gravelly sand.

The elevational middle of the section consisted of 

extensive silt beds separated laterally by gravel deposits. 

Eight thick beds of dark-brown (10YR 4/3 moist, 5/3 dry) 

sandy silt were present in the section. The color names (such 

as dark brown) and color codes (such as 10YR 4/3) used herein 

are standardized technical descriptions (Munsell Color, 2000) 

determined using soil color charts. These large silt beds were 

from 20 to 170 cm thick, ranged in length from 4 to 25 m, and 

spanned about 30 percent of the trench length. The bases of 

the silt beds were generally in planar-horizontal contact with 

underlying gravel, but lenticular or wedge-shaped silts also 

interfingered with underlying gravel. The silt beds contained 

(1) lenses of gravel, (2) a few lenses of peat (as much as 5 cm 

thick and less than 3 m long), (3) fourteen peat lamina (1 cm 

thick and 1–10 m long), (4) leaves and root casts possibly left 

by grasses, and (5) abundant twigs (presumably willow) some 

of which were in growth position. One twig was obviously 

chewed by a large mammal, possibly a beaver. Aside from 

those features, the silt beds appeared from a distance to be 

uniform and without sedimentary structures. Close inspection, 

however, indicated that the silt beds also contained very thin 

lenses of fine sand, irregular-shaped bands of climbing ripples 

composed of coarse silt, and 10-cm deep indentation markings left by large mammal hooves. The silt beds were either 

overlain conformably or were truncated by wavy or irregular 

erosional surfaces.

Also in the elevational middle of the section, channel-fill 

gravel deposits truncated the lateral extent of the thick silt beds 

at several locations. The former stream banks composed of silt 

were about 1 m high. The channel-fill deposits consisted of 

sandy gravel that contained (1) discontinuous parallel bedding, 

(2) medium, curved, parallel, trough crossbedding, (3) and lenticular or irregular-shaped deposits of coarse sand 10–40 cm 

thick. Nine of the sand lenses were dominated by angular 

grains of sulfide or gangue minerals (sphalerite, pyroxmangite, 

and pyrite); two of these lenses contained historical iron 

objects. The surfaces of pyroxmangite crystals were still pink, 

which is important because pyroxmangite darkens rapidly 

924 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 925

when exposed to oxygen. We inferred that these were deposits 

of stamp-mill tailings, and they ranged in size from medium 

sand to granule (0.25–4 mm).

The upper meter of the section consisted of sheets of 

sandy gravel and extensive beds of fine-grained tailings. The 

gravels were either horizontally bedded or trough crossbedded. 

These gravels contained lenses and two beds (10–40 cm thick 

and 3–11 m long) of coarse tailings, and numerous historical 

artifacts including sawn lumber and metal objects. The sheet 

gravels were exposed at the surface for about half the length 

of the trench. Elsewhere the surface consisted of extensive 

tabular beds of fine-grained tailings typically 10–50 cm thick. 

The beds of fine-grained tailings were generally without obvious sedimentary structures, but locally contained crossbedded 

laminations, and climbing ripples suggesting deposition from 

suspension. Although generally composed of silt-sized particles, the tailings also contained very fine sand and clay-sized 

particles. The colors of the fine-grained tailings were variable, 

but so unusual and vivid that they were diagnostic (fig. 11). 

The beds were typically white (10YR 8/2) or yellowish brown 

(7.5YR 5/9 to 10YR 5/8), and occasionally olive (5YR 5/3), 

where observed in fresh exposures. Where undisturbed, 

however, the surface of the tailings deposits was consistently 

colored light olive gray (5Y 6/2).

Locally the gravel and sand deposits were stained or 

cemented by compounds of manganese and iron (fig. 10; 

pl. 6). Typically the coatings and cement were bluish black 

(SPB 2.5/1) in color. The cemented or discolored zones were 

irregular in shape, with the only generalities being they were 

typically laterally elongate and the tops of the zones were 

usually located from 0.5 to 1 m below the ground surface. 

The bottoms of zones were located as much as 1 m above the 

ground-water surface (as it was at the time of trenching), or 

extended down to or beneath the ground-water surface. Thus 

the zones were generally restricted to within the elevation 

range of annual fluctuations of the water table. The boundaries of the cemented or discolored zones were not restricted 

to changes in hydraulic conductivity of the host sediment. 

Locally the boundaries conformed to stratigraphic contacts 

or internal bedding, but locally the boundaries also crosscut 

stratification or ended within a bed. The cementing compounds were precipitated from the alluvial ground water after 

deposition of the host gravel, much of which is historical in 

age. A portion of the surface of the gravel flood plain was also 

locally bluish black in color, as indicated on plate 6. In those 

locations the tops of surficial stones had continuous, 1-mm 

thick varnish-like coatings, which evidently precipitated from 

stream water or temporarily ponded water. These blackened 

gravel bars were evident in the 1945 aerial photographs.

Constraints on Ages of Sediment Deposition

Four types of evidence (historical artifacts, radiocarbon 

dating, willow growth rings, and photographs) were used 

to constrain the depositional ages of deposits exposed in 

the trench (pl. 6), and retrieved from a hole augered into the 

nearby Forest Queen wetland. That wetland is located on the 

east side of the valley floor, just downstream from the trench, 

and is discussed by Stanton, Fey, and others (this volume, 

Chapter E25) and Finger and others (this volume, Chapter F).

Numerous historical artifacts were discovered in the 

trench wall (pl. 6), in the middle and upper (elevational) 

portions of the exposed stratigraphy. These artifacts include 

iron objects (cans, pots, barrel hoops, and a stove leg), cloth 

fabric, sawn lumber, porcelain fragments, a rubber machinery belt, and a brick. We could not determine a refined 

manufacturing-age for any of these objects, but we can 

constrain when they were introduced to the area. We conclude that the sediment containing any historical artifact was 

deposited after 1871 and probably after 1899 when the first 

mill was constructed in Eureka. The tailings are like historical 

artifacts in that we know when they were manufactured, and 

that is also when they were introduced to the fluvial system. 

The coarse-grained stamp-mill tailings were first produced in 

1888; quantities increased and began being discharged adjacent to the Animas River starting in 1900. The use of stamp 

mills ended in 1918, but the tailings became increasingly 

finer as new technology was employed. For example, Jones 

(this volume) suggests that the “Gravity Milling era” ended 

in 1913, so we use that date for the end of, or diminished rate 

of, production of coarse tailings. The fine-grained tailings 

were produced between 1913 and 1930. Like any artifact, 

the age of tailings deposits must be younger than when the 

material was first produced or introduced to the area. The tailings, particularly the coarse-grained tailings, may have resided 

elsewhere in the landscape for a long or short period of time 

before being transported and deposited at what became the 

trench site.

The gravel bars blackened by compounds of manganese 

and iron (discussed in the preceding section) are evident in 

the 1945 aerial photographs. Thus gravel aggradation ceased 

before 1945 at those specific sites, if not in general.

Important age constraints were derived from a pair of 

willows that had been pushed over by flood waters, covered 

by sediment, and subsequently resprouted. This occurred 

repeatedly, and one of the plants is illustrated in figure 12. 

The plants were discovered alive and recently exhumed and 

exposed in a streambank 100 m downstream of the trench 

site. The stratigraphy at that site consisted of three gravel 

beds (labeled G1 to G3) covered by a stratified bed of finegrained tailings (labeled T). The tailings are thicker on the 

downstream side of the plant (fig. 12) than on the upstream 

side. Analysis of plant growth rings has long been used to 

determine the timing of tissue damage (scarring or bending) or burial, and thus the age of causal events such as 

floods. (See, for example, Sigafoos, 1964; Scott and others, 

1997.) Mike Scott of the USGS analyzed the growth rings of 

10 slabs cut from the two willow plants (written commun., 

1999) allowing the following conclusions to be made. The willow shown in figure 12 sprouted in 1915 (±3 years) on a gravel 

bar close to the present level of the stream. Thus the gravel at 

the base of the section (G1) was deposited before that, perhaps 
Figure 11. Fresh exposure of flood-plain sediment located near the trench site. The base of the 1-m high 

exposure was composed of dark-brown sandy silt. The middle of the exposure was composed of sandy silt 

containing very thin beds of fine-grained tailings. The top of the exposure was composed of thin to mediumthick beds of yellowish-brown, white, and olive-colored fine-grained tailings.

926 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 927

T 1941–1985G3–1927G2–19211915 3G1–1911 ?

±

1.4 m

FLOW

DIRECTION

Figure 12. Recently exposed willow bush that had been repeatedly pushed over and covered by sediment, only to resprout. Labeled dates are based on growth-ring analysis 

discussed in text. Willow was located about 100 m downstream of the trench (pl. 6).
during the 1911 flood of record. The previously undisturbed 

plant stems were permanently bent downstream around 1920; 

thus, gravel bed G2 may have been deposited during the 1921 

flood that is known to have caused damage in the area. Plant 

tissue from within gravel bed G3 showed signs of burial by 

1929; thus, bed G3 may have been deposited during the large 

1927 flood recorded at the Durango gauge. The aggradation of gravel at the site occurred during or shortly after the 

introduction of large quantities of coarse-grained tailings, and 

perhaps coincidentally during the period of huge production 

of fine-grained tailings (fig. 7). Willow stems showed signs 

of slowed growth caused by some disturbance around 1941, 

1949, possibly in the late 1950s, between 1970 and 1974, and 

during the first half of the 1980s. Thus the fine-grained tailings 

shown in figure 12 may have been deposited from suspension during the floods of 1949, 1957, 1973, and 1985. One 

other disturbance must be accounted for. At some point lateral 

migration of the stream, or incision, exhumed and presumably 

stressed the plants; this may have occurred during the 1973 or 

1985 floods.

Select twigs and peat samples were dated using standard 

radiocarbon techniques (data are presented in table 7). The 

results appear graphically in figure 13 and are discussed in the 

next section.

Stratigraphic Interpretations

The oldest dated sediments are under the Forest Queen 

wetland on the east side of the valley. Peat obtained from 

a depth of nearly 5 m below the surface is slightly more 

than 3,000 years old (fig. 13). Since that time sediment in 

the spring-fed wetland accumulated at the average rate of 

1.5 m/ka (meter per thousand years), with rates ranging from 

0.8 to 3.4 m/ka. These rates were calculated using calibrated 

radiocarbon ages (table 7). The rates were slowest (0.8 and 

1.1 m/ka) in the lowest 2 m of the section, perhaps in part due 

to compaction of those sediments. These rates are fairly rapid 

compared to other prehistorical rates of aggradation, based on 

measurements made in the area. For example, Carrara and others (1991) determined that several bogs in the area aggraded 

about 0.2 m/ka through the early and middle Holocene. 

Vincent and others (this volume) determined that peat wetlands on the flood plain of Cement Creek aggraded at rates 

on the order of 0.5 m/ka during the late Holocene. Vincent 

and others (this volume) also determined that the channel of 

Cement Creek aggraded as rapidly as 1.7 m/ka during the late 

Holocene. Three dates of about 1,100 years were obtained by 

this study from a depth of about 2 m below the present floodplain level (fig. 13), but approximately 1 m of that overlying 

sediment is historical in age. The average prehistorical 

aggradation rate for the Animas River and flood plain is thus 

on the order of 1 m/ka. These rates pale in comparison to that 

for the historical period, when more than a meter of sediment 

accumulated over several decades.

Extensive gravel deposits dominate the base of the 

stratigraphic section exposed in the trench. We interpreted 

this to represent a period of deposition and reworking by broad 

braided streams, with only localized deposition (or preservation) of fine-grained flood-plain deposits. This period apparently ended about a thousand years ago.

Two changes in the landscape are evident in the middle 

of the stratigraphic section. After 1,000 or 2,000 years ago, 

the brown silt flood-plain deposits began to accumulate (and 

were preserved) in between multiple channels of the Animas 

River at the trench site. The flood plain likely consisted of 

dense thickets of willow and areas of grasses between sparse 

willow bushes. The thin layers of peat within the silt beds 

suggest that beaver ponds were locally or occasionally present 

on the flood plain. This period of multi-threaded channels 

surrounded by vegetated flood plain persisted until the early 

20th century, based on the observations made from early 

historical photographs discussed previously. The existence of 

the multi-threaded channels evident in the ca 1904 photograph 

(fig. 2A), in particular, supports the stratigraphic interpretation 

of the nature of channels and flood plain. That general timing, 

of the development of the silty flood plain, is consistent with 

the results of the better dated study of Cement Creek (Vincent 

and others, this volume). Cement Creek was apparently not 

aggrading between about 6,000 and 1,500 years B.P., after 

which both the channel and flood plain began to aggrade. 

About 500 years B.P., Cement Creek incised 2–3 m, but if the 

Animas River in our study area incised at that time it was only 

by a small amount.

Rapid changes in the upper Animas River valley 

occurred during historical times. The channels were filled 

with gravel, broad sheets of gravel were deposited on the flood 

plain, and then fine-grained tailings were deposited over much 

of the valley floor at the trench site. The deepest stamp mill 

tailings (at 34 and 44 m on pl. 6) may have been deposited late 

in the 19th century or early in the 20th century, before rapid 

channel filling began. The two large beds of stamp-mill tailings (between 44 and 53 m, and between 87 and 99 m on pl. 6) 

were deposited on the flood plain before the channels were full 

of gravel, based on crosscutting relationships at the margins 

of the beds. This likely occurred after 1900, because only a 

modest quantity of tailings was produced before that (fig. 7). 

Channels at the trench site were not full of gravel in 1904 

(fig. 2A). The channels may have been partially filled with 

gravel during the 1911 flood, but this cannot be demonstrated 

conclusively. At the site of the dated willows (fig. 12) 100 m 

downstream of the trench, gravel at the base of the section was 

likely deposited in 1911, but the bulk of the streambed gravel 

was probably deposited during the 1921 and 1927 floods. 

Much of the gravel accumulation in the reach at the trench 

also likely occurred in the 1920s. The 1945 aerial photograph 

shows broad sheets of blackened gravel and also tailings at the 

trench site; thus, the period of rapid aggradation ended before 

that time. After 1945, the surficial tailings beds were locally 

reworked and transported downstream, as discussed in a later 

section of this chapter.

928 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 929

Table 7. Data for radiocarbon samples collected near the trench, downstream of the Eureka townsite.[Note that “B.C.” and “A.D.” are both placed before dates in this table, for better readability and quicker recognition]

Field No. Lab No. Material δ13C per mil 14C age1 years B.P.

Calibrated ages2 Depth below

surface,

m

Distance 

along

trench, 

m

Intercept (s)3 1-sigma range4 2-sigma range4

Samples from core in wetland at the Forest Queen mine

AVT-A2-1.5 GX-24744-AMS wood –25.8 930±50 A.D. 1040–1160 A.D. 1020–1180 A.D. 1000–1220 1.5 (6

)

AVT-A2-2.85 GX-24746 peat –27.3 1,220±75 A.D. 780 A.D. 690–890 A.D. 660–990 2.85 (6

)

AVT-A2-4.1 GX-24745 peat –28.0 2,305±70 B.C. 390 B.C. 400–230 B.C. 540–180 4.1 (6

)

AVT-A2-4.85 GX-24743 peat and wood –26.5 3,020±90 B.C. 1290–1260 B.C. 1400–1130 B.C. 1490–1000 4.85 (6

)

Samples from the flood-plain trench (pl. 6) downstream of Eureka

AVT-65.2-0.9 GX-25027 wood –26.6 Modern (102%)5 0.9 65.2

AVT-93-0.93 GX-24741 peat and twigs –25.1 Modern (102%)5 0.9 93AVT-69-2.2 GX-24739 wood –23.2 1,080±120 A.D. 980 A.D. 780–1030 A.D. 680–1220 2.2 69AVT-127-2.3 GX-24742-AMS charcoal –22.2 1,210±60 A.D. 780–800 A.D. 690–890 A.D. 670–980 2.3 127AVT-92-1.25 GX-24740 peat –25.5 2,130±145 B.C. 170 B.C. 380–A.D. 50 B.C. 480–A.D. 210 1.25 92

1Conventional radiocarbon age (δ13C corrected), based on the Libby half life (5,570 years) for 14C, as reported by Geochron laboratories in years before A.D. 1950 (years B.P.). Sample analyses done using 

accelerator mass spectrometry denoted with the letters AMS in the lab number. The error is ± 1 sigma as judged by the analytical data alone. The sample was crushed if necessary and dispersed in water. The eluted clay/organic fraction was treated in hot dilute 1N HCl to remove any carbonates. It was then filtered, washed, dried, and combusted in oxygen to recover carbon dioxide for analysis.

2Ages calibrated using the CALIB4.3 program based on Stuiver and Reimer (1993) with data from Stuiver and Braziunas (1993) and Stuiver and others (1998). The 1998 atmospheric decadal data set 

and laboratory error multiplier K=1 were used in the calculations.

3Maximum and minimum values given if the radiocarbon age intercepted the calibration curve at multiple locations. Age(s) rounded to the nearest decade if the standard deviation in the radiocarbon age 

was ≥50 years.

4Calibrated age range (or the maximum and minimum if there was more than one range) using the intercept method. Ages rounded to the nearest decade if the standard deviation in the radiocarbon age 

was ≥50 years. 1 sigma = square root of (sample standard deviation2 + curve standard deviation2); 2 sigma = 2 × square root of (sample standard deviation2+ curve standard deviation2).

5Sample with “modern” age had the given percentage of the A.D. 1950 14C activity, and is considered younger than A.D. 1850.6From Forest Queen site (fig. 13). Samples not on trench transect.
Figure 13. Summary of dated stratigraphy of the Animas River valley flood plain near the trench site (pl. 6).

Trench site

66 – 69 m

horizontal distance

Trench site

92 – 94 m

horizontal distance

Trench site

126 – 127 m

horizontal distance

Forest Queen

wetland site A-2

≈ 370 m

Ground water

9/26/98

930

yr B.P.

1,220

yr B.P.

2,305

yr B.P.

3,020

yr B.P.

DEPTH, IN METERS

DEPTH, IN METERS 1,210 yr B.P. 2,130 yr B.P. 1,080 yr B.P. Modern < 300 yrs

66 67 68 69 0

12

0123

12345

0123

Modern< 300 yrs

EXPLANATION

Sandy gravelSandTailings, fine-grainedSandy tailings, coarse-grainedBrown siltSilt, reduced

Peat or twigsOrange-mottled siltManganese nodulesSample locationCarbon 14 age estimate 1,080 yr B.P.

930 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 931

Sediment Chemistry

In order to understand the chemistry of sediment deposited in the distant and recent past, we obtained and analyzed 

82 samples of sediment. These were extracted from the trench 

wall or obtained from cores of tailings and silt beds adjacent to 

the trench.

Geochemical Methods and Results

The sediment samples extracted from the trench wall 

were generally about 1 L (liter) in volume, whereas samples 

from cores were somewhat smaller. Four 5-cm diameter cores, 

each approximately 1 m long, were taken adjacent to the 

trench. The cores were subsequently divided at visual changes 

in mineralogy or sedimentology into from 4 to 12 subsamples, 

with typical volumes of 50 cm3 (cubic centimeters).

Sample preparation and geochemical analysis followed 

the methods used by Fey and others (2000) in their regional 

study of the geochemistry of sediment in the Animas River 

watershed. All samples containing gravel were passed through 

a stainless steel sieve with 2 mm mesh opening, and the gravel 

clasts were discarded. All samples were air dried, and sieved 

to –100 mesh (<0.15 mm), and then crushed to –150 mesh 

(<0.1 mm), except for the early stamp-mill tailings, which 

were crushed to –150 mesh without sieving beyond 2 mm. 

The chemistry was thus determined for very fine sand, silt, 

and clay-sized particles in the samples of silt beds, finegrained tailings, and sandy gravel. For the stamp-mill tailings samples, only, the chemistry of the coarse sand and finer 

particles (<2.0 mm) was determined. The analytical results are 

therefore representative of the entire volume of brown silt beds 

and tailings deposits, but representative of only the fine fraction (<0.15 mm) of the sandy gravel samples.

After grinding, all samples were digested using a mixedacid procedure (Briggs, 1996). This procedure is very effective 

in dissolving most minerals, including silicates, oxides, and 

sulfides. Some refractory minerals such as zircon, chromite, 

and tin oxides are only partially attacked; however, elements 

contained in these minerals were not of concern in this study. 

The resulting solutions were analyzed by inductively coupled 

plasma–atomic emission spectrometry (ICP-AES) for major 

elements and trace elements. To monitor the quality of analyses, laboratory duplicates were analyzed to assess precision, 

and three standard reference materials were analyzed with 

the sample sets to assess analytical accuracy. The reference 

materials were NIST-2704, NIST-2709, and NIST-2711, available from the National Institute of Standards and Technology 

(NIST, 1993a, 1993b, 1993c). We discuss here results for zinc 

and vanadium, but all analytical results are found in the database of Sole and others (this volume, Chapter G).

In order to establish the trends in element concentrations in the streambed and flood-plain sediment, we ranked 

all samples by their stratigraphic position (relative age) using 

the trench log of plate 6. The horizontal axis of figure 14A

and B reflects that ranking. The 14C ages allowed identification of those sediments that were definitely deposited before 

the mining era (fig. 14C). The age constraints provided by 

the presence of tailings and historical artifacts, and the growth 

rings of willows, allowed identification of those sediments 

that were definitely deposited after milling began (fig. 14C). 

The geochemical data are presented using symbols for four 

deposit types, and the differences in the concentration of 

various metals in those deposit types are discussed in a few 

paragraphs.

The concentrations of vanadium in sediment deposited 

prior to mining at the trench site (fig. 14A) and elsewhere in 

reach B (Church, Fey, and Unruh, this volume, Chapter E12) 

average 140 ppm, which is close to the 160 ppm crustal 

abundance of vanadium (Emsley, 1991). The concentrations of vanadium in historical (post-1888) deposits average 

50 ppm, in contrast. The vanadium concentrations in some 

of the fine-grained tailings samples are close to 20 ppm.

The concentrations of zinc in sediment (fig. 14B) 

deposited prior to the mining era are uniform and close to 

1,000 ppm, which is an order of magnitude greater than the 

75 ppm crustal abundance of zinc (Emsley, 1991). Compared 

to this premining background, the concentrations of zinc in 

most (38 out of 46 samples) of the sediment samples from 

material known to have been deposited after mining began 

are elevated by as much as 10 times.

The tailings deposits contain other metals that are substantially elevated above concentrations in sediment deposited 

during prehistorical times (table 8). Compared to that background, lead concentrations are elevated about 40 times in 

fine-grained tailings and are elevated about 20 times in coarsegrained tailings. Cadmium, copper, and manganese concentrations are elevated above background by factors between 4 and 

14. In contrast, arsenic concentrations in tailings are equivalent 

to background. Nickel as well as vanadium concentrations are 

substantially less than that found in premining sediment.

In that light, the concentrations of metal in the two types 

of tailings are not substantially different from one another. The 

concentrations of zinc and cadmium are equivalent in the two 

tailings types. The coarse-grained tailings contain somewhat 

less (from 20 to 50 percent less) lead, arsenic, copper, and 

manganese than the fine-grained tailings.

Geochemical Conclusions

We draw the following conclusions from these 

geochemical results. Sediment known to have been deposited 

prior to the mining era has concentrations of certain metals 

that are substantially elevated above concentrations typical of 

the Earth’s crust, which is not surprising. That sediment was 

produced by erosion of hillslopes in the watershed, where 

locally the surficial bedrock contains high concentrations, even 

economical concentrations, of metal. The introduction of mill 

tailings into the fluvial system resulted in dramatic increases in 

concentration of certain metals (and dilution of other elements) 
ZINC, IN PARTS PER MILLION

DECREASING STRATIGRAPHIC AGE

10,000

8,000

6,000

4,000

2,000

0

Fine-grained tailings

Stamp-mill tailings

Brown sandy silt

Sandy gravel

Premining After start of ore milling

B

C

1,080 B.P.

2,130 B.P.

Age

relative

to

mining

uncertain

1930 AD

1970 AD

1940 AD

50

0

100

150

200

H

H

H H

H

H H

H

H H

H H H H

H H

HH

H

H

H

H

H

HH

H

D

D

D

D

D

D

D

D

D

D

I

I

I

I I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

J J J

J J

J

J

J J

J

J

J

J

J

J

J

J

J

J

J

J

JJ

J

J

J

J

J

JJ JJJ

J J

J

J J

J

J

J J

JJ

J

JJ

J

J

J

J

J

J J J

J JJ

J

J

J

H H H

H

H H

H

HHHHH H

H

H

HHHHH

H

H

H

D

D

DD

DD

D

D

D

III I I

I I

I

I

II I

I

I

I

IIII

VANADIUM, IN PARTS PER MILLION

A

Figure 14. Concentrations of vanadium (panel A) and zinc (panel B) in the fine fraction of sediment of various types, plotted 

against stratigraphic (relative) age. The relative age scale (panel C) is based on numerical ages from radiocarbon dating (fig. 13) 

and tree-ring analysis (fig. 12), and the stratigraphic position of sediment containing cultural artifacts (which provides the 

relative timing of sediment deposition after onset of ore milling). All samples were obtained from the trench (pl. 6) excavated into 

the Animas River flood plain 1.4 km downstream of Eureka townsite (fig. 1).

932 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 933

in historical sediment compared to the premining background 

sediment at the trench site (Vincent and others, 1999), and at 

sites downstream (Church, Fey, and Unruh, this volume).

The trend in zinc concentration shows a dramatic 

increase, by as much as 10 times (fig. 14B). This increase 

occurred both in sediment deposited in streambeds and bars, 

and in sediment deposited on brush-covered flood plains. 

The concentrations of zinc in sediment known to have been 

deposited prior to the mining era are uniform and close to 

1,000 ppm. The concentrations of zinc are also close to 

1,000 ppm in most (all but four) of the samples whose depositional ages are uncertain relative to the beginning of mining 

and milling (fig. 14B). The concentrations of zinc are elevated, 

however, in most (38 of 46 samples) of the samples of sediment known to have been deposited after mining began, and 

more specifically after mill processing of ore began in earnest. 

We infer, therefore, that the increase in zinc concentrations in 

streambed and flood-plain sediment was caused by the release 

of huge quantities of tailings from the mills, most notably the 

Sunnyside Eureka Mill (site # 164).

The trend in vanadium concentration is opposite to that 

for zinc (fig. 14A). The vanadium concentrations in premining sediment are similar to that typical of the Earth’s crust. In 

contrast, they are also, in general, three times higher than the 

concentrations in historical deposits. Mining did not cause an 

increase in the release of vanadium to the environment; rather 

it diluted the premining vanadium concentration with mill tailings. We conclude that vanadium can be used as a lithologic 

tracer for sediment derived from premining erosion of the 

watershed. The average vanadium concentration is 23 ppm, 

for the 10 tailings samples with the lowest values (out of 28), 

and we assume this is representative of pure tailings. The 

Table 8. Concentration of selected elements (in parts per million) in deposits of coarse-grained and fine-grained mill tailings.

[Samples were obtained in or near the trench (pl. 6) of this study. Data for prehistorical sediment are from Church, Fey, and Unruh (this volume, Chapter E12, 

sample sites B19–B23) from samples obtained within reach B. Crustal abundance values from Emsley (1991)]

 Element

Deposit type Prehistorical

sediment

n = 44

Crustal

abundance Coarse tailings

n = 9

Fine tailings

n = 19

Arsenic: Median 40 51 55

Mean 40 53 52 1.5

1 sigma 10 18 16

Range 28–61 28–100 23–85

Cadmium: Median 16 14 4

Mean 20 15 4 0.1

1 sigma 12 8 4

Range 9–42 2–34 1–18

Copper: Median 1,500 2,000 120

Mean 1,390 1,990 142 50

1 sigma 454 720 67

Range 530–2,000 760–3,100 92–480

Lead: Median 7,800 14,000 360

Mean 7,756 16,900 403 14

1 sigma 3,085 13,700 212

Range 3,800–14,000 5,500–65,000 150–1,600

Manganese: Median 28,000 35,000 3,700

Mean 27,778 33,632 4,448 950

1 sigma 4,295 10,084 1,737

Range 22,000–34,000 13,000–47,000 2,500–9,100

Nickel: Median 7 4 14

Mean 6 5 14 80

1 sigma 2 2 3

Range 3–8 2–8 9–20

Vanadium: Median 64 38 140

Mean 56 40 135 160

1 sigma 17 20 12

Range 30–78 12–77 120–160

Zinc: Median 3,200 4,500 940

Mean 4,411 4,585 1,022 75

1 sigma 3,209 2,437 258

Range 1,300–10,000 920–10,000 720–1,800
average vanadium concentration for the fine-grained fraction 

of premining gravel samples is 135 ppm, and is 60 ppm for 

that of historical gravel samples at the trench. Using a simple 

mixing equation, we conclude that the fine-grained fraction of 

historical sediment at the trench site is, in general, composed 

of two-thirds mill tailings and one-third premining sediment 

unrelated to mining or milling. This proportional tailings 

content is, if anything, an underestimate. Using a value larger 

than 23 ppm for the vanadium concentration of pure tailings 

would result in a larger estimate of the proportional tailings 

content. Given the way we use this mixing result in the section 

titled, “Current Location of Tailings,” however, we consider 

this estimate (two-thirds of the fine fraction is tailings) to be 

conservative.

The concentration of lead in fine-grained tailings is 

16,900 ppm, substantially higher than the 7,760 ppm for 

coarse-grained tailings (table 8). We conclude that lead can be 

used as a lithologic tracer for determining the provenance, and 

manufacturing age, of tailings. For example, the ratio of lead 

and vanadium concentrations is about 3 for premining sediment. The ratio of lead and vanadium concentrations is about 

140 for the older coarse-grained tailings, whereas it is about 

420 for the younger fine-grained tailings.

The concentrations of other metals in the two types 

of tailings are not substantially different from one another 

(table 8). The concentrations of copper, manganese, and zinc 

are somewhat higher in fine-grained tailings compared to 

that in coarse-grained tailings. More important, however, is 

the surface area of mineral grains available for geochemical 

reactions, per unit volume or mass of sediment, which depends 

on particle size. For example, silt-sized particles have about 

100 times more surface area (per unit mass) compared to 

medium sand (Birkeland, 1984). The fine-grained tailings are 

dominated by silt-sized particles and should be more reactive, 

compared to the coarse-grained tailings dominated by sand. In 

addition, most of the fine-grained tailings reside at the ground 

surface and thus are easily accessible, whereas most of the 

coarse-grained tailings reside in the subsurface. For that reason 

it might be prudent to base decisions for prioritizing mitigation 

of the two types of tailings on factors other than differences in 

chemistry.

Current Location of Tailings

The desire to remove mill tailings from the landscape for 

environmental reasons has been expressed. The environmental 

concern is that metal may be extracted from the sediment by 

geochemical processes and transported by water. That water 

may enter streams where the metal may have an adverse effect 

on aquatic life, or enter plants where the metal may have an 

adverse effect on wildlife using the plants as a food source. 

Although discussion of geochemical processes is beyond the 

scope of this project, the presence of tailings within stream 

sediment may result in increased metal concentrations in local 

water. Indeed the Animas River in our study area does have 

elevated zinc concentrations, comparable to that in Cement 

and Mineral Creeks (Kimball and others, this volume).

Therefore, we discuss where the tailings currently reside in 

the study reach, and how that has changed through time. The 

tailings reside in the landscape at the ground surface and in the 

subsurface, and these two deposit types need to be discussed 

separately, for practical reasons.

Beds of fine-grained tailings at the ground surface 

can be remediated relatively easily and efficiently, for three 

reasons. These beds of tailings are easy to identify because 

of their unique colors (fig. 11), and they are easy to excavate 

because they are at the surface. They can also be efficiently 

processed because they do not contain gravel clasts, which 

otherwise would presumably be separated from the tailings 

before shipment to a repository. The owners of the Sunnyside 

mine removed most of the surficial beds of tailings in reach B, 

proving the viability of remediation of that deposit type. In 

addition, unless those deposits are reclaimed, they are to 

some degree susceptible to being mobilized by streamflow 

and flushed downstream to be deposited in dilute form in 

inaccessible locations, as discussed next.

The spatial extent of beds of fine-grained tailings 

(table 9) was interpreted using the aerial photographs discussed previously, based on the gray tone and smooth texture 

of the ground surface typical of those beds. For an example 

see figure 4. The extent of tailings in 1945 was not interpreted 

because of the poor tonal quality of that photograph set. 

Tailings deposition was most extensive closest to the source 

mills in Eureka, namely in reach B. The apparent extent of 

tailings beds in reach D was and is relatively minor. The 

extent of the tailings beds decreased dramatically through 

time, in reaches B and C. Of the tailings present in 1951, 

by 1960 about 40 percent disappeared from reach B, and 

about a third disappeared from reach C (table 9). We presume that was largely the result of the three large floods that 

occurred in the 1950s (table 1). Between 1960 and 1987 the 

areas of tailings in reaches B and C continued to decrease, 

but at a less dramatic rate; and we attribute these changes to 

the actions of streamflow, which mobilized the fine-grained 

tailings and largely transported them out of the study reach. 

The decrease in reach B between 1987 and 1997, however, 

is attributed to the remediation actions of the owners of the 

Sunnyside mine.

Table 9. Spatial extent of surficial fine-grained tailings as 

interpreted from aerial photographs, by reach and year.

Reach-- B C D

Date Area of tailings, square kilometers

1951 0.23 0.06 0.01

1960 .14 .04 .02

1973 .13 .04 .01

1987 .09 .03 .01

1997 .03 .02 .00

934 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 935

In conclusion, streamflow has mobilized and exported 

tailings from the study area and will continue to do so. This 

is not without consequences. Church, Fey, and Unruh (this 

volume) studied the chemistry of streambed sediment at 

many locations along the Animas River. They demonstrated 

that stream sediment deposited in historical times contains 

elevated concentrations of several metals, compared to sediment deposited in prehistorical times. Just as we have done 

in this study, they attributed the elevated metal concentrations 

to the presence of mill tailings within the historical stream 

sediment. The geochemical signature of tailings is present in 

stream sediment at least as far downstream as Durango, Colo. 

The rate of export of tailings from the study area by streams, 

however, should be much slower in the future than it has been 

in the past, for two reasons. First, the extent of fine-grained 

tailings is currently much less than it was previously (table 9). 

Second, the remaining beds of fine-grained tailings reside 

at elevated sites, or are otherwise protected from common 

streamflows. In a previous section, we estimated the discharge 

needed to begin to inundate the tailings beds observed at the 

trench (pl. 6). The result was a discharge of 42 m3

/s, which 

is a rare discharge according to the measurements made at 

the Howardsville stream gauge (tables 1 and 2). As an annual 

instantaneous peak at that gauge, this discharge has a recurrence interval of about 10 years (table 2). The tailings beds 

could be locally mobilized by lateral migration of channels, 

as well as by flood inundation, but the calculation illustrates 

that the remaining tailings beds are relatively protected from 

erosion.

The coarse-grained tailings residing in the subsurface 

are even more protected from stream erosion, and remediation of that material would be an involved process. Relatively 

pure beds of tailings occur in the subsurface, but they are thin 

and are discontinuous, and their locations cannot be predicted accurately. For example, Campbell and Eckhart (2000) 

conducted geoelectrical studies along the trench transect just 

before the trench was excavated. They measured the local 

magnetic field and the electric conductivity and polarizability 

of the ground, and they did a ground-penetrating radar survey. 

The results were compared to the stratigraphy of plate 6, but 

they could not accurately or consistently predict the locations 

of tailings beds in the subsurface. Subsurface tailings also 

occur disseminated within other sediment, principally gravel 

deposits. The following simple calculation illustrates one 

aspect of remediating subsurface tailings.

We developed an estimate of the volumes and weights 

of the two types of tailings (table 10). The volume of surficial 

tailings was estimated by assuming an average thickness of 

the beds. Although not exhaustive, our field observations of 

the tailings beds visible in the trench (pl. 6) and other exposures suggest that they are 0.3 m thick on average. Using 

that thickness and the extent of the beds visible in the 1997 

photographs, the volume of the surficial tailings beds totals 

16,000 m3

 (21,000 cubic yards). The greatest uncertainty in 

this estimate is our assumption of average thickness.

Estimating the volume of tailings in the subsurface is 

more complicated (table 10). We assumed that areas where 

gravel is presently exposed at the surface are areas where the 

stream aggraded or reworked the substrate in historical times, 

and thus are areas with tailings in the subsurface. The spatial 

extent of exposed gravel, including the active streambed, was 

estimated from the 1997 photographs (table 10). The gravel 

containing tailings is approximately 1 m thick at the trench, 

and we assume this thickness for all exposed gravel in reach B. 

The aggradation or depth of reworking in reaches C and D 

was likely less than in reach B, so we assumed a thickness of 

0.5 m for all exposed gravel in those reaches. The limit to the 

accuracy of the results is probably these thickness assumptions. 

Accordingly the bulk volume of gravel containing tailings in the 

study area is on the order of 600,000 m3

 (780,000 cubic yards). 

Only a fraction of the gravel volume is tailings, however, so 

we assumed that 30 percent of the gravel deposits is sand and 

that two-thirds of the sand is tailings, which was derived in 

the section titled, “Geochemical Conclusions.” In other words 

we assumed that 20 percent of the gravel volume is tailings. 

Accordingly the bulk volume of tailings in gravel of our study 

area is on the order of 120,000 m3

 (160,000 cubic yards). If the 

tailings in the subsurface were to be reclaimed, presumably the 

gravel would be sieved, separating the coarse clasts from the 

sand, and all of the sand would be hauled to a repository.

In conclusion, in order to reclaim the 120,000 m3 of 

tailings, 600,000 m3

 of gravel would have to be excavated 

and sieved and 180,000 m3

 of material hauled to a repository. 

Reclaiming the 120,000 m3

 of coarse-grained tailings from 

the subsurface would thus be more involved than reclaiming 

the 16,000 m3 of surficial beds of fine-grained tailings. The 

fine-grained tailings are also more susceptible to geochemical 

reactions, all other factors being equal, because the surface 

area (per unit mass) of the silty tailings is so much larger 

(perhaps 100 times larger) than that of the sandy coarsegrained tailings. On the other hand, the coarse-grained tailings 

are saturated by ground water for a large portion of the year, 

increasing the opportunity for geochemical reactions, whereas 

only rain and snowmelt wet the fine-grained tailings. It is 

beyond our expertise to evaluate the geochemical consequence 

of these differences in wetting times.

We estimated tailings volumes in order to also understand how much of the original tailings released still reside 

in the study area. The production of mill tailings was given 

in terms of weight, so the volume of tailings just discussed 

was converted to weight using a bulk density (table 10). 

Bulk density is the average density of mineral grains reduced 

by the volumetric proportion that is void space (porosity). 

The typical density of various minerals is given herein, in 

parentheses, in grams per cubic centimeter. The rocks of 

the watershed largely consist of volcanics of intermediate 

to silicic composition, which means that they are dominated 

by quartz (2.65) and feldspars (≈2.6) and a minor amount of 

mafic minerals (3.0–3.5). A fraction of the rocks are sulfides 

and other economic minerals that are fairly dense (3.5 to 5 

and much greater). Thus the average density of bedrock may 

be close to 2.7 g/cm3

. Because of the presence of tailings, 
the dense minerals are more abundant in the sediment than 

they are in the bedrock. We assume an average mineral 

density of 3.5 g/cm3

 for the fine sediment containing tailings. We also assume a porosity of 25 percent, which yields 

a sediment bulk density of 2.6 g/cm3

. The results are given 

in table 10. Of the original tailings produced, the majority, perhaps 70–80 percent, has been flushed from the study 

reach by streamflow. The owners of the Sunnyside mine had 

“120,000 cubic yards” of tailings mechanically removed from 

reach B in 1997 (Rob Robinson, Bureau of Land Management, 

written commun., 2004), which constitutes a significant fraction, perhaps 10 percent, of the amount produced. The tailings 

remaining in the study area are also a fraction of the original, 

on the order of 12 percent, but are substantial in terms of 

their volume and weight (table 10). Of that remaining in the 

landscape, on the order of 10 percent exists at the surface as 

identifiable and accessible tailings beds, and the majority is in 

the subsurface dispersed within gravel deposits.

Summary and Conclusions

In this study we document the physical nature of the 

upper Animas River and its flood plain in several study 

reaches between Eureka and Howardsville (figs. 1, 3, 4), during the latest Holocene and in detail during the past century. 

The Animas River was likely never a classical (narrow, deep, 

tree-shaded, and single-threaded) meandering stream, based 

on comparison with meandering and braided streams in 

general (fig. 5). In addition, a sediment transport calculation 

indicated that a hypothetical single-threaded channel would 

be unstable: it would locally fill with gravel and thus force 

braiding to occur. This is the result of the strongly concave 

shape of the longitudinal profile in reaches A and B (fig. 3), 

which causes boundary shear stress (fig. 6), and thus bed 

load transport rates, to decrease rapidly downstream. Spatial 

variation in bed load transport rates causes local erosion and 

Table 10. Estimates of volumes and weights of tailings remaining in our study reach.

[Weights are expressed in metric tons (t)]

Reach B Reach C Reach D Total Footnote

Surficial beds of fine-grained tailings

Surface area, km2 0.030 0.018 0.005 0.053 1

Thickness, m 0.3 0.3 0.3 ng 2

Volume, m3 9,132 5,250 1,392 15,774 3

Volume, yd3 11,944 6,867 1,821 20,631

Tailings weight, t 23,742 13,650 3,619 41,011

Coarse tailings buried in gravel

Surface area, km2 0.429 0.168 0.174 0.771 1

Thickness, m 1 0.5 0.5 ng 2

Gravel volume, m3 429,000 84,000 87,013 600,013 3

Gravel volume, yd3 561,111 109,868 113,809 784,787

Sand volume, m3 128,700 25,200 26,104 180,004 4

Tailings volume, m3 84,942 16,632 17,229 118,803 5

Tailings volume, yd3 111,100 21,754 22,534 155,388

Tailings weight, t 220,849 43,243 44,794 308,887 6

Fraction of total tailings produced

Total weight, t 2,032,000

Removed by streamflow 76 percent

Reclaimed from reach B 12 percent

Surface beds remaining 1 percent

Within gravel remaining 11 percent

1

Estimated from 1997 aerial photographs.

2

Assumed based on field observations; ng, not given.

3

Bulk volume calculated as thickness times area.

4

Bulk volume of sand assumed to be 30 percent of gravel volume.

5

Bulk volume of tailings assumed to be 20 percent of gravel volume.

6

Assumes a bulk density of 2.6 g/cm3.

936 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Response Downstream from Eureka to Discharge of Mill Tailings 937

deposition that will persist until the channel morphology 

adjusts to remove the uneven transport rates. Given a strongly 

concave valley profile, the end result of uniform bed load 

transport down the Animas River inevitably involves a spatial 

transition of channel shape down the valley. Prior to the activities of miners, this involved a transition from a braided stream 

in reach A to multi-threaded channels at the downstream end 

of reach B. The channels transitioned from being braided 

(multiple shallow channels with largely indistinct banks) to 

what we call multi-threaded: more than one relatively stable 

channel with most having steep and relatively high banks. A 

century ago, reach A consisted of a braided and shifting channel (figs. 8 and 9A). Reach A is upstream of Eureka and thus 

upstream of the source of tailings from the mills associated 

with the Sunnyside mine.

The sediment transport model suggests that reach B is 

inherently susceptible to braiding, and other evidence demonstrates that the Animas River in reach B was alternately 

braided and multi-threaded during periods of the late Holocene. 

Extensive gravel deposits dominate the base of the stratigraphic 

section exposed in the trench (pl. 6). We interpreted this to represent a period of deposition and reworking by broad braided 

streams, with only localized deposition (or preservation) of 

fine-grained flood-plain deposits. This period apparently ended 

about a thousand years ago (fig. 13). After that, the brown silt 

flood-plain deposits began to accumulate (and were preserved) 

in between multiple channels of the Animas River at the trench 

site. The banks of these channels were on the order of 1 m 

high. The flood plain likely consisted of dense thickets of willow and areas of grasses between sparse willow bushes. Thin 

layers of peat within the silt beds suggest that beaver ponds 

were locally or occasionally present on the flood plain. This 

period of multi-threaded channels surrounded by vegetated 

flood plain persisted until the early 20th century, based on 

the observations made from early historical photographs. The 

multi-threaded channels evident in the ca 1904 photograph 

(fig. 2A), in particular, support the stratigraphic interpretation of the nature of channels and flood plain. The persistence 

of that channel configuration for perhaps a thousand years 

demonstrates a geomorphic equilibrium involving uniform bed 

load transport. This equilibrium condition ended abruptly. The 

stratigraphy (pl. 6) reveals that dramatic and rapid changes in 

the upper Animas River valley occurred during historical times. 

At least at the trench site, the channels were filled with gravel, 

after which the Animas River reverted to a braided condition 

and broad sheets of gravel were deposited over the formerly 

vegetated flood plain. The upper end of reach B also apparently 

aggraded, as that would explain the current absence of 1-m 

high banks that are evident in that area in the 1883–84 photograph, and which are faintly visible in the 1904 photograph 

(fig. 2A). Subsequently, beds of fine-grained tailings were 

deposited over much of the valley floor of reach B (fig. 4).

The timing of this episode of dramatic channel change 

is fairly tightly constrained. Coarse-grained tailings are locally 

found at the base of buried channels (pl. 6) and may have been 

deposited late in the 19th century or early in the 20th century. 

This deposition likely occurred after 1900, however. Only a 

modest quantity of tailings was produced before 1900 (fig. 7); 

and after 1900, coarse-grained tailings were produced in 

quantity directly adjacent to the Animas River at the head of 

reach B. Channels in reach B were not full of gravel in 1904 

(fig. 2A). The channels may have been partially filled with 

gravel during the 1911 flood, which is the flood of record. At 

the site of the dated willows (fig. 12), 100 m downstream of 

the trench, gravel at the base of the section was likely deposited in 1911, but the bulk of the streambed gravel at that site 

was probably deposited during the 1921 and 1927 floods. 

Much of the gravel accumulation in the reach at the trench also 

likely occurred in the 1920s. In the aerial photographs (fig. 4), 

the fine-grained tailings appear to have been purposely placed 

behind the gravel berms in reach B, and tailings stopped being 

produced in quantity in 1930. Thus channel aggradation may 

have ceased before 1930. In the field we observed large areas 

of ground surface consisting of blackened gravel or tailings 

beds at the trench site, and these features are evident in the 

1945 aerial photograph. Thus, at the latest, the period of rapid 

aggradation ended before 1945.

Why did the channels in reach B aggrade? The first argument involves a coincidence in space and in time. The Animas 

River underwent a dramatic change from a stable geomorphic 

condition and transformed into a braided reach that is unique 

in the region. Mining activities (but not milling) were extensive in the Cement Creek basin, for example, but Cement 

Creek did not experience dramatic physical changes during 

historical times (Vincent and others, this volume). The unique 

reach B is located immediately downstream of mills that 

produced between 35,000 and 330,000 metric tons per year of 

tailings. Furthermore, the channels began to fill immediately 

after the period when mills supplied large quantities of sandsized tailings directly to the fluvial system. This argument suggests that tailings played an important role in causing channel 

change.

Understanding the causal mechanism is tricky, however, 

and two issues must be explained. The first issue is the source 

of the gravel clasts filling the paleochannels of reach B (pl. 6). 

This is important, because the bulk of the gravel consists 

of subrounded stream-gravel clasts, unrelated to mining or 

milling. We believe that these clasts were derived from the 

substantial deposits in reach A, in part because the Animas 

River in reach A appears to have incised after 1906 (fig. 9). 

The second issue involves our explaining how the delicately 

balanced condition of uniform bed load transport through 

reach B became altered. We do not think that the fine-grained 

tailings played the major role. On the one hand, the tailings 

beds may have acted to increase the heights of banks, locally 

allowing flows to be deeper and exert higher boundary shear 

stress, resulting in anomalously high bed load transport rates. 

On the other hand, however, the beds of fine-grained tailings 

are dominated by silt, and field evidence (climbing ripples) 

demonstrated that silt was carried in suspension. As suspended 

load, silt could influence bed load transport by decreasing the 
fluid viscosity and decreasing the density contrast of the fluid 

and bed material. Suspended sediment rarely makes up even 

5 percent of the fluid volume, however, and we do not think 

that the influence on bed load transport is sufficiently strong 

to cause the observed changes. We suspect the sand-sized 

tailings altered bed load transport in the following way. Suspended transport of sand cannot be supported unless patches 

of sand are present on the bed, which essentially makes the 

bed smoother, effectively lowering the critical shear stress 

for transport of pebbles on the bed.

We offer the following sequence of events as a hypothesis. After 1900 sand-sized tailings were produced in abundance but largely resided near the mill until a major flood 

occurred. A major flood occurred in 1911, depositing tailings 

from this site in the Animas River canyon (Church, Fey, and 

Unruh, this volume, fig. 20). This flood mobilized gravel in 

reach A, but when the gravel encountered the abundant sand 

near the mills the sediment transport rate increased, perhaps 

causing local incision in the upper portion of reach B. As the 

excessive bed load encountered the decreasing gradient of 

the middle and lower portions of reach B, the transport rate 

decreased and gravel was deposited in the multi-threaded 

channels, but did not fill them. Sand-sized tailings continued 

to be produced until at least 1913, if we interpret the discussion by Jones (this volume) of the “Gravity Milling era” 

correctly. Alternatively, sand-sized tailings may have continued to be produced in abundance until the efficient Sunnyside 

Eureka Mill was completed in 1918. In any case the sand 

largely resided near the mill(s) until a large flood occurred. 

A large flood did occur in 1921, and an even larger flood 

occurred in 1927; and just as in 1911, gravel was removed 

from reach A and deposited in the lower portion of reach B, 

with one change occurring. As sediment was deposited in the 

channels at the downstream end of reach B, the gradient of 

the reach was reduced. The reduced gradient, and shallowing 

of the flow due to braiding, caused the locus of deposition 

to migrate upstream until all channels in reach B were filled. 

The original gradient was thus restored and uniform bed 

load transport resumed, just as our calculations suggest that 

transport is currently uniform in the subreach containing the 

surveyed cross sections at the downstream end of reach B. In 

any case aggradation did occur in reach B, whereas changes in 

reaches C and D, which have more uniform profile gradients, 

were comparatively minor.

Tailings became disseminated within streambed gravel 

in the subsurface (pl. 6) during aggradation, and are also found 

as easily identifiable beds at the ground surface (fig. 11). As 

a direct result of the presence of tailings, the concentrations 

of zinc (fig. 14) and other metals (table 8) are dramatically 

elevated compared to those of prehistorical sediment, whereas 

vanadium and other elements are more dilute. Streamflows 

have been reworking the tailings (table 9), and have transported between 70 and 80 percent of the tailings originally 

produced out of our study area (table 10). Of the tailings that 

remain in our study area, on the order of 10 percent exists 

as distinct beds of fine-grained tailings at the ground surface 

(table 10). If the assumptions of our calculations are correct, 

the majority (90 percent) is located in the shallow subsurface, 

and consists of predominantly sand-sized tailings particles 

mixed with gravel. It is our understanding that the environmental concern over tailings is that “water/rock” geochemical reactions will liberate metals from tailings particles and 

increase metal concentrations in ground water and (or) 

surface water. The concentrations of metals are similar in the 

two types of tailings (table 8), with the exception of lead, and 

manganese to a lesser degree, which have higher concentrations in the fine-grained tailings. Thus, the two deposit types 

are similar in that way. Yet, there are important differences 

in the two types of tailings deposits. It is our understanding 

that the rates of geochemical reaction with solid particles are 

directly proportional to the surface areas of those particles, 

all other factors being the same. The fine-grained tailings have 

about 100 times more surface area (per unit mass) compared 

to the coarse-grained tailings. It is also our understanding that 

the efficacy of geochemical reactions increases with increasing duration of the contact of water and solid particles. We 

presume that the duration of contact with water is greater for 

the coarse-grained tailings, as they are located in the subsurface and are presumably in contact with ground water for a 

large portion of the year. Thus, the rates of geochemical reactions of water with coarse-grained tailings may be relatively 

slow (per unit time), yet the coarse-grained tailings are far 

more abundant and the duration of their contact with water is 

longer, compared with that for the fine-grained tailings. On the 

other hand, the fine-grained tailings are susceptible to being 

mobilized by streamflow (but, at present, likely only by large 

floods), and being exported to areas outside of our study area 

where they may cause problems. The coarse-grained tailings 

are comparatively protected in the subsurface from mobilization by streamflow. It is beyond our expertise to balance these 

different factors and judge the relative environmental impact 

of the two tailings types, but we hope that the information that 

we have provided will enable others to do so.

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