Mo.) Peabody Coal Company (Saint Louis.

Progress report, hydrologic research at the Big Sky Mine online

. (page 1 of 5)
Online LibraryMo.) Peabody Coal Company (Saint LouisProgress report, hydrologic research at the Big Sky Mine → online text (page 1 of 5)
Font size
QR-code for this ebook


Dollhopf, D.J., W.D,


Hall, and G.W. Wendt


Progress Report:


Hydrologic Research at

the Big Sky Mine





D.J. Dollhopf, W.D. Hall, and G.W. Wendt '

R.L. Hodder
Program Leader: Reclamation Research Program

July 1977

Research conducted by the Montana Agricultural Experiment Station,
Reclamation Research Program, Montana State University, Supported
by the Peabody Coal Company, Land Use and Reclamation Division,
St. Louis 66, MO.


Soil Physicist - Principle Investigator, Hydrogeologist , and Research

Assistant in Soils, respectively, Montana State University, Bozeman,


AUG 10


3 0864 1002 1275 5


This report does not signify that the contents necessarily
reflect the views and policies of the Peabody Coal Company. Mention
of trade names or commercial products does not constitute endorsement
or recommendation for use by either the Peabody Coal Comapny or the
Montana Agricultural Experiment Station.



The authors thank the Big Sky Mine reclamation staff,
Mr. Alten Grandt , St. Louis office, Mr. Gene Tuma, Colstrip office,
and Mr. George Robinson, Denver office, for their excellent cooperation
during these studies. A special acknowledgement is in order to
Mr. Larry Fox, Big Sky Mine supervisor, for use of his personal time
and equipment during construction phases of this research.

The authors thank Dr. Richard Meyn, former M.A.E.S. watershed
scientist for his help in initial planning and manuscript preparation
on this project.








Surface Hydrology 6

Root Zone Hydrology 9

Ground-Water Hydrology 19

Summary 27


Reservoir History 29

Reservoir Monitoring Program 31

Results and Discussion ^0

Summary ^^






The Big Sky Mine, owned and operated by the Peabody Coal Company,
is located in southern Rosebud County about 9.6 km south of the town of
Colstrip, Montana (Figure 1). The mining operation will extract coal
from portions of Sections 13, 14, 15, 21, 22, and 23, within Coalbank
and Miller coulees drainage basin, tributaries to Rosebud Creek. The
area has considerable relief (Figure 2), ranging between 915 and 1065
meters above sea level. Local climate is characterized as continental,
warm in summer, cold in winter. Average annual precipitation is 40.1 cm
(^U.S, Dept. of Commerce).

The Montana Agricultural Experiment Station has been conducting
several hydrological studies in the Colstrip area (Arnold and Dollhopf,
1977; Dollhopf and Meyn, 1975). This interim report discusses two
hydrological studies presently being conducted at the Big Sky Mine, Colstrip,
Montana. In the Coalbank Coulee watershed (Figure 1) , which encompasses
about 1200 hectares, the Experiment Station is evaluating the present
state of the ground water, surface water, and soil water resources.
Approximately 325 hectares in this region will be strip mined during the
next several decades. Particular attention has focused on a 90 hectare
alfalfa field in the lower Coalbank Coulee basin, portions of which are
subirrigated. The basis of concern is that mining of the sub-bituminum
coal from upper portions of the watershed may interrupt the ground-water
regime, resulting in a water table drop sufficient to reduce production
in alfalfa fields in downstream alluvium. Such concerns stem from a
larger, more general theme regarding conflicts between western strip

» »

A A ■■.A^..4..j»^B|4Mllhmr. m. .

Figure 2. Upper and Lower Coalbank Coulee, respectively.
Colstrip, Montana.

mining and agriculture. This report presents a quantitative description
of the existing hydrological characteristics in Coalbank Coulee.

The second study discussed in this report pertains to the feasibility
of including a reservoir in spoil material as part of the Big Sky Mine
reclamation plan. Present State of Montana reclamation guidelines do not
encourage such a development, largely due to the unknown effects of such
an impoundment upon the environment and due to concern that a semiarid
climate may not sustain a viable reservoir on spoils.

In 1975 the Big Sky Mine was granted temporary permission to construct
a reservoir within recontoured spoils under condition that the hydrological
characteristics of the reservoir be monitored for a number of years. At
the end of this research period the State shall review the feasibility of
this reservoir on the basis of research results. The Miller Coulee
watershed encompasses about 2145 hectares and about half of this surface
drainage area is planned to be diverted into the reservoir (Figure 1).
The intent is to utilize intermittent surface flows from the Miller
watershed to sustain the reservoir. This report presents the first year
hydrologic data.


Surface Hydrology


The Coalbank Coulee sub-basin comprises an area of approximately
1200 hectares. The topography consists mostly of gently sloping, semiarid
rangeland. It lies to the north but adjacent to the Miller Coulee sub-
basin (Figure 1). The drainage channel at the confluence of Miller and
Coalbank Coulee sub-basins is an intermittently flowing channel. Coalbank
Coulee has numerous, but intermittent, third, second, and first order
channels entering it.

The elevation differential of the main drainage channel is about 155
meters. The highest point is roughly 1100 meters above sea level (a.s.l.)
while the lowest point at the injunction with Miller Coulee is about 940
meters a.s.l. The total channel length is approximately 5940 meters, and
the average channel gradient is 2.6 percent. Approximately 100 meters
of the main channel have been disturbed by mining. The average gradient
along the channel above the mined area is 6 percent; in contrast, the
average gradient below the mine averages 1.3 percent (Figure 1).

Drainage from Coalbank Coulee is to Miller Coulee which in turn flows
in a southerly direction to Rosebud Creek. Rosebud Creek turns north
and flows into the Yellowstone River near Forsyth, Montana, approximately
56 km from Coalbank Coulee. The uppermost boundary of Coalbank Coulee
is well defined by a rim of weather resistant clinker and sandstone. Sheer
cliffs can be found at this point. Beyond the rim of cliffs or bluffs is
a plateau of small areal extent.


The vegetation of the Coalbank Coulee sub-basin consists of primarily
native bunchgrasses , shrubs, f orbs , and trees. The gently sloping and
lower portion of the basin is predominantly a grassland type with occasional
trees. Cottonwoods (Populus deltoides) are found in and along the lower
drainage channel where the soil is moist. The upper portion of the basin
consists of grasslands interspersed with a pine woodlands type. Ponderosa
pine (Pinus ponderosa) and juniper (Juniperus spp.) are found on the rocky
upland slopes and on the plateau area.

Differentiation between vegetative species growing within the drainage
channel and upon the adjacent slopes reflects past grazing management
practices (Dr. D.F. Ryerson, Montana State University, personal communica-
tion, 1975). No severe climatic changes have occurred within the past 50
years which could explain the vegetative development that has occurred.
Colonies of prairie sandreed grass (Calajvolvilfa longifolia) , a warm season
grass, have spread from the upper ends of the drainage channel, down the
sides, and onto the channel bottom of Coalbank Coulee. This species spreads
primarily by rhii:omes (subterranean stems) and requires many years (possibly
decades) for such expansion to take place. Such growth would be unlikely
with active channel erosion. In a similar manner, colonies of little
bluestem {Schiza-^hrium scopariurn) have spread to a distance of 100 meters
or more into denselv vegetated rangeland. Such encroachment does not readily
occur with grazing or with land disturbance. Thus, the vegetation of
Coalbank Coulee provides a basis for estimating the liistorv of land use,
the level of land disturbance, and the period of time since major surface
drainage activitv has occurred.

Channel Morphology

The channel areas below the mined site and the adjacent basin slopes
are well stabilized with a dense cover of vegetation. There are no signs
of active erosion or massive slippage. The stability of the drainage
channel is considered excellent when compared to other drainage channels
in the local area and when considering the limitations of a semiarid
climate on plant growth.

The channel of Coalbank Coulee from the mined area to the confluence
with Miller Coulee shows no visible evidence of active or erosive flow
in 10 or more years, possibly 50 or more years. There are no deposits of
plant and soil debris as one would find in an active, but intermittent,
drainage channel. There are no visible signs of scouring. The channel
below the mine is apparently being slowly raised by silt deposition from
small surface flows during the spring. This conclusion is supported by
an average 1.3 percent gradient, and in places a reverse gradient, in the
channel below the mine as compared to a general basin gradient of 2.6
percent at the upper slope position.

Root Zone Hydrology

In Montana there are about 202,350 hectares (500,000 acres) of dryland
alfalfa production. Most of this acreage is used for Iiay production, but
approximately 40,500 hectares are used for seed production. Economic value
of alfalfa grown in Montana for seed can range between $100 and $1,000 per
acre depending upon yield and market price. Most of the root biomass of
dryland alfalfa is found within 3 m of the surface and the viable roots are
found within 1.4 m of the surface. Occasional reports (i.e., Hughes,, 1962), liave indicated alfalfa rooting deptlis of 9 m, or, in some
rare instances, over 60 m wliere a root has followed a crack in an attempt
to seek soil water. However, in Montana it has been shown that dryland

alfalfa will generally root in the surface 3 m of soil and extrr.ct its water

requirements within the upper 5 m of soil.

In Coalbank Coulee, the 90 hectare alfalfa field, owned by Mr. Snider,

a local Colstrip rancher, has been grown for seed in recent years. In order

for the alfalfa to produce seed it must slow or stop its vegetation production

and progress into a reproductive state. This change is triggered by a soil

water stress which, for maximum seed production, would ideally occur in late

June. Harvesting of the seed would generally take place in mid-August.

Personal communication with Dr. Raymond Ditterline, forage researcher and

Asst. Professor, Plant & Soils Dept . , Montana State University.


Unpublished data from salt seep research in the Highwood Bench area of

Montana by Dr. P.L. Brown, Soil Scientist, Agriculture Research Service,

Highwood, Montana.

Variation from this phenological scheme can decrease seed production and,
if the plant water stress period is late, then seed development may extend
into September when freezing becomes damaging. Therefore the soil water
characteristics of this field are very important and are discussed in this


Neutron access tubes were installed in or near the lower part of the
alfalfa field (Figure 1) next to Coalbank Coulee during December of 1974 to
monitor the changes in soil water content as a function of time. The neutron
scattering method was chosen for its ability to nondestructively sample a
given point at frequent intervals.

The reader is referred to Schultz (1965) for detail on the theory of
the neutron method as well as its applicability in estimating water content
of the unsaturated zone. In brief, a radioactive source (contained in a
probe) is lowered into the ground. High speed neutrons are emitted from
the source into the adjacent soil. Upon striking water or other hydrogenous
materials, these neutrons are slowed down and become low speed neutrons.
Some of these low speed neutrons are deflected and make their way back to
the probe where a gas-filled, electronically-amplified detector counts
them as a function of time. The percent of water in the soil is then
determined by comparison of the number of slow neutrons counted (field count)
as compared to a reference of known water content (shield count). For
example, a neutron probe reading may indicate a 50 percent moisture content.
A soil with volumetric moisture percentage of 50 percent would then equal
15 cm of water per 30 cm of soil.


Five neutron access tubes were installed on a transect extending from
native range, across Snider' s alfalfa field, to a point on disturbed range
(Figure 1). The disturbed rangeland site is a result of construction
activity to build a landfill railroad pass over Coalbank Coulee (Figure ]).
Measurements began in January 1975, and were taken subsequently in monthly
intervals. Identification, location and approximate depth of installation
of neutron probe access tubes in Coalbank Coulee are listed in Tabje 1,

Table 1. Neutron probe access tubes in the lower Coalbank Coulee region, 1976.















Maximum Depth
(cm) (inches)












Disturbed range
Alfalfa field
Alfalfa field
Alfalfa field
Native range

Soil water content data alone can leave some question as to whetlier a
determination, i.e. at 40%, is at near saturation (0 bars), or near field
capacity (=.3 bars), or near the wilting point (-15 bars). In order to
substantiate the interpretation of water content data, they are best
presented with the desorption characteristics of the soil material, which
indicate the water content percentage at various soil water potentials,
i.e. 0.0 bars, 0.3 bars, and 15 bars. Soil water desorption characteristics

1 3 4 5

Online LibraryMo.) Peabody Coal Company (Saint LouisProgress report, hydrologic research at the Big Sky Mine → online text (page 1 of 5)