U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3004

February 2015

Prepared in cooperation with the Yellowstone River Conservation District Council and the U.S. Army Corps of Engineers

Effects of Water-Resource Development on Yellowstone 

River Streamflow, 1928–2002

Streamflow in the Yellowstone 

River

Major floods in 1996 and 1997 

intensified public concern about the 

effects of human activities on the Yellowstone River in Montana. In 1999, the 

Yellowstone River Conservation District 

Council (YRCDC), whose members are 

primarily representatives from the conservation districts bordering the main stem 

of the Yellowstone River, was formed to 

promote wise use and conservation of the 

Yellowstone River’s natural resources 

(Yellowstone River Conservation District 

Council, 2014). No major dams or reservoirs are located on the main stem of the 

Yellowstone River making it one of the 

longest free-flowing rivers in the United 

States (fig. 1); however, many uses of 

the river alter streamflows. The YRCDC 

is working with the U.S. Army Corps of 

Engineers (USACE) to understand the 

cumulative hydrologic effects of waterresource development in the Yellowstone 

River Basin (fig. 1). Hydrologic effects 

are a critical component of the overall 

cumulative effects studies, which have 

been designed to evaluate the collective 

effect of a wide variety of human activities on streamflows in the river. 

Since the 19th century, water from 

the Yellowstone River Basin has been 

diverted for irrigation, mining, municipalities, and others uses (PBS&J, 2009). 

Streamflows have been measured by the 

U.S. Geological Survey (USGS) and 

other Federal and State agencies since the 

late 1800s at sites throughout the Yellowstone River Basin (Chase, 2014). At 

any given time, streamflows measured 

by streamgages are affected by water 

withdrawals for irrigation, tributary 

reservoirs, and other water uses, as well 

as water added back into the river by 

tributary reservoir releases and irrigation 

return flows (Chase 2013, 2014). Irrigation and tributary reservoirs affect both 

the timing and quantity of streamflow. In 

the Yellowstone River Basin, irrigation 

systems and tributary reservoirs generally 

hold (or divert) water in the spring and 

summer and release water to the stream 

in the fall and winter (U.S. Army Corps 

of Engineers, 2011a, 2011b). Therefore, 

streamflows for some months, typically 

in the spring and summer, are lower than 

they would be naturally because of irrigation diversions or the temporary holding of water in reservoirs upstream, and 

because of water losses due to evaporation and seepage. Conversely, in the fall 

Figure 1. The Yellowstone River Basin consists of the main stem of the Yellowstone 

River and four major tributaries: Bighorn River, Clarks Fork of the Yellowstone River, 

Powder River, and Tongue River. Eight U.S. Geological Survey streamgages were 

selected to analyze flows at various points in the river and are described in this 

fact sheet.

Yellowstone River

Yellowstone River

Yellowstone

Lake

111° 110° 109° 108° 107° 106° 105° 104°

43°

44°

45°

46°

47°

48°

0 50 100 KILOMETERS

0 50 100 MILES

Base modified from U.S. Geological Survey Digital Line Graph (DLG), 1973, 1:2,000,000 

Lambert Conformal Conic projection, standard parallels 45°N and 49°N, central meridian 109°30’W

North American Datum of 1983 (NAD 83) 

Livingston

Gardiner

Billings

Cody

Sheridan

Riverton

Lander

Sidney

Miles

City Forsyth

Locate

Corwin

Springs

EXPLANATION

YELLOWSTONE 

NATIONAL PARK

MONTANA

WYOMING

NORTH

DAKOTA

Billings

Study area

Glendive

Yellowstone 

River Basin

Streamgage

River

Powder River

Boysen

Reservoir

Tongue R

Reservoir

Bighorn

Lake

River

Wind

River

Tongue

GreybullRiver

Bighorn

River

Yellowstone R

Buffalo

Bill

Res

Stillwater R

LittlePowder River

ShoshoneR

Little Bighorn R.

Powder

RosebudCre e k

Sarpy Creek

BoulderRiver

Lake

outlet

Bighorn

River

Basin

Yellowstone River Cla rks F k

Bighorn River Missouri River

Missouri River

Big Timber

Crazy MountainsCrazy Mountains

Intake
and winter, when water is released from 

irrigated fields or a reservoir upstream, 

streamflows are higher than they would 

be naturally. In addition to the streamflow 

that typically leaves the Yellowstone 

River Basin when the Yellowstone River 

flows into the Missouri River, each year, 

some water is removed from the Yellowstone River Basin because of evapotranspiration from reservoirs and irrigated 

fields and seepage from reservoirs and 

irrigation canals.

Calculating Streamflow Statistics 

Using Data from 1928 to 2002

The USACE, YRCDC, and USGS 

began cooperatively studying the Yellowstone River in 2010, publishing four 

reports describing streamflow information for selected sites in the Yellowstone 

River Basin,1928–2002 (U.S. Army 

Corps of Engineers, 2011a, 2011b; Chase, 

2013, 2014). Detailed information about 

the methods used, as well as summary 

streamflow statistics, are available in 

the four reports. The purpose of this fact 

sheet is to highlight findings from the 

published reports and describe the effects 

of water use and structures, primarily 

dams, on streamflow in the Yellowstone 

River Basin. The definitions of technical 

terms used in this fact sheet are shown in 

the sidebar on page 3. 

Observed streamflow records from 

USGS streamgages (fig. 2) served as the 

basis for all calculations. In order to estimate the effects of water use, streamflow 

datasets for a daily time-step were constructed (using the observed streamflow 

records) to represent streamflow for two 

different scenarios. Datasets constucted 

to represent streamflow without waterresource development are defined as 

“unregulated flow,” and the datasets representing streamflow with water-resource 

development are defined as “regulated 

flow.” Streamflow statistics, such as 

annual mean streamflows, mean low 

flows, and peak streamflows, were then 

calculated for unregulated and regulated 

conditions for sites on the Yellowstone 

River, and on the Bighorn, Tongue, and 

Powder Rivers for the years 1928–2002 

(U.S. Army Corps of Engineers, 2011a, 

2011b; Chase, 2013, 2014). In this fact 

sheet, comparisons between unregulated 

and regulated streamflow statistics for a 

few selected Yellowstone River locations 

(fig. 1) are used to describe the effects of 

water-resource development on the Yellowstone River Basin.

Figure 2. Stream stage is measured every 15 minutes by instrumentation at a streamgage and transmitted by satellite to 

U.S. Geological Survey computers; the data collected are referred to as real-time data. Periodic stream stage and streamflow 

measurements are made by U.S. Geological Survey hydrographers to ensure accuracy of the real-time data. Streamflow information is 

displayed on the web and can be accessed at http://waterdata.usgs.gov/nwis. (Details about the entire process of displaying real-time 

streamflow data on the web can be found in Olson and Norris, 2005). 

Data recorder transmission to satellite

Wading rod

Current meter

Orifice line

Gage house

Wading rod

Current meter

Orifice line

Gage house
Comparing Streamflow Changes 

During the Course of a Year

Comparing unregulated and regulated hydrographs (graphs showing 

daily streamflows during the course of 

a year) can provide insight into how 

water-resource development in the Yellowstone River Basin affects streamflows 

in the Yellowstone River. Unregulated 

(without water-resource development) 

and regulated (with water-resource 

development) streamflows are compared 

for the Yellowstone River streamgages 

at Billings and at Forsyth for a typical 

water year (October 1–September 30) 

in 1979 (fig. 3). At both locations, the 

regulated hydrograph, which includes the 

effects of water-resource development, 

shows differences when compared to 

the unregulated hydrograph because of 

stream diversions during the spring and 

summer for irrigation, reservoir storage, 

and other uses. 

Unregulated and Regulated Low 

Flows along the Yellowstone River

Low streamflows in a river are of 

particular concern to irrigators needing 

to apply sufficient water for crops as well 

as for municipalities to support domestic, 

Definitions of Technical Terms

observed streamflow 

records Streamflow data measured using a streamgage

unregulated flow Calculated 

streamflow without water-resource 

development

regulated flow Calculated streamflow with water-resource development

annual mean flow Mean streamflow for a single year

7-day 50-percent annual nonexceedance probability (ANEP) 

flow The lowest mean streamflow 

over a consecutive 7-day period 

with a 50-percent chance of not 

being equaled or exceeded in a 

given year. Sometimes called the 

7-day 2-year low flow (or 7Q2).

7-day 20-percent ANEP flow The 

lowest mean streamflow over a 

consecutive 7-day period with a 

20-percent chance of not being 

equaled or exceeded in a given year. 

Sometimes called the 7-day 5-year 

low flow (or 7Q5).

30-day 50-percent ANEP flow

The lowest mean streamflow over 

a consecutive 30-day period with 

a 50-percent chance of not being 

equaled or exceeded in a given year. 

Sometimes called the 30-day 2-year 

low flow (or 30Q2).

30-day 20-percent ANEP flow

The lowest mean streamflow over 

a consecutive 30-day period with 

a 20-percent chance of not being 

equaled or exceeded in a given year. 

Sometimes called the 30-day 5-year 

low flow (or 30Q5).

50-percent annual exceedance 

probability (AEP) flood Streamflow that has a 50-percent chance 

of being equaled or exceeded in any 

given year. Sometimes called the 

2-year flood.

20-percent AEP flood Streamflow 

that has a 20-percent chance of 

being equaled or exceeded in any 

given year. Sometimes called the 

5-year flood.

1-percent AEP flood Streamflow 

that has a 1-percent chance of 

being equaled or exceeded in any 

given year. Sometimes called the 

100-year flood.

Figure 3. Differences between daily unregulated and regulated streamflow can be 

seen in hydrographs for October 1978 through September 1979 at the A, Yellowstone 

River at Billings, Montana; and B, Yellowstone River at Forsyth, Montana, sites. The 

regulated hydrograph shows slightly higher streamflow compared to unregulated 

streamflows at Forsyth in the fall and winter because of water released back into the 

stream from irrigation return flows and from upstream reservoir releases. Conversely, 

because of upstream water use, regulated streamflows are lower than unregulated 

streamflows in the spring and early summer as much as 5,000 cubic feet per second 

at Yellowstone River at Billings and as much as 16,000 cubic feet per second at 

Yellowstone River at Forsyth.

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

Streamflow, in cubic feet per second

Regulated

Unregulated

Observed

EXPLANATION

A. Billings

Oct. 1978

Nov. 1978

Dec. 1978

Jan. 1979

Feb. 1979

Mar. 1979

Apr. 1979

May 1979

June 1979

July 1979

Aug. 1979

Sept. 1979

B. Forsyth

Month and year

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000
The Bighorn River provides irrigation water to users 

in the arid Bighorn Basin.

Melting snow from the Crazy Mountains, shown 

in the background, will reach this section of the 

Yellowstone River near Big Timber, Montana.

commercial, and industrial water use. 

Low streamflows also affect ecological 

conditions such as water quality, fish 

populations, and a host of other biological communities associated with the river. 

Two primary variables are used to 

describe low-flow characteristics. The 

first variable is the duration of the lowest 

flow period during any given year. For 

example, the 7-day low flow is the lowest 

mean streamflow for any consecutive 

7-day period during a given year, and 

the 30-day low flow is the lowest mean 

streamflow for any consecutive 30-day 

period. 

The second variable is the probability, such as 50 percent or 20 percent, 

of a given low-flow duration occurring 

in any given year. The combination of 

these two variables, the duration of the 

low flow and the probability of occurrence, allows the computation of different statistics to understand how low 

streamflows might be and is referred to 

as the low-flow annual non-exceedance 

probability (ANEP). For example, there 

is a one in two chance that flows will 

be lower than an ANEP of 50 percent, 

whereas for 20-percent low flows, there 

is a one in five chance that flows will be 

lower than this value each year. The values for unregulated and regulated 7-day 

and 30-day low-flow ANEPs for eight 

streamgages along the Yellowstone River 

are shown in figure 4. 

Unregulated and Regulated 

Peak Streamflows along the 

Yellowstone River

Peak streamflows are important 

to the health of the Yellowstone River 

because they improve fish habitats, saturate wetlands, and propagate seedlings. 

Peak streamflows also can substantially 

change the stream channel through 

scouring, transport, and deposition of 

streambed sediments and can potentially 

cause damage to communities and river 

infrastructure. Peak streamflows typically are analyzed and described in terms 

of how often they might be expected to 

occur, which relates to a probability or 

chance of occurrence. At a site, streamflow that has a 50-percent chance of 

being equaled or exceeded in any given 

year is referred to as a 50-percent annual 

exceedance probability (AEP) flood. 

Figure 4. Differences between unregulated and regulated streamflows in the 

Yellowstone River can be seen in four different calculated annual non-exceedance 

probabilities. A, the 7-day 50-percent and 7-day 20-percent probability. B, the 30-day 

50-percent and 30-day 20-percent probability. These probabilities indicate that low 

flows for regulated conditions are higher than for unregulated conditions. In particular, 

the low flows in the Yellowstone River downstream from Billings would be lower for a 

7-day period or 30-day period if no regulation occurred in the Yellowstone River Basin. 

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

Streamflow, in cubic feet per second

EXPLANATION

20-percent unregulated

20-percent regulated

50-percent unregulated

50-percent regulated

A. Annual 7-day low flow

B. Annual 30-day low flow

River mile

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

600 500 400 300 200 100 0 Glendive Sidney Forsyth Miles City Billings Livingston Corwin Springs Yellowstone Lake outlet
Yellowtail Dam on the Bighorn River, a tributary 

of the Yellowstone River, creates Bighorn Lake in 

northern Wyoming. Photograph by National Park 

Service.

U.S. Geological Survey personnel survey a 

diversion structure on the Yellowstone River at 

Intake, Montana. The structure (modified after the 

photograph was taken in 2006) channels water into 

a canal to be used for irrigation.

Similarly, streamflows with a 20-percent 

or a 1-percent chance of being equaled or 

exceeded in any given year are referred to 

as a 20-percent AEP flood or a 1-percent 

AEP flood, respectively. Values for 

unregulated and regulated peak streamflows for the 50-, 20-, and 1-percent AEP 

floods for eight sites along the Yellowstone River are shown in figure 5. 

At the Yellowstone Lake outlet, 

Corwin Springs, and Livingston sites 

(figs. 1 and 5), unregulated and regulated 

streamflows are nearly equal because 

little water-resource development occurs 

in the upstream part of the Yellowstone 

River Basin. At the Billings site, unregulated peak streamflows are slightly higher 

than regulated peak streamflows, similar 

to the 1979 hydrograph in figure 3. The 

differences between unregulated and 

regulated peak streamflows are much 

larger downstream from Billings, primarily because of the effects of the Bighorn 

River, a highly regulated stream, which 

joins the Yellowstone River upstream 

from Forsyth. 

Depletions

Much of the water in the Bighorn 

River Basin is used for agriculture, and 

reservoirs have been constructed to store 

water. The U.S. Bureau of Reclamation (1999, 2005) estimated depletions, 

changes to natural streamflow, which 

result from water-resource development 

during 1928–2002. Average annual depletions for 1928–2002 estimated for areas 

upstream from each USGS streamgage 

on the Yellowstone River and for the 

area upstream from the Bighorn confluence are shown on figure 5. These mean 

annual depletions mostly represent water 

that leaves the Yellowstone River Basin 

by evapotranspiration from reservoirs 

and from irrigated fields, and by seepage 

from reservoirs and irrigation canals into 

the groundwater system. Water-resource 

development steadily increases from 

Livingston to Billings, with a larger 

increase at the Bighorn River confluence. The depletions depicted in figure 5 

represent water that annually leaves the 

Yellowstone River Basin (1) because of 

irrigation and (2) because of the combination of irrigation, and water stored 

in the major reservoirs in the Bighorn 

River Basin (Boysen Reservoir, Buffalo Bill Reservoir, and Bighorn Lake). 

Figure 5. Six different calculated annual exceedance probabilities for eight sites along the Yellowstone River show different peak 

streamflows based on the probability of occurrence and whether the streamflow was unregulated or regulated. The greatest differences 

between unregulated and regulated peak streamflows in the Yellowstone River can be seen at the Forsyth, Miles City, and Glendive sites. Streamflow, in cubic feet per second Depletions, in thousands of acre feet

EXPLANATION

1-percent unregulated

1-percent regulated

Cumulative depletion from irrigation

Cumulative depletion from irrigation and reservoirs

20-percent unregulated

20-percent regulated

50-percent unregulated

50-percent regulated

River mile

0

50,000

100,000

150,000

200,000

250,000

300,000

600 500 400 300 200 100 0

-12,000

-10,500

-9,000

-7,500

-6,000

-4,500

-3,000

-1,500

0

1,500

3,000

4,500

6,000

Sidney

Glendive

Miles City

Billings

Forsyth

Downstream from estimated confluence

of Yellowstone and Bighorn Rivers

Livingston

Corwin Springs

Yellowstone Lake outlet
Comparison of the water that annually 

leaves because of irrigation with water 

that leaves because of the combination of irrigation and water stored in the 

reservoirs, indicates that storage in the 

Bighorn River Basin reservoirs makes up 

a large part of water that leaves the Yellowstone River Basin.

Using the Information

Scientists are using the unregulated 

and regulated streamflow statistics to 

understand how water-resource development has altered streamflows in the 

Yellowstone River Basin and how these 

changes have affected river flood-plain 

extents, low-flow hydraulics, fish habitat, 

and water quality. Water managers are in 

the process of developing protocols and 

practices that will increase the health of 

the river while balancing the multiple 

uses and needs of the river (Johnson, 

2014). These studies and data will help 

managers to understand the Yellowstone 

River system, allowing them to make 

informed decisions about the future of 

the river to balance the multiple uses and 

needs of the river. 

References

Chase, K.J., 2013, Streamflow statistics 

for unregulated and regulated conditions for selected locations on the Yellowstone, Tongue, and Powder Rivers, 

Montana, 1928–2002 (ver. 1.1, June 

2014): U.S. Geological Survey Scientific Investigations Report 2013–5173, 

183 p. [Also available at http://dx.doi.

org/10.3133/sir20135173.]

Chase, K.J., 2014, Streamflow statistics for unregulated and regulated 

conditions for selected locations on 

the Upper Yellowstone and Bighorn 

Rivers, 1928–2002: U.S. Geological 

Survey Scientific Investigations Report 

2014–5115, 117 p. [Also available at 

http://dx.doi.org/10.3133/sir20145115.]

Johnson, C., 2014, Yellowstone River 

study heading into home stretch: Billings, Montana, The Billings Gazette, 

5 p., accessed December 3, 2014, 

at http://yellowstonerivercouncil.

org/news/wordpress-content/

uploads/2014/10/Yellowstone-Riverstudy-heading-into-home-stretch.pdf.

Olson, S.A., and Norris, M.J., 2005, 

U.S. Geological Survey Streamgaging...from the National Streamflow 

Information Program: U.S. Geological 

Survey Fact Sheet 2005–3131, 4 p. 

[Also available at http://pubs.usgs.gov/

fs/2005/3131/.]

PBS&J, 2009, Irrigation in Montana–A 

Preliminary Inventory of Infrastructure Condition: Missoula, Mont., 

PBS&J Report, 91 p., accessed 

January 12, 2015, at http://dnrc.

mt.gov/cardd/ResourceDevelopment/

IrrigationDevelopment/docs/InventoryI

rrigationInfrastructureMontana.pdf.

U.S. Army Corps of Engineers, 2011a, 

Yellowstone River corridor study, 

Lower Bighorn River hydrology: 

Omaha, Nebr., U.S. Army Corps of 

Engineers Engineering Division, 

Hydrologic Engineering Branch, 

Hydrology Section, Final Report, 22 p.

U.S. Army Corps of Engineers, 2011b, 

Yellowstone River corridor study, 

Upper Yellowstone River hydrology: Omaha, Nebr., U.S. Army Corps 

of Engineers Engineering Division, 

Hydrologic Engineering Branch, 

Hydrology Section, Final Report, 63 p.

U.S. Bureau of Reclamation, 1999, 

A study to determine the historic 

and present-level streamflow depletions in the Missouri River Basin 

above Hermann, Missouri, Appendix F–C of Upper Mississippi River 

system flow frequency study: Billings, Mont., U.S. Department of the 

Interior, Bureau of Reclamation, Water 

Resources Service Group, Great Plains 

Regional Office, [variously paged].

U.S. Bureau of Reclamation, 2005, 

A study to determine the historic 

and present-level streamflow depletions in the Missouri River Basin 

for the period 1929 to 2002: Billings, Mont., U.S. Department of the 

Interior, Bureau of Reclamation, Water 

Resources Service Group, Great Plains 

Regional Office, [variously paged].

Yellowstone River Conservation District Council, 2014, About YRCDC: 

accessed June 20, 2014, at http://www.

yellowstonerivercouncil.org/about.php.

For more information, 

contact:

Katherine Chase 

U.S. Geological Survey 

3162 Bozeman Avenue 

Helena, MT 59601 

406-457-5957, kchase@usgs.gov

By Cheryl A. Eddy-Miller and Katherine J. Chase

Yellowstone River at Lake outlet, Wyoming.

ISSN 2327-6932 (online) 

http://dx.doi.org/10.3133/fs20153004