Estimated 100-Year Peak Flows and Flow 

Volumes in the Big Lost River and Birch 

Creek at the Idaho National Engineering 

Laboratory, Idaho

By L.C. Kjelstrom and Charles Berenbrock

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 96-4163

Prepared in Cooperation with the

U.S. Department of Energy

Boise, Idaho 

1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL $URVEY 

Gordon P. Eaton, Director

For additional information write to:

District Chief 

U.S. Geological Survey 

230 Collins Road 

Boise, ID 83702-4520

Copies of this report can be purchased from:

U.S. Geological Survey 

Information Services 

Box 25286 

Federal Center 

Denver, CO 80225
CONTENTS

Abstract................................................................................................................................................................................ 1

Introduction ..................................................................................................................................................................^ 1

Purpose and scope......................................................................................................................................................... 3

Description of study area.............................................................................................................................................. 3

Previous investigations ................................................................................................................................................. 5

Historical and recorded peak flows...................................................................................................................................... 7

Estimation of 100-year peak flows....................................................................................................................................... 8

Big Lost River............................................................._^ 9

Routing peak flows in the Big Lost River through Box Canyon.................................................................................. 12

Birch Creek......................................................................................................................_ 13

Estimation of 100-year flow volumes .................................................................................................................................. 16

Big Lost River............................................................................................................................................................... 17

Birch Creek ......................................................................................................................................................^^ 17

Summary..........................................................................................................................................^ 20

References cited .................................................................................................................................................................. 22

FIGURES

1. Map showing locations of study area, selected basins, and streamflow-gaging stations on the Big Lost River, 

Birch Creek, and nearby streams..................................................................................................................................... 2

2. Map showing locations of Big Lost River Basin, subbasins, and streamflow-gaging stations........................................ 4

3. Map showing locations of Birch Creek Basin, subbasins, and streamflow-gaging stations............................................ 6

4. Graph showing simulated 48-hour streamflow hydrographs for a 100-year peak flow at the Arco gaging station

routed through Box Canyon to the INEL boundary......................................................................................................... 13

5. Graph showing simulated 2-hour streamflow hydrographs for a 100-year peak flow at the Arco gaging station

routed through Box Canyon to the INEL boundary......................................................................................................... 14

TABLES

1. Estimates of 100-year peak flow from subbasins of the Big Lost River between the Howell Ranch (13120500) 

and Arco (13132500) gaging stations.............................................................................................................................. 10

2. Estimated peak flows at selected gaging stations and from contributing subbasins in the Big Lost River Basin

and estimated channel-infiltration losses from the Big Lost River for selected recurrence intervals.............................. 11

3. Estimates of 100-year peak flow from subbasins of Birch Creek between the Reno gaging station (13117000)

and Birch Creek Sinks ..................................................................................................................................................... 15

4. Estimated peak flows at gaging stations and other selected sites in the Birch Creek Basin

at selected recurrence intervals........................................................................................................................................ 16

5. Values for 60-day flow hydrographs and daily estimates of 100-year peak flow volumes at selected

gaging stations and from contributing subbasins in the Big Lost River Basin................................................................ 18

6. Values for 60-day flow hydrographs and daily estimates of 100-year peak flow volumes at the

Medicine Lodge Creek at Ellis Ranch, near Argora, gaging station and at selected gaging stations and sites

in the Birch Creek Basin.................................................................................................................................................. 19

Contents iii
CONVERSION TABLE

Multiply

acre

acre-foot (acre-ft)

cubic foot per second (ft3/s)

foot (ft)

foot per mile (ft/mi)

inch (in.)

mile (mi)

square mile (mi2)

By

4,047

1,233

0.02832

0.3048

0.1894

25.4

1.609

2.590

To obtain

square meter

cubic meter

cubic meter per second

meter

meter per kilometer

millimeter

kilometer

square kilometer

Sea Level: In this report "sea level" refers to the National Geodetic Vertical 0atum of 1929 a geodetic datum derived from a general 

adjustment of the first-order level nets of the United States and Canada, formejrly called Sea Level Datum of 1929.

iv 100-Year Peak Flows, INEL, Idaho
Estimated 100-Year Peak Flows and Flow 

Volumes in the Big Lost River and Birch Creek 

at the Idaho National Engineering Laboratory, 

Idaho

By L.C. Kjelstrom and Charles Berenbrock

Abstract

Peak flows and flow volumes with recurrence 

intervals of 100 years for the Big Lost River and 

Birch Creek were estimated so that the extent of 

the 100-year flood plain at the Idaho National 

Engineering Laboratory could be delineated. 

Flows entering the Idaho National Engineering 

Laboratory area were estimated from flood-frequency analysis of data at gaging stations, from 

regional regression equations, and from channelinfiltration losses. The one-dimensional flow 

model FOURPT was used to route peak flow 

through a deep, basalt gorge between the Arco 

gaging station and the Idaho National Engineering 

Laboratory boundary. Results indicated that no 

adjustments to attenuation of the peak were 

needed at the Idaho National Engineering 

Laboratory's western boundary.

Estimates of flow volumes entering the Idaho 

National Engineering Laboratory area were made 

by using representative hydrographs at selected 

gaging stations. Representative hydrographs were 

developed by fitting the 1-, 3-, 7-, 15-, 30-, and 60- 

day mean flows having a recurrence interval of 

100 years to a smooth curve. Estimated peak flow 

for a recurrence interval of 100 years entering the 

boundary of the Idaho National Engineering Laboratory from the Big Lost River was 7,260 cubic 

feet per second. The estimated volume of flow 

for a 60-day period for a recurrence interval of 

100 years was 390,000 acre-feet. For Birch Creek, 

the estimated peak flow for a recurrence interval of

100 years entering the Idaho National Engineering 

Laboratory area was 700 cubic feet per second, 

and the estimated 60-day volume of flow with a 

recurrence interval of 100 years was about 

10,600 acre-feet.

In the next phase of this flood-plain delineation 

study, the 100-year peak flow will be routed downstream to spreading areas and playas in the Idaho 

National Engineering Laboratory area using a 

computer model to delineate the extent of the 

100-year flood plain.

INTRODUCTION

The Big Lost River and Birch Creek flow onto the 

eastern Snake River Plain (fig. 1) and terminate in playas, sinks, and spreading areas at the Idaho National 

Engineering Laboratory (INEL). Floods occur when 

streambanks can no longer contain streamflow. Although these streams rarely flood, peak flows and volumes from these rare periods of high streamflow need 

to be determined so the extent of the flood plain can be 

delineated to evaluate the potential severity of floods at 

INEL facilities.

The Big Lost River is the most likely source of 

flooding at the INEL. Natural water yield from the Big 

Lost River Basin is about seven times greater than that 

of the Birch Creek Basin (Kjelstrom, 1986, sheet 2). 

Mackay Reservoir, 45 mi upstream from the INEL, 

stores water from the Big Lost River for irrigation. 

Most of the water stored in the reservoir and most of 

the tributary inflow between the reservoir and Arco are 

diverted for irrigation or lost by infiltration through the

Abstract 1
m 

a0)

114° 30' 114° 00' 113° 30' 113° 00' 112" 30' 112° 00'

44° 30' -

44° 00' -

EXPLANATION

A Streamflow-gaging 

13132500 station and number

i I Idaho National 43° 30' 

Engineering Laboratory

 . Basin boundary

'- i I FREMONT

Base from U.S. Geological Survey 

1:100,000 quadrangles

0 5 10 15 20 MILES 0 5 10 15 20 KILOMETERS

Figure 1. Locations of study area, selected basins, and streamflow-gaging stations on the Big Lost River, 

Birch Creek, and nearby streams.
channel bottom to the alluvial valley fill. Thus, the 

streamflow-gaging station, Big Lost River near Arco 

(13132500), records little or no streamflow throughout 

most of the year. Streamflow that does reach this gaging station is depleted further by infiltration through the 

channel bottom downstream from Arco.

The Little Lost River flows onto the eastern Snake 

River Plain near Ho we (fig. 1) from a drainage basin 

between the Big Lost River and Birch Creek Basins. 

Topographic maps indicate that flood water from the 

Little Lost River would flow eastward toward the Big 

Lost River play as. However, flow past the sinks at the 

terminus of the Little Lost River is not likely because 

of large irrigation diversions and infiltration losses in 

the valley upstream from the sinks. No INEL facilities 

exist in the Little Lost River drainage basin, and the 

terminus of the river is outside the INEL boundary. 

Thus, for the purposes of this study, the Little Lost 

River was considered to have a negligible potential for 

flooding facilities at the INEL.

Birch Creek flows southward through a long, alluvium-filled valley to the eastern Snake River Plain. 

Birch Creek has several small diversions for irrigation 

and one diversion to a power-generation facility. As in 

the Big Lost River channel, infiltration losses to alluvial valley fill are large. As a result of diversions and infiltration losses, Birch Creek rarely flows into the Birch 

Creek Playa at the north end of the INEL until flood 

conditions prevail.

In 1994, the U.S. Department of Energy at the 

INEL entered into a cooperative agreement with the 

U.S. Geological Survey to delineate the extent of possible flooding at the INEL from peak flows with recurrence intervals of 100 years from the Big Lost River 

and Birch Creek.

Four steps are planned for the flood-plain delineation study: (1) Determine 100-year recurrence interval 

peak flows and associated flow volumes. (2) Survey 

flood-plain cross sections to provide data needed for 

one- and two-dimensional models that will characterize 

the flow capacity of the stream channels and floodplain areas. (3) Use the one-dimensional model WaterSurface PROfile (WSPRO) to calculate the water-surface profiles and delineate the 100-year flood plain of 

Birch Creek, and to determine the areal extent of the 

grid for the two-dimensional model. (4) Use a twodimensional surface-water model to calculate watersurface profiles and delineate the 100-year flood plain 

of the Big Lost River.

Purpose and Scope

The purpose of this report is to provide estimates 

of the 100-year peak flows and flow volumes that could 

enter the INEL area from the Big Lost River and Birch 

Creek. The 100-year peak flows for the Big Lost River 

and Birch Creek are needed as input data for models 

that will be used to delineate the extent of the 100-year 

flood plain at the INEL. The methods, procedures, and 

assumptions used to estimate the 100-year peak flows 

and flow volumes are described in this report.

Description of Study Area

The Big Lost River and Birch Creek Basins are located on the north side of the eastern Snake River Plain 

(fig. 1), about 50 mi west of Idaho Falls. These basins 

trend northwest-southeast and are bounded by mountains except at their southeastern ends, where they 

merge with the eastern Snake River Plain. Big Lost 

River terminates in four playas (the last playa is the 

Birch Creek Playa) with connecting channels that are 

in the INEL area (fig. 1). Birch Creek terminates at the 

Birch Creek Playa near Test Area North (TAN) and 

Loss of Fluid Test (LOFT) facilities (fig. 1). Basaltic 

lava flows underlie much of the Snake River Plain and 

prevent streamflow from these basins from reaching 

the Snake River. Consequently, all streamflow that 

flows onto the eastern Snake River Plain from the Big 

Lost River and Birch Creek either evaporates or infiltrates into the ground.

Much of the 1,410-mi2 drainage area in the Big 

Lost River Basin upstream from the gaging station near 

Arco (13132500, fig. 2) is mountainous. The elongate 

valley is about 2 to 10 mi wide, 50 mi long, and partly 

filled with alluvium. The valley floor is relatively flat. 

The valley width between Mackay Reservoir (fig. 2) 

and Arco ranges from 0.3 mi just below the dam to 

6 mi near Arco. Land surface altitude ranges from 

about 5,300 ft above sea level near Arco to more than 

12,600 ft in the Lost River Range (fig. 1). Although the 

mean altitude of the basin is 7,700 ft and the mean precipitation is about 20 in/yr, the mean altitude of the valley is about 6,000 ft and mean precipitation is about 

10 in/yr. Precipitation is fairly evenly distributed 

throughout the year. Flow in the Big Lost River near 

Arco is ephemeral because of climate, irrigation diversions, and channel infiltration.

Description of Study Area 3
R. 21 E. /\

44° 15' f'l

22 E

Mu"r^~~X 24 £w \"*~L/ oqV 'XfX B S ^- -.^40 / OO .Owi H '

Base from U.S. Geological Survey 

1:100,000 quadrangles

EXPLANATION

A Streamflow-gaging station 13132500 and number

Boundary of subbasin and 

33 number (modified from 

Crosthwaite and others, 

1970)

Boundary of drainage 

basin

i i i i i i i 

02468 KILOMETERS

113° 30' 113° 15'

Figure 2. Locations of Big Lost River Basin, subbasins, and streamflow-gaging stations.
The communities of Mackay, Leslie, Moore, Arco, 

and Butte City (fig. 2) had a combined population of 

about 2,800 in 1992. The town of Arco, with a population of about 1,030, covers 1 mi2 in the southeastern 

part of the basin.

Mackay Reservoir, about 30 mi northwest of Arco, 

is about halfway between the headwaters of the Big 

Lost River and Arco. Most surface-water inflow to 

Mackay Reservoir is the result of melting snowpacks. 

Active storage of Mackay Reservoir has decreased 

from 43,500 acre-ft in 1956 to about 38,500 acre-ft 

in 1980 (Williams and Krupin, 1984; U.S. Army 

Corps of Engineers, 1991) because of sedimentation. 

Water is stored in Mackay Reservoir to irrigate about 

33,000 acres during the irrigation season (April 

through October).

Several miles downstream from the Arco gaging 

station, the Big Lost River enters Box Canyon, a deep, 

narrow gorge with nearly vertical walls cut into basaltic rocks. The canyon is about 7.5 mi long and averages 

about 125 ft wide and 75 ft deep. After exiting Box 

Canyon, the channel cuts through alluvial fill covering 

basalt and crosses the western boundary of the INEL.

In 1958, a flood-control diversion system was constructed at the INEL to reduce the threat of floods from 

the Big Lost River. Water from the diversion channel 

can be stored in a connected series of spreading areas. 

About 18 mi downstream from the diversion dam, the 

Big Lost River channel strikes north to the Big Lost 

River Sinks and terminates in a series of playas that are 

connected by branching channels.

The Birch Creek Basin lies between the Lemhi 

Range on the west and the Bitterroot Range on the east. 

The Birch Creek Valley is about 2 to 10 mi wide and 

about 40 mi long (fig. 3). A low ridge separates the 

Lemhi River Valley on the north and the Birch Creek 

Valley on the south. Because prevailing winds are from 

the west, precipitation in the Birch Creek Basin is less 

than in the Big Lost River Basin. Mean annual precipitation is less than 10 in. in Birch Creek Valley, 17 in. 

in the uplands and mountains of the Lemhi Range, and 

15 in. in the uplands and mountains of the Bitterroot 

Range. Birch Creek is a perennial stream that is fed by 

springs upstream from the Birch Creek near Reno gaging station (13117000). Communities in the Birch 

Creek Basin include Reno, Kaufman, and Blue Dome. 

Their combined population in 1992 was about 300.

About 15 mi downstream from the Reno gaging 

station, diversion works divert flow from Birch Creek 

to a channel that leads to a hydroelectric power-generating facility several miles east of Birch Creek. Flow to 

this power facility during the nonirrigation season is returned to Birch Creek several miles south of Highway 

22 at a gravel pit.

About 10 mi southeast from where Birch Creek 

crosses the INEL boundary, the channel transforms into 

a large, shallow playa known as the Birch Creek Playa. 

The LOFT and TAN facilities are located near the playa's perimeter. The area between the gravel pit that is 

about 1 mi north of LOFT and the Birch Creek Playa is 

known as "Birch Creek Sinks" (fig. 3). No flow from 

Birch Creek has reached the Birch Creek Sinks since 

1969, when several feet of water accumulated in the 

playa as a result of snowmelt from the lower Birch 

Creek Valley (Koslow, 1984). In response, the INEL 

built channels in 1969 to divert Birch Creek to several 

gravel pits and from there to sinks northeast of the playa. Dikes also were built to protect TAN and LOFT.

Previous Investigations

The earliest study of surface-water resources in the 

Big Lost River Basin was by Wright (1903). He briefly 

discussed gains and losses in the Big Lost River, including the influence of irrigation. The first comprehensive hydrologic study was by Stearns and others 

(1938). They described water resources of the Big Lost 

River Basin and estimated surface- and ground-water 

outflow from the area as part of a study of the eastern 

Snake River Plain. Mundorff and others (1964) discussed ground- and surface-water relations in the Big 

Lost River Basin and the basin's importance as a 

source of recharge to the eastern Snake River Plain. 

Barraclough and others (1967) studied mainly the 

ground-water resources of the INEL area but also discussed surface- and ground-water relations of the Big 

Lost River in the INEL area. Lamke (1969) developed 

stage-discharge relations for the Big Lost River at the 

diversion channel and the channels between each 

spreading area and playa. Crosthwaite and others 

(1970) described and provided new information on the 

geohydrology of the Big Lost River Basin. They also 

determined surface-water outflow from 44 subbasins to 

develop ground-water budgets for the valley. Williams 

and Krupin (1984) investigated erosion and sediment 

transport in the Big Lost River upstream from Mackay 

Reservoir. Bennett (1986) updated the stage-discharge 

relation at the diversion channel because the diversion

Previous Investigations 5
113° 15 WV- " "

X"~\ 113° oo' 

 "=^"'- '""-- V- K ' '

EXPLANATION

^ Streamflow-gaging station 

13117020 and number

 .. Subbasin boundary and 

6 number

. . . Idaho National Engineering 

Laboratory boundary

Base from U.S. Geological Survey, Big Lost 

River Sinks, 1969; Blue Dome, 1969; Circular 

Butte, 1969; Circular Butte NW, 1969; Copper 

Mountain, 1969; Eightmile Canyon, 1969; 

Richard Butte, 1969; Scott Butte, 1969; 1:24,000 

Diamond Peak, 1957; Gilmore, 1957; Nicolia, 

1956; Scott Peak, 1957; 1:62,500. 

Universal Transverse Mercator (UTM) projection, 

Zone12

10 MILES

44° 00'

10 KILOMETERS

43°45'-

43° 45'

113° 00

Figure 3. Locations of Birch Creek Basin, subbasins, and streamflow-gaging stations.

6 100-Year Peak Flows, INEL, Idaho
channel was enlarged, and the dam and dikes were 

raised in 1984 to provide additional flood control. Bennett (1990) described channel-infiltration losses along 

the Big Lost River at the INEL. Koslow (1984) analyzed flooding from the Birch Creek Basin near LOFT 

and TAN. She determined that the 500-year flood 

would not cause water to pond in Birch Creek Playa 

near LOFT and TAN, but for the probable maximum 

flood, water would be within 2 ft of the escape tunnel at 

LOFT in the Birch Creek Playa.

The U.S. Army Corps of Engineers (1967) described the conditions and extent of the June-July 1967 

flood in the Big Lost River. The analysis did not extend 

downstream from Arco. The Corps (1991) also investigated the feasibility of reducing flood damages in the 

Big Lost River Valley. Carrigan (1972), Druffel and 

others (1979), Noble (1980), and Koslow and Van 

Haaften (1986) examined the hypothetical failure of 

Mackay Dam and the behavior of flood waves downstream by using flood-routing techniques. For the principal case of dam failure, the attenuated peak at the 

INEL boundary (about 45 mi downstream from the 

dam) was estimated to be about 45,000 ftVs by Koslow 

and Van Haaften (1986) and 54,000 ftVs by Druffel and 

others (1979). Rathburn (1989 and 1991) estimated 

that a paleoflood discharge of Big Lost River in Box 

Canyon and overflow areas ranged from 2 to 4 million ft3/s. She estimated that the depth of water at the 

Radioactive Waste Management Complex (RWMC) 

resulting from this large paleoflood was 50 to 60 ft.

Several regional analyses of flood magnitude and 

frequency in Idaho were used in this study (U.S. Army 

Corps of Engineers, 1967; Koslow, 1984; Koslow and 

Van Haaften, 1986; and Stone and others, 1993). Kjelstrom and Moffatt (1981) described a regional method 

of estimating flood magnitudes and frequencies by regression equations that used basin characteristics as independent variables. Quillian and Harenberg (1980) 

evaluated the stream-gaging network in Idaho and also 

developed regression equations for peak flows and average discharge. Harenberg (1980) presented regression equations that used channel geometry to estimate 

peak flows. In a study that included Arizona, Nevada, 

Utah, and parts of California, Colorado, Idaho, New 

Mexico, Oregon, Texas, and Wyoming, Blakemore and 

others (1994) developed regression equations that used 

drainage area and annual mean precipitation to estimate the magnitude and frequency of floods at ungaged 

sites.

HISTORICAL AND RECORDED PEAK 

FLOWS

Peak flows in the Big Lost River (tabulated in a 

report by Stone and others, 1993) occur mostly during 

May, June, and July because of melting snowpacks, often associated with rainstorms, in the mountains. Notable peak flows in the Big Lost River have been determined at several gaging stations. For example, the 

peak flow on June 12, 1965, in the Big Lost River 

at Howell Ranch, near Chilly, gaging station (13120500) 

was 3,450 ftVs. On June 29, 1965, a peak flow of about 

2,500 ftVs was estimated by indirect methods at the 

Arco gaging station, and a peak flow of about 2,220 ftVs 

was estimated at the INEL diversion dam (Barraclough 

and others, 1967). The different peak flow dates for the 

1965 flood at the two gaging stations are probably due 

to the effects of reservoir storage, diversions for irrigation, channel-infiltration losses, and differences in the 

timing and location of rainstorms during the June 12- 

29 period.

The highest recorded peak flow in the Big Lost 

River was 4,420 ftVs at the Howell Ranch gaging station on May 25, 1967. By June 22, when another peak 

occurred (3,850 ftVs), Mackay Reservoir was nearly 

full and only a small attenuation of the second peak 

was recorded as it passed through the reservoir. Mean 

daily discharge was about 1,600 ftVs at the Howell 

Ranch gaging station from May through July, and the 

volume of flow was about 293,000 acre-ft. As a result 

of the long duration and high magnitude of streamflow 

upstream, the river overflowed its banks downstream 

from Mackay Reservoir and inundated about 7,000 

acres; ground-water levels in the valley rose high 

enough to flood an additional 7,000 acres (U.S. Army 

Corps of Engineers, 1967). Flooding was caused by 

rapid snowmelt from above-normal air temperatures, 

several intense rainstorms interspersed throughout the 

period, and little reservoir storage capacity following 

an earlier peak on May 25. The recorded peak flow at 

the Arco gaging station was 1,890 ftVs on July 5, 1967.

Peak flows in Birch Creek (tabulated in a report by 

Stone and others, 1993) occur mostly during April, 

May, and June because of melting snowpacks in the 

mountains. Annual peaks were recorded at the Reno 

gaging station (13117000) in 1911-12, 1921-22, 

1952-60, and 1962-63. This gaging station was discontinued in 1963. Two other gaging stations at Blue 

Dome Inn, near Reno (13117020), and at Eightmile

Historical and Recorded Peak Flows 7
Canyon Road, near Reno (13117030), were operated 

for part of the year (usually April through October) 

from 1967 to 1981 and 1985 to 1991, and from 1967 

to 1981 and 1984 to 1988, respectively. The highest recorded peak flow in Birch Creek was 220 ftVs at the 

! Reno gaging station on April 1,1962; larger peak flows 

may have occurred in other years when the station was 

not operated. The highest recorded peak flows at the 

Blue Dome Inn and Eightmile Canyon Road gaging 

stations were 149 ftVs on July 30, 1969, and 122 ftVs 

on April 18, 1984, respectively.

ESTIMATION OF 100-YEAR PEAK FLOWS

The 100-year peak flows for the Big Lost River 

and Birch Creek were estimated using flood-frequency 

curves and regression equations. Flood-frequency 

curves at gaging stations with at least 10 years of 

record generally provide reliable estimates of peak 

flow for recurrence intervals up to 100 years. The estimates are less reliable where the natural peak flows 

have been significantly altered because of storage and 

diversion structures.

Flood-frequency curves for all gaging stations 

were developed by fitting the array of recorded annual 

peak flows to a theoretical probability distribution. As 

recommended by the Interagency Advisory Committee 

on Water Data (1982), the three-parameter log-Pearson 

Type III distribution was used for this analysis. The 

three parameters that define the probability distribution 

and resultant flood-frequency curve at each station are 

the mean, standard deviation, and skew coefficient of 

the logs of annual peak flow. Because gaging-station 

values of skew coefficients are unreliable for shortterm records, the Interagency Advisory Committee on 

Water Data (1982) recommended that station skew coefficients be weighted with a generalized skew coefficient that is applicable to all sites in a region. A generalized skew coefficient of -0.3, previously determined 

for the mountainous areas of the Big Lost River and 

Birch Creek Basins (Kjelstrom and Moffatt, 1981), was 

used for this study.

Flood-frequency information at gaging stations can 

be used to estimate flood frequency at ungaged sites by 

the development of regional regression equations based 

on climatic and basin characteristics. Regional floodfrequency equations are developed using ordinary and 

generalized least-squares multiple-regression analyses

8 100-Year Peak Flows, INEL, Idaho

to relate peak flows for selected recurrence intervals 

(dependent variable) to selected climatic and basin 

characteristics (independent variables). Regional regression equations have been developed by several investigators to provide estimates of peak flows at ungaged sites. A channel geometry method (Harenberg, 

1980) uses a regression equation that incorporates 

bankfull width as the independent variable to make estimates. The channel geometry method is applicable to 

alluvial stream channels and requires a visit to the 

stream. Estimates from channel geometry methods are 

based on the concept that the size and shape of an alluvial channel are a measure of the volume of water discharge and sediment transport. Quillian and Harenberg 

(1982) developed regression equations on the basis of 

only drainage area to estimate the 100-year peak flow; 

Blakemore and others (1994) developed equations on 

the basis of both drainage area and mean annual precipitation as independent variables. Crosthwaite and others (1970) showed that mean altitude was related to annual mean flow from subbasins of the Big Lost River 

Basin. Kjelstrom and Moffatt (1981) noted that, in 

Idaho basins where 80 percent of the area was higher 

than 6,000 ft, peak flows resulted primarily from snowmelt.

The regional regression equations and procedures 

given by Kjelstrom and Moffatt (1981) were used in 

this report because the equations were based on different geographic regions and drainage area sizes and 

generalized skew coefficient maps developed for 

Idaho. The regression equations used three independent variables: drainage area, mean annual precipitation, and mean altitude.

Peak flow is poorly represented by regional regression equations for streams such as the Big Lost River 

and Birch Creek that flow over highly permeable rocks, 

in braided or small channels, or in large, hydraulically 

rough flood plains. Large amounts of streamflow can 

be lost by infiltration through permeable materials, and 

the magnitude of peak flow can be significantly reduced. Therefore, regional regression equations were 

used to calculate peak flow only for mountainous areas 

in the Big Lost River and Birch Creek Basins, and regression results were reduced to account for channelinfiltration losses.
Big Lost River

Two methods were used to calculate the 100-year 

peak flow at the Big Lost River near Arco gaging station (13132500): (1) flood-frequency analysis for the 

Arco gaging station; and (2) the addition of results 

from flood-frequency analysis for the Big Lost River 

at Howell Ranch, near Chilly (13120500), and the 

Lower Cedar Creek above diversions, near Mackay 

(13128900), gaging stations to estimates from regression equations for ungaged tributaries.

Flood-frequency analysis resulted in a computed 

100-year peak flow of 5,480 ftVs at the Arco gaging 

station. A peak flow of 2,500 ftVs in 1965, which was 

estimated using indirect methods when the Arco gaging station was not operated, was included with the recorded annual peak flows at the station. The upper and 

lower 95-percent confidence limits for the estimated 

100-year peak flow were 11,600 and 3,150 ftVs, respectively. This large difference between upper and lower 

limits was attributed to the damping of most peaks as 

a result of storage in Mackay Reservoir and low-lying 

areas in the valley and diversion of streamflow that 

does not return to the river. Previous estimates of the 

100-year peak flow were 4,480 ftVs (U.S. Army Corps 

of Engineers, 1991) and 3,700 ftVs (Stone and others, 

1993). Differences in the estimates are the result of using data from different periods of record, omission of 

the 1965 peak flow in the estimates by the U.S. Army 

Corps of Engineers (1991) and Stone and others 

(1993), and use of different skew coefficients.

Results from the flood-frequency analysis for the 

Arco gaging station were considered to show an unacceptably high level of uncertainty because of the wide 

range in confidence limits. Therefore, a second estimation method was employed, whereby results from the 

flood-frequency analysis for the Howell Ranch and 

Lower Cedar Creek gaging stations in subbasin 30 

(fig. 2) were added to results from regional regression 

equations for 22 other subbasins in the basin and adjusted for channel-infiltration losses.

The Howell Ranch gaging station recorded 85 annual peaks through 1993. These peaks were not 

significantly affected by storage or diversions. Floodfrequency analysis resulted in a 100-year peak flow of 

4,880 ftVs at the Howell Ranch gaging station and 

compared favorably with the highest recorded peak 

flow of 4,420 ftVs on May 25, 1967. The upper and 

lower 95-percent confidence limits were 5,660 and

4,320 ftVs, respectively, and indicated that the error of 

the estimated peak was considerably smaller than that 

of the estimated peak at the Arco gaging station. Previous estimates of the 100-year peak flow at the Howell 

Ranch gaging station were 4,800 ftVs (U.S. Army 

Corps of Engineers, 1991) and 4,690 ftVs (Stone and 

others, 1993). These estimates are similar and differences are due to different periods of record used. 

Flood-frequency analysis for the Lower Cedar Creek 

gaging station (13128900), which had 15 years of 

streamflow data, resulted in an estimated 100-year 

peak flow of 338 ftVs for subbasin 30. The highest recorded peak flow of 310 ftVs for this station occurred 

on June 22, 1982.

Regional regression equations were used to estimate 100-year peak flows for the other 22 subbasins 

that drain mountainous and foothill areas downstream 

from the Howell Ranch gaging station (fig. 2). Subbasins on the valley floor were not considered to contribute peak flows to the Big Lost River and were not included in the regression analysis. Regional regression 

equations and procedures presented by Kjelstrom and 

Moffatt (1981) were used to estimate 100-year peak 

flows (table 1) for 22 subbasins shown in figure 2.

No adjustments to flood frequency or regression 

results were made for storage and diversion effects. 

Mackay Dam was assumed to be full, and fields were 

assumed to be completely saturated. These conditions 

existed during the 1967 floods (U.S. Army Corps of 

Engineers, 1991).

Large channel-infiltration losses were measured 

between the Howell Ranch gaging station and Mackay 

Reservoir (Crosthwaite and others, 1970, p. 46-47). 

Most of the losses occurred in a 5-mi reach known 

as Chilly Sinks (fig. 2), which is completely dry for 

about 8 months each year (Williams and Krupin, 1984, 

p. 36). Average measured losses in the Chilly Sinks 

reach were 120 ftVs and were greater than 1,000 ftVs 

after extended periods of dryness (Williams and 

Krupin, 1984, p. 36). A channel-infiltration rate for the 

Chilly Sinks reach of 3.4 in/hr was calculated to estimate losses for the 100-year peak flow by dividing representative reach loss by the product of reach length 

and width. First, average losses and losses after extended dry periods for the reach were averaged to produce a 

representative reach loss that could be expected 

through the duration of peak flow conditions soon after 

the dry period ended. Next, the peak flow of 4,420 ftVs 

on May 25, 1967, was used to calculate channel width 

with the following equation for stabilized flow in an alBig Lost River 9
luvial channel as described by Dawdy (1979): 

W=9.5QOA, where W is channel width (in feet) and Q is 

flow in the channel (in cubic feet per second). The recurrence interval of that peak flow was 95 years. An estimated loss of 580 ftVs for a flow of 4,880 ftVs at the 

Howell Ranch gaging station (table 2) was calculated 

from the product of reach length, width, and channelinfiltration rate.

Infiltration rates for subbasin streams were estimated by adjusting the rate for the Chilly Sinks reach 

(3.4 in/hr) according to the rock types underlying each 

stream (Crosthwaite and others, 1970, fig. 10). Miscellaneous discharge measurements, no-flow observations, and gaging-station records, primarily from 1967, 

when the 95-year peak flow occurred at the Howell

Ranch gaging station, were used as a guide in making 

the adjustments. For example, channel-infiltration rates 

were decreased as low as 1.6 in/hr where the stream 

flowed over consolidated alluvium and were increased 

as high as 4.2 in/hr where the stream flowed over jointed carbonaceous rocks. For several subbasins, mainly 

on the eastern side of the valley, poorly developed 

channels in many of the steep canyons composed of 

carbonaceous rocks indicated that surface flows were 

rare. Adjustments to the channel-infiltration rate for 

these subbasins were made so that average flow from 

each subbasin would not reach the Big Lost River, but 

a small part of the 10-year peak flows would. For more 

develpped channels composed of consolidated alluvium, or where flows had been measured, the infiltration

Table 1. Estimates of 100-year peak flow from subbasins of the Big Lost River between the Howell Ranch (13120500) and Arco 

(13132500) gaging stations

[mi2, square miles; in., inches; ft, feet; in/hr, inches per hour; ft3/s, cubic feet per second; , not applicable; locations of subbasins and gaging stations shown 

on figure 2; regression estimates obtained from equations in a report by Kjelstrom and Mpffatt (1981)]

Drainage 

Subbasin area2 

No.1 (mi2)

9

12

19

20

21

22

23

24

25

26

27

30

31

32

33

36

37

38

39

40

41

42

43

50

18

2.6

8.8

4.4

14

9.9

9.6

12

14

8.8

25

59

16

3.6

28

45

93

19

33

73

22

Mean 

annual 

precipitation2 

(in.)

14.1

13.8

30.7

21.8

11.9

11.7

13.3

17.6

11.9

20.4

14.3

23.2

22.6

22.9

25.4

21.9

16.0

27.5

30.4

20.8

17.7

17.0

Mean Infiltration 

subbasin rate at 

altitude2 peak 

(ft) (in/hr)

7,380

7,220

9,860

8,430

9,230

9,520

9,060

7,950 !

7,000

8,250

7,010

8,130

7,850

8,410

8,840

8,230

7,000

7,960

7,580

7,850

6,860

6,900

Total of peak flows from subbasins:

2.9

2.9

3.2

3.2

3.2

3.2

3.2

2.7

3.1

3.2

3.7

3.7

4.2

4.1

2.8

2.9

3.7

1.6

1.6

1.6

2.5

3.7

100-year peak flows

Regression After adjustments 

estimate for channel-infiltration 

(ft3/s) losses (ft3/s)

704

347

101

229

102

223

192

232

251

313

218

3338

501

925

356

127

519

716

1,370

488

595

1,060

451

250

138

30

75

35

76

65

91

101

114

70

186

187

253

106

34

188

194

541

153

196

349

76

3,510

Subbasin numbers from Crosthwaite and others (1970). 

2Basin characteristic values from Crosthwaite and others (1970). 

3 Value determined from flood-frequency analysis of data from the Lower Cedar Creek above diversions, near Mackay, gaging station (13128900).

10 100-Year Peak Flows, INEL, Idaho
Table 2. Estimated peak flows at selected gaging stations and from contributing subbasins in the Big Lost River Basin and 

estimated channel-infiltration losses from the Big Lost River for selected recurrence intervals

[Locations of gaging stations, subbasins, and reaches shown on figure 2]

Gaging stations, subbasins,

Peak flows at selected recurrence intervals 

and concurrent channel-infiltration losses 

(cubic feet per second)

or location of losing reach

Big Lost River at Howell Ranch, near Chilly (13120500) ............

Contributing subbasins ..................................................................

Losses at Chilly Sinks....................................................................

Losses from Mackay Reservoir to Arco gaging station..... ............

Big Lost River near Arco (13132500) ...........................................

5-year

2,960

350

480

360

2,470

10-year

3,470

950

510

420

3,490

100-year

4,880

'3,510

580

550

7,260

500-year

5,710

5,200

620

610

9,680

'From table 1.

rate was adjusted so that a small part of the 5-year peak 

flow reached the river. Final channel-infiltration rates 

for the subbasin streams are shown in column 5 on 

table 1.

Channel-infiltration losses for each subbasin were 

calculated by multiplying infiltration rate, stream distance, and channel width. The approximate stream distance from the mouth of the subbasin to the Big Lost 

River was estimated from 7.5-minute topographic 

maps, and channel width was calculated with the 

Dawdy equation. Although the assumption of stabilized flow does not apply to the tributaries and to the 

Big Lost River, the increase in channel depth and width 

between the 10- and 100-year peak flows at the Lower 

Cedar Creek gaging station is 0.35 and 5 ft, respectively; Dawdy's equation increases channel width by 10 ft. 

After subtraction of channel-infiltration losses, the 

resulting subbasin values for the 100-year peak flows 

that reach the Big Lost River are shown in column 7 

on table 1. Total peak flow from all subbasins was 

3,510 ftVs (table 1).

Downstream from Mackay Reservoir, the Big Lost 

River is generally a losing stream, but along some 

reaches, the river gains water. Infiltration rates for the 

Big Lost River and diversion channels were based on 

measurements during 1966-68 (Crosthwaite and others, 1970) and 1985 (S.A. Goodell, U.S. Geological 

Survey, written commun., 1985). Measurements of 

flow in all channels every 3 mi downstream in east-towest lines straight across the valley indicated an average infiltration rate of about 0.7 in/hr. These measurements were made during the nonirrigation season when 

water was being released from the reservoir; the infiltration rate may be greater or less during a flood in the 

valley. The lower infiltration rate of 0.7 in/hr in this 

reach, compared with the infiltration rate of 3.4 in/hr in 

the Chilly Sinks area, is attributed to finer grained deposits on the valley floor relative to the coarser deposits 

in the mountainous subbasins. Using Dawdy's equation 

(W=9.5(2°-4), average channel width between Mackay 

Reservoir and the Arco gaging station was calculated 

to be about 350 ft for the 100-year peak flow. 

In mapping the flood plain of the Big Lost River, the 

U.S. Army Corps of Engineer's (1991) delineation of 

the 100-year flood showed that the width ranged from 

about 200 ft to more than 1,000 ft; therefore, an average width of 350 ft probably is representative for the 

Big Lost River on the valley floor. Estimated channelinfiltration losses for the reach from Mackay Reservoir 

to the Arco gaging station were 550 ftVs for the 100- 

year peak flow (table 2). During high flow at the Arco 

gaging station in late June and early July of 1967, the 

average difference in the volume of streamflow released from the reservoir and measured at the Arco 

gaging station was about 700 ftVs. However, irrigation 

diversions may account for some of the difference so 

that the 550-ft3/s loss for the 100-year peak flow seems 

reasonable and probably is somewhat conservative because of additional tributary flow. Thus, the calculated 

100-year peak flow in the Big Lost River near the Arco 

gaging station was 7,260 ftVs (table 2). The method 

used to estimate the 100-year peak flow at the Arco 

gaging station also was used to estimate the 5-year, 10- 

year, and 500-year peak flows (table 2).

The assumptions used to estimate the peak flows 

shown in table 2 were: (1) Mackay Reservoir was cornBig Lost River 11
pletely full, and the flows were not affected by reservoir storage and regulation; (2) water was not diverted 

for irrigation; (3) flow in the Big Lost River occurred 

only in a single, unbraided channel; (4) peak flows 

from the Howell Ranch gaging station and subbasins 

arrived simultaneously at the Arco gaging station; 

(5) estimated infiltration losses were representative of 

losses along the channel during flooding; and (6) the 

peak flows were not attenuated. The assumption that 

Mackay Reservoir was full or nearly full was realized 

during 1965 and 1967 floods when the reservoir was 

nearly full. The assumption that peak flow arrived simultaneously at the Arco gaging station from the Howell Ranch gaging station and subbasins is also probably 

reasonable. In 1967, peak flows from the Lower Cedar 

Creek (13128900), Alder Creek (13129800), Antelope 

Creek (13130900), and Howell Ranch (13120500) gaging stations occurred within a 31.5-hour period. The 

computer model CONROUT (Doyle and others, 1983) 

was used to simulate simultaneous hydrographs of 

peak flow for the four gaging stations. Model results 

showed that the simulated 100-year peak near Arco 

was about 200 ftVs less than the value shown in table 2. 

The longest travel time of the peak, from the Howell 

Ranch gaging station to the Arco gaging station, was 

about 6 hours. Results from this simulation and the 

1967 flood illustrate that peak flows are not significantly attenuated, travel times are relatively fast, and subbasin peaks occur within a relatively short period of 

time; thus, the assumption that subbasin peaks occurred simultaneously is probably reasonable.

1 To minimize effects of reservoir regulation inherent in flood-frequency analysis of the Arco gagingstation record, estimates of peak flow made from floodfrequency analyses for the two gaging stations upstream 

from Mackay Reservoir were added to estimates made 

from regional regression equations for intervening tributaries between the reservoir and the Arco gaging station. Although this method may have substantial error 

and produced an estimate of the 100-year peak flow 

that was 32 percent greater than the estimate made 

from flood-frequency analysis, the variable and generally indeterminate effects of reservoir regulation in the 

flow record for the Arco gaging station were eliminated.

12 100-Year Peak Flows, INEL, Idaho

Routing Peak Flows in the Big Lost River 

Through Box Canyon

A potential exists for the peak flow to be attenuated 

during travel through Box Canyon from the Big Lost 

River near Arco gaging station to the INEL boundary. 

Attenuation decreases the magnitude and lengthens the 

duration of a peak flow. The amount of attenuation that 

may occur through Box Canyon could have significant 

effects on the flood plain at the INEL. For example, if 

an earthen structure were designed for very attenuated 

flows but no attenuation occurred, earthen structures 

could fail or be eroded because of greater velocities 

and higher stages caused by larger peaks.

The computer model FOURPT (DeLong, 1993) 

was used to simulate streamflow conditions from the 

Arco gaging station to the INEL boundary. FOURPT is 

based on one-dimensional, partial-differential equations o( continuity and momentum, which govern unsteady open-channel flow. For computational purposes, 

these equations are replaced by finite-difference equations, which approximate the actual solution and are 

solved by using a four-point-implicit method.

To demonstrate the effect of attenuation on peak 

flow in Box Canyon, the 100-year peak flow at the 

Arco gaging station was simulated. The channel length 

is about 7.5 mi, from the Arco gaging station to the 

INEL boundary. Channel geometry is regular and the 

cross-sectional area is rectangular. Rathburn (1989, 

p. 4) estimated average channel width and canyon 

depth to be about 125 ft and 75 ft, respectively. Channel slope was estimated from 7.5-minute topographic 

maps 19 be about 17 ft/mi.

Streambed materials in Box Canyon consist of 

coarse sand to gravel with cobbles and boulders composed mainly of basalt. The channel is sinuous and 

would 9ontain the 100-year peak flow with no overbank flow. On the basis of a study by Rathburn (1989, 

p. 15), a value of Manning's n of 0.050 was used for all 

cross sections through the canyon.

Two simulations were completed to illustrate the 

changes caused by attenuation as the peak flow traveled through Box Canyon. Both simulations used modifications of the 100-year flow hydrograph developed 

for the Arco gaging station and described in the section 

"Estimation of 100-Year Flow Volumes." For the first 

simulation, several days around the maximum daily 

mean flow of the 100-year peak flow hydrograph and 

the 100-year peak flow (table 2) were used as the up-
stream boundary condition to the FOURPT model. 

Attenuation of the peak flow, as shown in figure 4, 

was essentially nonexistent. For the second simulation, 

the hydrograph was shortened to a period of 2 hours, 

and all other parameters were held constant. The simulated peak flow was attenuated through Box Canyon 

(fig. 5) and was reduced by about 170 ftVs at the INEL 

boundary.

Results of these simulations illustrate that peak 

flows in Box Canyon were not significantly attenuated 

when the duration of the streamflow hydrograph is relatively long and were attenuated only slightly for a 

peak flow of short duration. If infiltration losses in Box 

Canyon are assumed to be balanced by possible local 

runoff from precipitation, peak flow at the INEL 

boundary can be considered the same as that at the 

Arco gaging station.

Birch Creek

Birch Creek at the INEL boundary and Birch 

Creek at the Birch Creek Sinks are ungaged sites. Peak 

flow data from the Birch Creek near Reno gaging station (13117000) are available for the upper part of 

Birch Creek Basin (subbasin 1, fig. 3). Flood-frequency analysis of annual peak flows at the Reno gaging 

station determined that the 100-year peak flow was 

about 260 ft3/s (Stone and others, 1993). This value is 

a poor representation of the 100-year peak flow because no large peak flows were recorded and the period 

of record was short. The value of 260 ftVs probably is 

more representative of shorter (5- to 10-year) recurrence intervals.

Flow data for Birch Creek are available from 

two other gaging stations Birch Creek at Blue 

Dome Inn, near Reno (13117020), and Eightmile 

Canyon Road, near Reno (13117030). The periods 

of record for these gaging stations are too short for 

reliable flood-frequency analysis.

Because the period of record of annual peak flows 

at gaging stations was not adequate to estimate the 

100-year peak flow with flood-frequency analysis, 

regional regression equations were used to estimate 

100-year peak flows. The regional regression equations 

and procedures presented by Kjelstrom and Moffatt 

(1981) were used to estimate the 100-year peak flows 

(table 3) from the mountainous areas upstream from 

the Reno gaging station (subbasin 1) and for 18 other 

subbasins (fig. 3). Drainage areas east of the junction of 

the Blue Dome Inn gaging station and Highway 28 and 

at the southern end of the Lemhi Range were not included because their channels are not tributary to Birch

7,400

7.200

7,000

6,800

1/3 

OS

cu

H 

U

NM 6,600

pa

6,400

O 6,200

1/3 » i

G

6,000

5,800

Arco gaging 

station 

(0 miles) INEL boundary 

(7.5 miles)

10 15 20 25 30 35

TIME, IN HOURS

40 45 50 55 60

Figure 4. Simulated 48-hour streamflow hydrographs for a 100-year peak flow at the Arco gaging station routed 

through Box Canyon to the INEL boundary. (Locations shown on figure 1)

Birch Creek 13
Q 

Z

w

CU

H

W

ta

7,400

7,200

7,000

6,800

HH 6,600

U

£ 6.400

HH

O 6,200

HH

(J 6,000

in

5.800

INEL boundary 

(7.5 miles)'

1.5 2.0 2.5 3.0 3.5

TIME, IN HOURS

4.0 4.5 5.0 5.5 6.0

Figure 5. Simulated 2-hour streamflow hydrographs for a 100-year peak flow at the Arco gaging station routed 

through Box Canyon to the INEL boundary. (Locations shown on figure 1)

Creek; any ephemeral flow from these drainage areas 

flows directly onto the eastern Snake River Plain.

Channel-infiltration losses are included and 

estimated peak flows are reported in tables 3 and 4. 

Channel-infiltration losses upstream from the Reno 

gaging station were determined by adjusting the infiltration rate until the 10-year peak estimated from regression equations equaled the 10-year peak estimated 

from flood-frequency analysis of the gaging station data 

(130 ft3/s, table 4). The resulting infiltration rate was 

2.3 in/hr. Infiltration rate, stream distance, and stream 

width were used to determine channel-infiltration losses as previously described for the Big Lost River, and 

the resulting losses were used to correct regression estimates of the 100- and 500-year peak flows at the Reno 

gaging station (table 4).

Estimation of the infiltration rate for channels from 

the 18 other subbasins tributary to Birch Creek was 

derived from two observations and one assumption: 

(1) Except for a few of the subbasins, channels are 

quite small or not discernible, and flow does not ordinarily reach Birch Creek; (2) flow at Birch Creek Sinks 

may occur at a 10-year recurrence interval, but not at a 

5-year interval; and (3) some flow from most subbasins 

Should reach Birch Creek during especially rare peak 

flows. An infiltration rate of 2.6 in/hr allows streamflow to reach Birch Creek from 16 of the 18 subbasins

14 100-Year Peak Flows, INEL, Idaho

for a 500-year peak flow but allows no streamflow to 

reach the Birch Creek Sinks for a 5-year peak flow. The 

two subbasins that would not contribute flow to Birch 

Creek for the 500-year peak flow have small drainages 

and long channels, indicating limited runoff and large

channel-infiltration losses. I 

Peak flows between the Reno gaging station and

Birch Creek Sinks also were adjusted for channel-infiltration losses. From 1988 to 1994, 17 sets of gain-loss 

discharge measurements were made along Birch Creek 

from the Blue Dome Inn gaging station to Reno Ditch, 

about 10 mi downstream. The average infiltration rate 

from these measurements was calculated to be about 

1.4 in/hr. Although infiltration rates in this reach were 

based on average conditions, they were considered to 

be comparable to rates during a 100-year flood. Including peak flows from subbasins 1 through 6 and adjusting for channel-infiltration losses, the resulting 100- 

year peak flow at the Blue Dome Inn gaging station 

was 980 ft3/s (table 4). Including peak flows from subbasins fe and 9 and adjusting for channel-infiltration 

losses, the resulting 100-year peak flow at the Reno 

Ditch diversion was 820 ftVs (table 4).

Peak flows also were adjusted for water being diverted to Reno Ditch. The maximum flow capacity for 

the Reno Ditch is 75 ftVs and, during flooding, flow in 

the ditch is assumed to be at maximum capacity. The
Table 3. Estimates of 100-year peak flow from subbasins of Birch Creek between the Reno gaging station (13117000) and 

Birch Creek Sinks

[mi2, square miles; in., inches; ft, feet; ft3/s, cubic feet per second; locations of subbasins shown on figure 3; regression estimates obtained from equations in a 

report by Kjelstrom and Moffatt (1981)]

Birch 

Creek 

subbasin 

No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Drainage 

area 

(mi2)

6.91

1.45

6.40

34.0

2.16

6.65

1.41

.93

.93

2.00

1.07

1.49

3.70

1.87

3.47

5.63

2.62

5.16

Mean 

annual 

precipitation 

(in.)

17.1

14.8

16.1

28.1

24.9

28.4

27.3

25.2

25.8

26.9

23.0

25.8

24.0

22.5

22.5

22.5

18.2

16.7

Mean 

subbasin 

altitude 

(ft)

6,800

7,300

7,800

8,500

7,600

8,200

8,100

7,500

8,000

8,800

7,800

8,000

8,500

6,800

8,000

7,400

6,200

7,200

Regression 

estimates 

(ft3/s)

204

62

171

654

98

215

72

55

53

89

54

71

130

78

121

169

91

140

Total of peak flows from subbasins contributing to Birch Creek:

100-year peak flows

After adjustments 

for channel-infiltration 

losses (ft3/s)

490

116

36

136

316

6

0

10

1

1

7

0

0

36

9

0

29

12

44

759

resulting peak flow at the INEL boundary after adjusting for channel-infiltration losses and water diverted to 

Reno Ditch was 700 ftVs (table 4).

The 100-year peak flow in Birch Creek at the Reno 

Ditch diversion also was calculated using channel geometry methods (Harenberg, 1980) that require estimates of bankfull width. Using an average bankfull 

width of 38 ft, a 100-year peak flow of 1,120 ft3/s 

was calculated. Although this value is greater than the 

estimate of 895 ft3/s (820 ft3/s shown in table 4 plus the 

75 ft3/s diverted into the ditch), the difference can be 

attributed to overestimating bankfull width and to regression errors.

Subbasins 10 through 19 can contribute flow to 

Birch Creek between the INEL boundary to the terminus of Birch Creek at Birch Creek Sinks. The infiltration rate of 1.4 in/hr used for the upper reaches of Birch 

Creek also was used to calculate channel-infiltration 

losses for this reach. The resulting peak flow at Birch 

Creek Sinks was 590 ftVs (table 4).

Peak flows in Birch Creek estimated at Birch 

Creek Sinks likely would be reduced considerably because of water ponding north of Highway 22 and the 

routing of water through constructed channels and 

gravel pits south of Highway 22. Peak flows shown in 

table 4 were not adjusted to account for these conditions.

The 5-, 10-, and 500-year peak flows (table 4) were 

estimated with the same method used to estimate the 

100-year peak flow. Stream and channel lengths and infiltration rates were constant for calculation of all peak 

flows, but channel width for each peak flow was calculated using Dawdy's equation.

The assumptions used to estimate the peak flows 

shown in table 4 were: (1) Water was not diverted for 

irrigation; (2) 75 ft3/s was diverted to the Reno Ditch; 

(3) flow in Birch Creek occurred only in a single, unbraided channel; (4) peak flow from the Reno gaging 

station and subbasins arrived simultaneously at the 

Blue Dome Inn gaging station, Reno Ditch diversion, 

INEL boundary, and Birch Creek Sinks, respectively; 

(5) estimated infiltration losses were representative of 

losses along the channel during flooding; (6) the peak 

flows were not attenuated; and (7) Highway 22 and

Birch Creek 15
Table 4. Estimated peak flows at gaging stations and other selected sites in the Birch Creek Basin at selected recurrence 

intervals

[Locations of gaging stations and sites shown on figure 3; peak flows include adjustments for channel-infiltration losses]

Gaging stations and 

other selected sites

Peak flows at selected recurrence intervals 

(cubic feet per second)

on Birch Creek 5-yea

teno gaging station (13117000) .................................................... 110

Jlue Dome Inn gaging station (13117020).................................... 240

teno Ditch diversion ..................................................................... 150

NEL boundary .............................................................................. 55

Birch Creek Sinks .......................................................................... 0

r 10-year

130

370

260

150

46

100-year

490

980

820

700

590

500-year

740

1,570

1,380

1,240

1,170

constructed channels and gravel pits south of Highway 

22 did not affect peak flows.

ESTIMATION OF 100-YEAR FLOW 

VOLUMES

The volume of runoff can be determined by computing the area under a streamflow hydrograph for a 

particular length of time. For the Big Lost River and 

Birch Creek, a length of 60 days was used because this 

period approximates the length of floods that were recorded in 1965 and 1967 at the Big Lost River at Howell Ranch gaging station. Daily mean flow from these 

floods was greater than 1,000 ftVs from May 16 

through July 22, 1965 (68 days), and from May 19 

through July 19,1967 (62 days). Any flows outside the 

60-day period likely would be diminished by storage 

and diversions. Peak flow within the 60-day period was 

selected to be on the 24th day, on the basis of the 1965 

flood in the Big Lost River. The assumptions used to 

estimate peak flows in the Big Lost River and Birch 

Creek also were used to estimate the 100-year flow volumes. Two additional assumptions were that (1) for the 

Big Lost River, the 100-year flow volumes described 

by hydrographs for the Howell Ranch gaging station 

and subbasins in the Big Lost River Basin arrived simultaneously at the Arco gaging station; and (2) for 

Birch Creek, the 100-year flow volumes described by 

hydrographs for the Reno gaging station and subbasins 

arrived simultaneously at the Blue Dome Inn gaging 

station, Reno Ditch diversion, INEL boundary, and 

Birch Creek Sinks, respectively. These assumptions 

would produce the largest possible flow-volume estimates for this method.

16 100-Year Peak Flows, INEL, Idaho

In this report, 100-year flow hydrographs for 

60-day periods at selected gaging stations were 

developed using the following steps:

1. The 1-day 100-year mean flow, obtained from 

published high-flow analysis of gaging-station 

data, was assigned for the daily mean flow (q) 

on the 24th day of the hydrograph on the basis 

of the 1965 flood.

2. The primary unit flow (Q') was computed by 

(dividing the 1-day 100-year mean flow by 60, 

kvhich is the duration, in days, of a typical peak 

flow hydrograph for the Big Lost River.

3. The ascending and descending limbs of the 

hydrograph were derived from published annual 

maximum D-day mean flows having a 100-year 

recurrence interval for D-days of 1, 3,7, 15, 30, 

and 60. For instance, the 30-day 100-year mean 

flow encompassed a 30-day period that began 

somewhere on the ascending limb and ended 

somewhere on the descending limb. The hydrograph was skewed to the right, as indicated by 

the peak occurring on day 24 (40 percent of the 

60-day duration), rather than on day 30 (50 percent of the 60-day duration). Therefore, using 

the 30-day flow as an example, the first day of 

the 30-day period began on the ascending limb, 

approximately 12 days (40 percent) prior to day 

24. The last day of the 30-day period ended on 

the descending limb, approximately 18 days 

(60 percent) after day 24. This approach was 

repeated for the 3, 7, and 15 D-day 100-year 

mean flows.

4. Each published D-day 100-year mean flow was 

divided by the primary unit flow (Q') to obtain 

an associated initial unit-flow coefficient (n).
The initial coefficients were adjusted for days 1 

through 23 and 25 through 60 in an iterative 

manner until the published maximum D-day 

100-year mean flows were approximated and a 

smooth transition in daily mean flow from one 

day to the next was obtained. For example, the 

initial unit-flow coefficient of 54.8 for the 7-day 

100-year mean flow of 4,250 ft3/s at the Big 

Lost River at Howell Ranch, near Chilly (table 

5) was adjusted for days 22 through 28 so that 

the 100-year mean flows for 3 and 7 days were 

approximated and the ascent to and descent 

from the 1-day peak flow on day 24 were 

smooth. This iterative approach was repeated 

for the 3, 15, 30, and 60 D-day 100-year mean 

flows.

5. Estimated daily mean flows (q) were calculated 

by multiplying the unit-flow coefficient (ri) by 

the primary unit flow

For ungaged sites, the 100-year flow hydrographs 

for 60-day periods were developed using the following 

steps:

1. A gaging-station flow hydrograph that likely 

represented the ungaged site was selected, or a 

composite of several gaging-station hydrographs in the region was used.

2. The 1-day mean flow (q) at the ungaged site was 

determined by multiplying the 100-year peak 

flow at the ungaged site (tables 1 and 3) by the 

ratio of the 1-day mean flow and peak flow at 

the representative gaging station.

3. The primary unit flow (Q*) was computed by 

dividing the 1-day mean flow by 60.

4. The unit-flow coefficients (ri) at the representative gaging station for the other 59 days were 

used to determine the daily mean flow (q) at the 

ungaged site.

Big Lost River

Representative hydrographs developed for the 

Howell Ranch and Lower Cedar Creek gaging stations 

(table 5) were used to develop 100-year flow hydrographs for 60 days for the upper Big Lost River Basin 

because flows for the Arco gaging station were affected 

by upstream storage and diversions.

Mean flows at the Howell Ranch gaging station for 

1, 3, 7, 15, 30, and 60 consecutive days were 4,650, 

4,520, 4,250, 3,840, 3,230, and 2,640 ft3/s, respectively, for a 100-year recurrence interval (Stone and others, 

1993, p. 24). The hydrograph for the Howell Ranch 

gaging station was used to determine a 60-day flow 

volume of 310,000 acre-ft (table 5).

The 60-day representative hydrograph developed 

from the data for the Lower Cedar Creek gaging station 

(Stone and others, 1993, p. 28) was used to make representative hydrographs for all 23 contributing subbasins 

(table 5). Consequently, the unit-flow coefficient (ri) for 

Lower Cedar Creek was multiplied by the primary unit 

flow (Q*) for the 23 subbasins to obtain daily mean 

flow (q). The primary unit flow for the subbasins was 

determined by multiplying the ratio of the 1-day mean 

and peak flows from Lower Cedar Creek (0.84) by the 

peak flow from each subbasin that reaches the river and 

dividing the result by 60. The estimated 60-day flow 

volume from the 23 contributing subbasins was 

165,000 acre-ft (table 5).

Flow volumes for a 60-day hydrograph for the Arco 

gaging station (table 5) were determined by adding the 

mean daily flows (q) at the Howell Ranch gaging 

station and the total from the 24 subbasins and subtracting calculated channel-infiltration losses. Channelinfiltration losses were calculated using the same infiltration rates and distances used in the calculation of 

peak flow. Average channel width changed according 

to the flow for each day. The estimated total volume 

of flow at the Arco gaging station for 60 days was 

390,000 acre-ft (table 5).

Birch Creek

Peak flows that reach Birch Creek Sinks could 

overflow playas shared with the Big Lost River and 

threaten facilities downstream. No representative hydrograph was developed for the Reno gaging station 

because the record was short and no data were available during years when flooding occurred in the Big 

Lost River. Because hydrologic and hydraulic conditions in Medicine Lodge Creek Basin (fig. 1) are similar to those in the Birch Creek Basin, a 60-day representative hydrograph developed for Medicine Lodge 

Creek at Ellis Ranch, near Argora (13116000), was 

used to determine flow volumes at sites in the Birch 

Creek Basin.

Big Lost River 17
Table 5. Values for 60-day flow hydrographs and daily estimates of 100-year peak flow volumes at selected gaging stations and 

from contributing subbasins in the Big Lost River Basin | |

[n, unit-flow coefficients applied to day; Q', primary unit flow, which is the 100-year daily mean flow, in cubic feet per second, divided by 60; q, daily mean 

flow, in cubic feet per second; contributing subbasins are listed in table 1; locations of gaging stations and subbasins shown on figure 2]

Big Lost River 

at Howell Ranch, 

near Chilly (131 20500)

Day

i

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

n

24

24.2

24.4

24.6

24.8

25

25.2

25.4

25.7

26

26.5

27

28

30

33

36

40

44

46

48

50

53

57

60

58

54

51

49

47

46

45

43

41

39

37

35

33

32

31

30

29

28

27.6

27.3

27

26.8

26.6

26.4

26.2

26

25.8

25.6

25.4

25.2

25

24.8

24.6

24.4

24.2

24

Q1

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

77.5

Total volume, in acre-feet:

q

,860

,880

,890

,910

,920

1940r

,950

,970

1,990

2,020

2,050

2,090

2,170

2,330

2,560

2,790

3,100

3,410

3,570

3,720

3,880

4,110

4,420

4,650

4,500

4,190

3,950

3,800

3,640

3,570

3,490

3,330

3,180

3,020

2,870

2,710

2,560

2,480

2,400

2,330

2,250

2,170

2,140

2,120

2,090

2,080

2,060

2,050

2,030

2,020

2,000

1,980

1,970

1,950

1,940

1,920

1,910

1,890

1,880

1,860

310,000

Lower Cedar Creek 

above diversions, 

near Mackay (13128900)

n

16

16.5

17

17.5

18

18.5

19

19.5

20

20.5

21

22

23

26

29

33

37

41

44

46

48

50

54

60

56.5

52

49

47

46

45

44

41

38

35

33

31

29

27

26

25

24

23

22.5

22

21.5

21

20.8

20.6

20.2

20

19.8

19.6

19.3

19

18.5

18

17.5

17

16.5

16

Contributing 

subbasins

Q' q Q'

4.73 76

4.73 78

4.73 80

4.73 ! 83

4.73 85

4.73 88

4.73 90

4.73 92

4.73 95

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

97

99

104

109

123

137

156

175

194

208

218

227

237

4.73 255

4.73 284

4.73 267

4.73 246

4.73 232

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

222

218

213

208

194

180

166

156

147

137

128

123

118

114

109

106

4.73 104

4.73 102

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

4.73

99

98

97

96

95

94

93

91

90

4.73 88

4.73 85

4.73 83

4.73 80

4.73 78

4.73 76

16,300

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

48

q

770

790

820

840

860

890

910

940

960

980

1,010

1,060

1,100

1,250

1,390

1,580

1,780

1,970

2.110

2,210

2,300

2,400

2,590

2,880

2,710

2,500

2,350

2,260

2,210

2,160

2,110

1,970

1,820

1,680

1,580

1,490

,390

,300

,250

,200

,150

,100

1,080

1,060

1,030

1,010

1,000

990

970

960

950

940

930

910

890

860

840

820

790

770

165,000

Big Lost River 

near Arco (131 32500)

q

2,060

2,090

2,130

2,160

2,190

2.240

2,270

2,300

2,350

2,390

2,450

2,520

2,630

2,900

3,230

3,600

4,060

4,500

4,760

4,990

5,220

5,510

5,950

6,430

6,140

5,670

5,320

5,110

4,930

4,810

4.710

4,440

4,160

3,900

3,670

3,460

3,230

3,070

2,960

2,850

2,740

2,630

2,580

2,540

2,490

2,470

2,450

2,420

2,390

2,370

2,340

2,320

2,290

2,270

2,240

2,190

2.160

2,130

2,090

2,060

390,000
Table 6. Values for 60-day flow hydrographs and daily estimates of 100-year peak flow volumes at the Medicine Lodge Creek at 

Ellis Ranch, near Argora, gaging station and at selected gaging stations and sites in the Birch Creek Basin

[n, unit-flow coefficients applied to day; Q', primary unit flow, which is the 100-year daily mean flow, in cubic feet per second, divided by 60; q, daily mean 

flow, in cubic feet per second; locations of gaging stations and sites shown on figures 1 and 3]

Medicine Lodge Creek 

at Ellis Ranch, 

near Argora 

(13116000)

Day

i

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

n

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

46

60

47.5

39

36

35

34

33

32

31

30

29

28.5

28

27.5

27

26.5

26

25.5

25

24.5

24

23.5

23

22.5

22

21.5

21

20.5

20

19.5

19

18.5

18

17.5

17

16.5

16

Q'

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

4.3

q

69

73

77

82

86

90

95

99

103

108

112

116

120

125

129

133

138

142

146

151

155

159

198

258

204

168

155

151

146

142

138

133

129

125

123

120

118

116

114

112

110

108

105

103

101

99

97

95

92

90

88

86

84

82

80

77

75

73

71

69

Total volume, in acre-feet:

Reno 

(13117000)

q

92

97

103

109

115

120

126

132

138

143

149

155

160

166

172

178

183

189

195

201

206

212

264

344

272

223

206

201

195

189

183

178

172

166

163

160

158

155

152

149

146

143

140

138

135

132

129

126

123

120

117

115

112

109

106

103

100

97

95

92

18,300

Blue Dome 

Inn 

(13117020)

q

179

190

201

212

223

235

246

257

268

279

290

302

313

324

335

346

357

369

380

391

402

413

514

670

531

436

402

391

380

369

357

346

335

324

318

313

307

302

296

290

285

279

274

268

262

257

251

246

240

235

229

223

218

212

207

201

195

190

184

179

35,800

Birch Creek

Reno Ditch 

diversion

q

142

151

160

169

177

186

195

204

213

222

231

239

248

257

266

275

284

293

302

310

319

328

408

532

421

346

319

310

302

293

284

275

266

257

253

248

244

239

235

231

226

222

217

213

208

204

200

195

191

186

182

177

173

169

164

160

155

151

146

142

28,400

INEL 

boundary

q

52

61

69

77

85

93

102

110

119

127

136

143

152

160

169

177

186

195

204

211

220

228

306

426

318

246

220

211

204

195

186

177

169

160

157

152

148

143

139

136

131

127

123

119

114

110

106

102

98

93

90

85

81

77

73

69

65

61

56

52

10,600

Birch 

Creek Sinks

q

3

10

15

21

29

35

43

51

58

66

74

81

88

95

104

112

121

129

138

145

154

161

239

366

252

179

154

145

138

129

121

112

104

95

94

88

85

81

76

74

68

66

61

58

54

51

46

43

40

35

33

29

26

21

18

15

12

10

6

3

9.800

Birch Creek 19
The same procedure that was used to estimate the 

100-year peak flow at gaging stations was used to develop the 60-day representative hydrograph for the 

Medicine Lodge Creek at Ellis Ranch gaging station. 

The magnitude and frequency of annual peak flows 

were based on 28 years of record from 1942 to 1969. 

Mean annual flows for periods of 1, 3, 7, 15, 30, and 

60 consecutive days for the 100-year recurrence interval were 258, 220,181, 165, 139, and 116 ft3/s, respectively (Kjelstrom and others, 1996, p. 258). The primary unit flow (£0 was 4.3 ft3/s. The peak flow again was 

selected to be on the 24th day. Results of calculations 

are shown in columns 1 through 4 on table 6.

The unit-flow coefficient developed for the Medicine Lodge Creek at Ellis Ranch gaging station (column 2 on table 6) was used for the Reno gaging station 

and subbasins (2-22) contributing flow to Birch Creek. 

Unit-flow coefficients were determined using the ratio 

(0.70) of the 1-day mean flow and peak flow for the 

100-year recurrence interval of the Medicine Lodge 

Creek at Ellis Ranch gaging station. This ratio was 

multiplied by the 100-year peak flows (table 4) to determine the 1-day mean flow at the Reno gaging station 

and from subbasins. Channel-infiltration losses were 

adjusted daily on the basis of the daily mean flows for 

the sites on Birch Creek shown in table 4. Infiltration 

losses were estimated in a manner similar to estimation 

of losses for peak flow. Distances between these sites 

and the infiltration rate of 1.4 in/hr were held constant. 

Average channel width changed according to the flow 

in the reach for each day. Hydrograph data estimated 

for the Reno gaging station, Blue Dome Inn gaging station, Reno Ditch diversion, INEL boundary, and Birch 

Creek Sinks are shown in table 6.

The hydrograph data for the INEL boundary and 

Birch Creek Sinks were determined by adjusting the 

daily mean flows at the Reno Ditch for diversions, 

Channel-infiltration losses, and inflow from subbasins 

(10-22). Flow to the Reno Ditch was assumed to be 

75 ftVs for the 60-day period. The 100-year volume of 

flow entering the INEL area was estimated to be 

10,600 acre-ft (table 6). The 100-year volume of flow 

to the Birch Creek Sinks was estimated to be about 

9,800 acre-ft (table 6). Flow to the Birch Creek Sinks, 

however, would decrease because of storage and infiltration losses in gravel pits and diversion channels. 

I Koslow (1984) estimated that the diking system in 

the Birch Creek Playa can contain 13,000 acre-ft; thus, 

the 100-year flow volume from Birch Creek is not significant unless the Birch Creek Playa receives water

20 100-Year Peak Flows, INEL, Idaho

from other sources, such as the Big Lost River or from 

melting snowpacks combined with rainfall on frozen 

ground (Koslow, 1984). Water available for runoff 

from snowmelt and precipitation in the eastern Snake 

River Plain in Idaho for 4- to 15-days' duration for a 

100-year recurrence interval was estimated to be 5 and 

8 in., respectively (Frederick and Tracey, 1976, p. 25, 

29). For the approximately 20 mi2 that likely would 

contribute water to the Birch Creek Playa under these 

conditions, the volume in the first 4 and 15 days would 

be 5,300 and 8,500 acre-ft, respectively. Consequently, 

the 100-year localized flood solely from snowmelt 

combined with rainfall and frozen ground would be 

contained by the present diking system in the Birch 

Creek Playa. However, combination of the 100-year 

volume of flow from Birch Creek to the Birch Creek 

Sinks with either the 4- or 15-day localized runoff 

would exceed the capacity of the diking system.

SUMMARY

Peak flows and flow volumes with recurrence intervals of 100 years for the Big Lost River and Birch 

Creek were estimated so the extent of the 100-year 

flood plain at the Idaho National Engineering Laboratory (INEL) could be delineated. Future studies should 

be able to use models to route peak flows through the 

INEL area to spreading areas and playas. This report 

quantifies peak flows and flow volumes that will be 

used in future modeling studies to delineate the flood 

plain.

The Big Lost River is the most likely source of 

flooding at the INEL. Annual peak flows at the Big 

Lost River near Arco gaging station are affected by reservoir storage and diversion to irrigation canals. As a 

result, flood-frequency analysis of data at the Arco 

gaging station probably does not represent the range of 

natural flow variability, and other methods were used. 

Flood-frequency analyses at two upstream gaging stations in the Big Lost River Basin were combined with 

regional regression equations to estimate peak flows 

and flow volumes at the Arco gaging station.

Estimates of peak flows were obtained by a floodfrequency analysis of 85 annual peaks in the Big Lost 

River at Howell Ranch gaging station. The 450-mi2 

drainage area upstream from this gaging station produced an estimated peak flow of 4,880 ftVs for a recurrence interval of 100 years. The highest recorded peak
was 4,420 ftVs on May 25,1967. Because flooding in 

the Big Lost River is caused primarily by snowmelt in 

mountainous areas, only mountainous areas between 

the Howell Ranch and Arco gaging stations would likely contribute runoff. On the basis of that assumption, 

23 subbasins were selected, one of which (Lower Cedar Creek) has a gaging-station record of 15 years. 

Rood-frequency analysis of annual peaks at the Lower 

Cedar Creek gaging station and regional regression 

equations at the other 22 subbasins produced estimates 

of peak flows with a recurrence interval of 100 years. 

Because infiltration losses in the river channel and on 

the alluvial fans between the subbasins and the river 

are large, channel-infiltration losses were subtracted 

from peak flows.

Channel-infiltration losses in the Big Lost River 

were determined by discharge measurements and 

gaging-station records upstream and downstream from 

losing reaches. Infiltration rates were estimated by calculating the water-surface area between upstream and 

downstream measuring sites by using approximate 

channel distances and average widths. In Chilly Sinks 

between the Howell Ranch gaging station and Mackay 

Reservoir, the infiltration rate was 3.4 in/hr; between 

Mackay Reservoir and the Arco gaging station, the infiltration rate was 0.7 in/hr. Estimates of infiltration 

rates in channels from the subbasins to the river ranged 

from 1.6 to 4.2 in/hr. Estimates were based on rock 

type and observations of no flow and discharge measurements in 1967.

The estimated 100-year peak flow in the Big Lost 

River near the Arco gaging station was 7,260 ftVs. The 

primary assumptions used to estimate this peak were: 

(1) The peak flow was not affected by storage in Mackay 

Reservoir, (2) water was not diverted to irrigation canals, (3) peak flows from different parts of the basin arrived simultaneously at the Arco gaging station, (4) infiltration losses were representative during flooding, 

and (5) the peak flow was not attenuated.

The computer model FOURPT was used to simulate streamflow from the Big Lost River near Arco gaging station to the INEL boundary. This is a 7.5-mi 

reach through a steep-walled canyon about 75 ft deep 

and 125 ft wide. Model simulations indicated that peak 

flows were not significantly attenuated. Thus, peak 

flow determined at the INEL boundary is assumed to 

be the same as that at the Arco gaging station.

Because gaging-station records of Birch Creek 

were not adequate to estimate the 100-year peak flow, 

flow estimates were made using regional regression

equations. These estimates were adjusted for channelinfiltration losses. Hooding in Birch Creek is caused 

primarily by snowmelt in mountainous areas. The 

100-year peak flow in Birch Creek at the Reno gaging 

station was estimated to be 490 ftVs. Downstream from 

the Reno gaging station, 18 mountainous subbasins 

were selected as likely contributors to the 100- and 

500-year peak flows in Birch Creek. Only 8 of the 18 

subbasins had simulated peak flows that reached Birch 

Creek for the 100-year flood because of channel-infiltration losses.

Before Birch Creek reaches the INEL boundary, up 

to 75 ftVs is diverted to Reno Ditch, which supplies a 

power-generation plant several miles east of Birch 

Creek. After accounting for diversion, channel infiltration, and inflow from subbasins, the 100-year 

peak flow at the INEL boundary was estimated to be 

about 700 ftVs and, at Birch Creek Sinks, to be about 

590 ftVs. The primary assumptions used to estimate 

these peaks were: (1) The Reno Ditch diverted 75 ftVs, 

(2) peak flows from different parts of the basin arrived 

simultaneously, (3) channel-infiltration losses were 

representative of losses during flooding, and (4) the 

peak flow was not attenuated.

Flow volumes from the Big Lost River and Birch 

Creek also were determined so the extent of the flood 

plain could be delineated. Representative hydrographs 

at selected gaging stations were developed for 60-day 

periods. Hydrographs were developed by creating 

smooth curves that contain the computed mean flow 

volumes having a recurrence interval of 100 years for 

1-, 3-, 7-, 15-, 30-, and 60-day periods. Records of the 

Big Lost River at Howell Ranch gaging station were 

used to provide a representative hydrograph for that 

part of the Big Lost River Basin, and records of the 

Lower Cedar Creek gaging station were used to provide a representative hydrograph for the contributing 

subbasins downstream from the Howell Ranch gaging 

station. Because gaging stations in the Birch Creek Basin did not have sufficient record, a gaging-station 

record of Medicine Lodge Creek at Ellis Ranch, which 

drains a basin hydrologically similar to that of Birch 

Creek, was used to develop a representative hydrograph. The volume of flow entering the INEL area 

from the Big Lost River during a 60-day period was 

390,000 acre-ft and, from Birch Creek, was about 

10,600 acre-ft. However, flow from Birch Creek reaching the Birch Creek Play a probably would be reduced 

because of channel-infiltration losses. The Birch Creek

Summary 21
Playa probably could receive flood water from the Big 

Lost River and from localized floods.

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