oin

nori

kili

mumak

merry

shelob

scatha

sauron

olorin

noldor

nazgol

maggot

shagrat

saruman

samwise

lumpkin

lugdush

jocko 8

jocko 5

rohirrim

jocko 15

jocko 12

jocko 10

shadowfax

oliphaunt

mithrander

ringwraiths

oakenshield

ori

roac

orald

legolas

jocko 1

quickbeam

radogastbrown

H56

83

MAP SEGMENT 3

N

84

85

86

87

88

89

90

91

92

92.0193

9495

96

97

98

99

100

101

102

103

106

104

105

107

108

109

110

111

112

B

B'

93

93

93

Highway 93 bridge

Old Highway 93bridge

Arlee

0.75 mile

Jocko

River

Jocko

River

FinleyCreek

Fishhatchery

200

200

200

0

DISTANCE FROM LEFT EDGE, IN FEET

2,960

2,958

2,962

ELEV

ATIO

N,I

NFE

ET(N

AVD

88)

200 800 1,000400 600 1,200

2,946

2,944

2,948

2,952

2,950

2.954

2,956

Ground surface

100 700 900300 500 1,100 1,300

Mai

nch

anne

l

Figure 7. Cross section 92, which is typical of split-flow conditions in Reach 7 along the Jocko River, western Montana. The water-surface elevations for the 100-year and 500-year flood discharges are shallow, but indeterminate between the main channel and themeander channel.

Mea

nder

chan

nel

Flood plain

Main and meander channels (existing and hypothetical conditions)Water surface generally is indeterminate between main and meander channels

Main and meander channels (existing and hypothetical conditions)Water surface generally is indeterminate between main and meander channels

CALCULATED WATER SURFACE FOR 500-YEAR FLOOD DISCHARGE

CALCULATED WATER SURFACE FOR 100-YEAR FLOOD DISCHARGE

20

27

R.21W.T.18N.

T.17N.T.16N.

1 6

36

6

36 31

DIAGRAM SHOWING RELATIVEPOSITION OF MAP SEGMENTS

AND MATCHLINES ALONGTHE JOCKO RIVER

A

A'

B

B'

EXPLANATION

JOCKO RIVER STUDY AREA

A A' MATCHLINE--Line separatingMap segments 1-3

1 MAP SEGMENT NUMBER--Redsegment number representssegments shown on this sheet

R.20W.

T.17N.T.18N.

R.20W.

R.20W. R.19W.

7 8 9

18 16

22 23

29 28 25 30 29

34 35 32 33

45

97

18 17 16 15

20 22

28 26

34

3

11 12 7

2

1

3

11'

47 15'

114 15'o 14'

13' 12' 11'

114 10'o

16'17'18'

09'

08'

07'

06'

05' 04'

19'

18'

17'

16'

o

14'

13'

12'

21

17

26

8

21

27

35

2

1,000 1,100 1,200 1,3002,960

2,970

Groundsurface

Irrig

atio

ndi

tch

Ground surfaceused for hypothet-ical (no-levee)condition

Figure 8. Cross section 96, which is typical ofnarrow flood-plain and channel conditionsin Reach 8 along the Jocko River, westernMontana.

DISTANCE FROM LEFT EDGE, IN FEET

ELEV

ATIO

N,I

NFE

ET(N

AVD

88)

2,972

2,964

2,966

2,962

2,968

EXPLANATION

Mai

nch

anne

l

Leve

e

Existing conditionHypothetical (no-levee) condition

Existing conditionHypothetical (no-levee) condition

CALCULATED WATER SURFACE FOR 500-YEARFLOOD DISCHARGE

CALCULATED WATER SURFACE FOR 100-YEARFLOOD DISCHARGE

0 200 400 600 8003,002

3,004

3,006

3,008

3,010

3,012

100 300 500 700DISTANCE FROM LEFT EDGE, IN FEET

ELEV

ATIO

N,I

NFE

ET(N

AVD

88)

Figure 9. Cross section 108, which is typical of wide flood-plain conditions andmultiple channels in Reach 8 along the Jocko River, western Montana.

Ground surface

Existing condition

Existing and hypothetical (no-levee) conditions

Hypothetical (no-levee) condition

EXPLANATION

Mai

nch

anne

l

Mea

nder

chan

nels

Mea

nder

chan

nels

CALCULATED WATER SURFACE FOR 500-YEAR FLOOD DISCHARGE

CALCULATED WATER SURFACE FOR 100-YEAR FLOOD DISCHARGE

Ground surface used forhypothetical (no-levee)condition

Leve

e

Leve

e

100-year flood limits without encroachment

Floodway

Flood-fringe encroachment100-year flood elevation

100-year flood elevation with encroachment

Figure 10. Theoretical cross section with floodway and water-surface elevations for the 100-year flood discharge with and withoutencroachment in the flood fringe.

Flood-fringe encroachment

Flood fringe Flood fringe

0.5 foot or less

U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

SCIENTIFIC INVESTIGATIONS MAP 2912(Sheet 3 of 3)

Chase and Parrett, 2006, Water-surface elevations and boundaries of the100-year and 500-year floods and floodway for the Jocko River, western Montana

WATER-SURFACE ELEVATIONS AND BOUNDARIES OF THE 100-YEAR AND 500-YEAR FLOODS AND FLOODWAY FOR THE JOCKO RIVER, WESTERN MONTANABy

Katherine J. Chase and Charles Parrett

Aerial-photographic base(scanned) from Water Consulting,Inc., for the Confederated Salish andKootenai Tribes of the Flathead Nation,1:3,600, 2002. Imagery flown by MAP, Inc.Lambert Conformal Conic Projection State PlaneCoordinate System (SPCS) Standard parallels 45 00'and 49 00'; central meridian -109 30'. North AmericanDatum of 1983 (NAD 83)

0

0 0

For additional information write to:

Director, Water Science CenterU.S. Geological Survey3162 Bozeman AvenueHelena, MT 59601

For sale by U.S. Geological Survey, Information ServicesBox 25286, Denver Federal CenterDenver, CO 80225-0286

For more information about the USGS and its products:Telephone: 1-888-ASK-USGSWorld Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptivepurposes only and does not imply endorsement by the U.S. Government.

U.S. Geological Survey, Reston, Virginia: 2006

Chase, K.J., and Parrett, Charles, 2006, Water-surface elevations and boundaries of the 100-yearand 500-year floods and floodway for the Jocko River, western Montana: U.S. Geological SurveyScientific Investigations Map 2912, 3 sheets, scale 1:12,000.

Suggested citation:

0 1 MILE0.25 0.50 0.75

SCALE 1:12,000

0 1 KILOMETER0.25 0.50 0.75

0 1,000 METERS250 500 750

0 5,000 FEET1,000 2,000 3,000 4,000

Printed on recycled paper

838485868788899091

77.2878.1879.3381.3482.9984.2284.8786.0087.52

2,875.32,880.12,883.92,891.42,900.02,905.42,912.82,925.32,934.0

2,880.82,886.42,890.52,897.02,904.42,914.22,918.72,931.32,940.7

2,880.02,886.22,890.02,897.12,904.42,914.22,918.72,931.32,940.7

2,881.42,887.32,891.32,898.02,904.92,915.12,919.42,932.22,941.6

2,880.32,887.02,891.12,898.02,905.02,915.12,919.52,932.22,941.6

92 89.28 2,947.7 32,952.7 32,952.7 32,953.1 32,953.12,953.2 2,960.6

2,960.2 2,964.32,960.8 2,966.22,963.1 2,967.42,964.1 2,969.82,964.3 2,972.22,965.4 2,973.12,966.1 2,975.02,969.1 2,976.42,972.7 2,979.82,972.8 2,980.42,972.8 2,980.22,972.8 2,980.82,973.6 2,982.72,974.5 2,982.62,974.5 2,982.52,974.5 2,982.62,977.0 2,984.02,994.9 3,000.83,003.2 3,008.83,016.5 3,022.43,026.0 3,032.33,041.6 3,046.8

93

949596979899100101102103103 DS 2

103 US 2

104105105 DS 2

105 US 2

106107108109110111112

90.54

91.3791.7192.0392.2992.6092.8493.2093.5093.9394.0994.1094.1394.2594.3794.3894.3994.5496.4897.5698.8599.99

101.48103.12 3,054.5

2,960.8

2,965.12,967.72,969.02,971.52,972.72,973.42,975.02,976.42,979.82,980.42,980.22,980.92,982.72,982.62,982.52,982.62,983.93,000.93,008.83,022.63,032.53,046.83,059.9 3,059.9

2,961.6

2,965.72,969.22,970.02,972.72,974.12,975.02,976.92,978.42,982.22,982.12,980.82,980.82,984.42,984.12,983.92,984.12,986.13,001.43,009.73,023.83,033.33,047.73,060.6

2,961.6

2,965.02,967.42,968.52,970.82,973.82,974.82,976.92,978.42,982.22,982.12,980.82,980.82,984.42,984.12,983.92,984.12,986.13,001.43,009.63,023.43,033.23,047.73,060.6

1 Data for some interpolated cross sections are omitted from the table.2 Cross section not shown on map segment.3 Calculated water-surface elevations in split-flow reaches are for main channel only.

Reach 6 (Map segment 3) Continued—

—Reach 7 Split-Flow 4 (Map segment 3)

Reach 8 (Map segment 3)

Table 4. Streambed- and calculated water-surface elevation data for the main channel of the Jocko River,western Montana.

1Cross

section

Distanceupstream

from crosssection 1,

inthousands

of feet

Minimumstreambedelevation,

in feet

100-year flood discharge 500-year flood discharge

Existingcondition

(with levee)

Hypotheticalcondition(no levee)

Existingcondition

(with levee)

Hypotheticalcondition(no levee)

Calculated water-surface elevation, in feet, forindicated flood discharge and condition

Continued—

[Distance upstream from cross section 1 measured along channel centerline shown on U.S. Geological Survey 7.5-minute quadrangle maps. Vertical coordinateinformation referenced to North American Vertical Datum of 1988 (NAVD 88). Abbreviations: DS, downstream side of bridge; US, upstream side of bridge]

Table 5. Streambed- and calculated water-surface elevation data for split-flow reaches for the 100-yearflood discharge on the Jocko River, western Montana.

[Distance upstream from cross section 1 measured along channel centerline shown on U.S. Geological Survey 7.5-minutequadrangle maps. Vertical coordinate information referenced to North American Vertical Datum of 1988 (NAVD 88).Symbol:

2,952.7--

2,952.7--

2,946.92,952.8

2,946.92,952.8

9292.01

1

Crosssection

89.2890.08

Distanceupstream

from crosssection 1,

inthousands

of feet

2,947.7--

Minimumstreambedelevation,

in feet

Main channel Left-flood plain

Existingcondition

(with levee)

Hypotheticalcondition(no levee)

Existingcondition

(with levee)

Hypotheticalcondition(no levee)

Calculated water-surface elevation, in feet, for 100-yearflood discharge on indicated portion of cross section and for

indicated condition

—Reach 7 Split-Flow 4 (Map segment 3)

, no data]--

Cross section represents left-flood plain only.1

Continued

838485868788899091

2,0501,950

270880450740340690950

400580230880450430190170400

2,880.82,886.42,890.52,897.02,904.42,914.22,918.72,931.32,940.7

2,881.12,886.42,890.52,897.12,904.42,914.22,918.72,931.52,940.9

.3

.0

.0

.1

.0

.0

.0

.2

.2

949596979899100101102103103 DS 2

103 US 2

104105105 DS 2

105 US 2

106107108109110111112

170250170110

80707070

3008070

60300110

9090

310600630950750600600

170200110

8080707070

2208070

60300110909080

30030010090

200260

2,965.12,967.72,969.02,971.52,972.72,973.42,975.02,976.42,979.82,980.42,980.2

2,980.92,982.72,982.62,982.52,982.62,983.93,000.93,008.83,022.63,032.53,046.83,059.9

2,965.12,967.82,969.02,971.82,972.92,973.52,975.12,976.42,979.82,980.52,980.4

2,980.92,982.72,982.62,982.52,982.62,983.93,001.33,008.83,022.63,032.73,046.83,060.1

.4

.1

.0

.3

.2

.1

.1

.0

.0

.1

.2

.0

.0

.0

.0

.0

.0

.4

.0

.0

.2

.0

.2

Reach 6 (Map segment 3)

Reach 8 (Map segment 3)

Width ofthe 100-yearflood area,

in feet

Calculatedwater-surface

elevation for the100-year flood

discharge,in feet

Calculatedwater-surfaceelevation withencroachment,

in feet

Increase inwater-surface

elevation,in feet

Floodwaywidth,in feet 1

1

Table 6. Floodway data for the Jocko River, western Montana.

[Floodway determined for Reaches 1, 4, 5, 6, and 8 for existing (1999-2001) conditions. Floodway width was expanded to equal the width ofthe 100-year flood area at all cross sections in split-flow Reaches 2, 3, and 7. Vertical coordinate information referenced to North AmericanVertical Datum of 1988 (NAVD 88). Abbreviations: DS, downstream side of bridge; US, upstream side of bridge]

Crosssection

(Mapsegments

1-3)

Continued

1 May include small areas that are not inundated by the 100-year flood discharge and may include some areas where water is ponded inmeander channels (no effective conveyance of floodwater).2 Cross sections not shown on map segments.

Looking downstream at Highway 93 bridge at cross section 103from Old Highway 93 bridge at cross section 104.

Reach 6

Reach 7

Reach 8

Prepared in cooperation withTHE CONFEDERATED SALISH AND KOOTENAI TRIBES

OF THE FLATHEAD NATION

Conversion Factors, Datum, and AcronymsMultiply By To obtain

Length

foot (ft) meter (m)

inch (in.) centimeter (cm)

inch (in.) millimeter (mm)

mile (mi)

0.3048

2.54

25.4

1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

square mile (mi )2 259.0 hectare (ha)

square mile (mi )22.590 square kilometer (km )2

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Hydraulic gradientfoot per foot (ft/ft) 1.0 meter per meter (m/m)

Velocityfoot per second (ft/s) 0.3048 meter per second (m/s)

Temperature in degrees Celsius (°C) may be converted to degrees

°F=(1.8×°C)+32

Horizontal coordinate information is referenced to the North AmericanDatum of 1927 (NAD 27) or to the North American Datum of 1983 (NAD 83)as noted. Surveyed vertical coordinate information is referenced to theNorth American Vertical Datum of 1988 (NAVD 88), unless otherwise noted.

Water year definition:Water year refers to the 12-month period beginning October 1 and endingSeptember 30. It is designated by the calendar year in which it ends.

CSKT Confederated Salish and Kootenai Tribes of the FlatheadNation

NAVD 88 North American Vertical Datum of 1988

NGVD 29 National Geodetic Vertical Datum of 1929

USGS U.S. Geological Survey

Fahrenheit (°F) as follows:

Acronyms used in this report:

Arcement, G.J., Jr., and Schneider, V.R., 1989, Guide for selectingManning’s roughness coefficients for natural channels and floodplains: U.S. Geological Survey Water-Supply Paper 2339, 38 p.

Barnes, H.H., Jr., 1967, Roughness characteristics of naturalchannels: U.S. Geological Survey Water-Supply Paper 1849, 213 p.

Coon, W.F., 1998, Estimation of roughness coefficients for naturalstream channels with vegetated banks: U.S. Geological SurveyWater-Supply Paper 2441, 133 p.

Henderson, F.M., 1966, Open channel flow: New York, MacMillanCo., 522 p.

Interagency Advisory Committee on Water Data, 1982, Guidelines fordetermining flood flow frequency—Bulletin 17 B of the HydrologySubcommittee: U.S. Geological Survey, Office of Water DataCoordination, 183 p.

Kendy, Eloise, and Tresch, R.E., 1996, Geographic, geologic, andhydrologic summaries of intermontane basins of the Northern RockyMountains, Montana: U.S. Geological Survey Water-ResourcesInvestigations Report 96-4025, 233 p.

Montana Administrative Rules, 1995, 36.15.502: Montana Departmentof Natural Resources and Conservation.

Parrett, Charles, and Johnson, D.R., 2004, Methods for estimating floodfrequency in Montana based on data through water year 1998: U.S.Geological Survey Water-Resources Investigations Report 03-4308,101 p.

Rantz, S.E, and Riggs, H.C., 1949, Floods of May-June 1948 inColumbia River basin with a section on Magnitude and frequency offloods: U.S. Geological Survey Water-Supply Paper 1080, 476 p.

U.S. Army Corps of Engineers, 2002a, HEC-RAS river analysissystem, user’s manual version 3.1: Davis, Calif., HydrologicEngineering Center [variously paged].

U.S. Army Corps of Engineers, 2002b, HEC-RAS river analysissystem, hydraulic reference manual version 3.1: Davis, Calif.,Hydrologic Engineering Center [variously paged].

U.S. Army Corps of Engineers, 2002c, HEC-RAS river analysissystem, applications guide version 3.1: Davis, Calif., HydrologicEngineering Center [variously paged].

U.S. Census Bureau, 2005, American FactFinder, accessed July 5, 2005at http://factfinder.census.gov/home/saff/main.html?_lang=en.

U.S. Geological Survey, 1955, Compilation of records of surface watersof the United States through September 1950, Part 12, Pacific Slopebasins in Washington and upper Columbia River basin: U.S.Geological Survey Water-Supply Paper 1316, 592 p.

U.S. Geological Survey, 2004, Water resources data, Montana, wateryear 2003, v. 2, Yellowstone and upper Columbia River basins andground-water levels: U.S. Geological Survey Water-Data ReportMT-03-2, 437 p.

Western Regional Climate Center, 2003a, Montana climate summary,accessed November 25, 2003 at http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?mtstig.

Western Regional Climate Center, 2003b, Montana average annualprecipitation map, accessed November 25, 2003 athttp://www.wrcc.dri.edu/pcpn/mt.gif.

References Cited

Looking upstream at cross section 109 from high bank on south side ofriver between Highway 93 and the fish hatchery.

EXPLANATION

AREA INUNDATED BY THE 100-YEARESTIMATED FLOOD DISCHARGE--Includesfloodway where floodway data were available

AREA INUNDATED BY THE 500-YEAR ESTIMATEDFLOOD DISCHARGE--Includes the area inundated bythe 100-year flood

FLOODWAY--Existing conditions (1999-2001)

CHANNEL CENTERLINE

CROSS SECTION AND NUMBER--Red cross-section symbol representscross section in split-flow reach

NATIONAL GEODETIC SURVEY BENCH MARK AND NAME

APPROXIMATE LOCATION OF LEVEE OR BANKLINE--Shown only in split-flow reaches

APPROXIMATE AREAS OF SHALLOW FLOODING WITH INDETERMINATE WATER-SURFACEELEVATIONS

ELEVATION-CONTROL POINT AND NAME--Data from Confederated Salish and Kootenai Tribes of the Flathead Nation

91 92

roac

H56

SummaryThe Jocko River flows across the Flathead Indian Reservation

in western Montana and provides irrigation water for several thousandacres of hay and grain that are produced throughout the Jocko Rivervalley. Much of the agricultural land along the river is being consideredfor residential and commercial development. In addition, proposedlevee modifications and stream-restoration work could further affect theJocko River channel and its aquatic habitat. The potential for increaseddevelopment highlighted the need for a better understanding of theboundaries of the 100- and 500-year floods.

The 100- and 500-year flood discharges for the Jocko River atDixon were estimated by applying the log Pearson Type III probabilitydistribution to the recorded and estimated annual-peak discharges forthe period of record from 1907-1910, 1948, and 1986 to 2001. Thelarge annual-peak discharges of 1908 (7,500 ft3/s) and 1948 (3,700ft3/s) are considered to be the largest during the entire period from 1907to 2001; thus, these values were treated as historic peak discharges(largest peak discharges in 94 years).

The 100- and 500-year flood discharges for the Jocko Rivernear Arlee were estimated by applying the log Pearson Type IIIprobability distribution to the recorded and estimated peak discharges atthe USGS and CSKT gaging stations for the period of record from1908, 1916, 1948, and 1990 to 2001. The peak discharges for 1908(6,200 ft3/s) and 1948 (2,660 ft3/s) also were treated as historic peakdischarges (largest peak discharges in 94 years).

HEC-RAS, version 3.1.1, a one-dimensional, hydraulic-flowmodel developed by the U.S. Army Corps of Engineers, was used tocalculate water-surface elevations at cross sections for the 100-year and500-year flood discharges and for the 1997 peak discharge. Channeland flood-plain geometry for 113 cross sections were surveyed between1999 and 2001 for use in the model. Bridge geometries at 9 of the 113sections located just downstream from bridge crossings also weresurveyed. Surveyed cross section data downstream from the bridgeswere used to estimate channel and flood-plain geometry at the upstreamand downstream edges of the bridges for use in the HEC-RAS model.For clarity, only the surveyed cross section immediately downstreamfrom each bridge is shown on the map segments. Interpolated cross-section data were used at 15 more locations where the hydraulic-modelresults indicated that additional channel and flood-plain geometry wererequired. For clarity, only 5 of the 15 interpolated sections are shownon the map segments. Overall, a total of 146 cross sections (113surveyed cross sections, 15 interpolated cross sections, and 18 crosssections at the upstream and downstream bridge edges) were used forhydraulic analyses. Data for the cross sections are available in files atthe USGS Montana Water Science Center office in Helena, Mont.

Although berms and levees are present in all reaches of theJocko River, only in Reaches 2, 3, and 5 are they long enough and highenough to create split-flow conditions. The berms and levees inReaches 2, 3, and 5 tend to keep floodwater confined to the mainchannel, but overtopping floodwater from upstream is confined behindand along the left side of the berms and levees. In addition, thecalculated water-surface elevations of the 100- and 500-year flooddischarges are greater than the ground elevations along the top of theberms and levees in many locations in Reach 2. Thus, the amount offlood discharge on the left-flood plain continually increases in thedownstream direction in Reach 2 as more floodwater from the channelspills over the top of the levees.

The flood boundaries along the Jocko River define areas thatwould be inundated by the 100- and 500-year flood discharges (Mapsegments 1, 2, and 3). The flood boundaries were delineated usingwater-surface elevations calculated at each cross section. Betweencross sections, the flood-profile graphs were used to determine water-surface elevations, which in turn were used to interpolate floodboundaries based on interpretation of aerial photography, limited onsitesurveys, and onsite visual inspection. The calculated water-surfaceelevations and flood-profile graphs are based on unobstructed flow andare valid only if structures remain clear of debris and do not fail. Someground elevations within the flood boundaries might be higher than thecalculated water-surface elevations. In these instances, the small areasof high-elevation ground generally cannot be shown due to lack ofdetailed topographic data and the map scale. Flood boundaries fortributary streams and irrigation ditches are not shown.

The boundaries of the floodway (for existing conditions) were

calculated by incrementally simulating encroachment on the 100-yearflooded area from each side of each cross section and re-running thehydraulic model until calculated water-surface elevations for the 100-year flood discharge increased by a maximum of 0.5 foot at all sections.Areas within the 100-year flood plain, where high depths (greater than3 ft) and high velocities (greater than 3 ft/s) occurred, were included inthe floodway even if the increase in the water-surface elevation was lessthan 0.5 ft. The floodway width was expanded to include the entire

flood plain at all cross sections in split-flow reaches 2, 3, and 7.flood plain at all cross sections in split-flow Reaches 2, 3, and 7.

The boundaries of the floodway (for existing conditions) were

Data for floodway widths and changes in water-surfaceelevations for the 100-year flood discharge as a result of floodwaydelineation are presented in table 6 (sheets 1, 2, and 3). Floodwayboundaries are shown together with the 100- and 500-year floodboundaries (Map segments 1-3).

Flooding on the Jocko River, June 4, 2005. View is looking northwest (downstream) from near cross section 71 (sheet 2) about 3 miles upstream from Ravalli, Montana.Photograph by Les Evarts, the Confederated Salish and Kootenai Tribes of the Flathead Nation.

Water-Surface Elevation DataWater-surface elevations were calculated for the 100- and 500-

year flood discharges for all surveyed and several interpolated crosssections in the study area for both existing and hypothetical (no-levee)conditions (table 4). Also presented in table 4 are the minimumstreambed elevations at the cross sections and the locations of bridges.Water-surface elevations were calculated for the 100-year flooddischarge for both existing and hypothetical (no-levee) conditions forthe main channel and left-flood plain for cross sections in the split-flowreaches (table 5). Also presented in table 5 are the minimum streambedelevations at the cross sections. For all cross sections shown in tables 4and 5, cumulative distances upstream from cross section 1 (mostdownstream cross section) also are presented.

The water-surface elevation data in tables 4 and 5 can be usedwith the distance data to construct profiles of 100- and 500-year flooddischarge throughout the study area. Flood profiles are useful forestimating water-surface elevations at selected locations between crosssections. Graphs showing flood profiles for the main channels of theJocko River based on channel and flood-plain conditions at the time ofthe surveys (1999-2001) are included on sheet 2. The calculated water-surface elevations (tables 4 and 5) and flood-profile graphs are based onunobstructed flow and are valid only if hydraulic structures remainclear of debris and do not fail.

Boundaries of the 100- and 500-year Flood DischargesThe flood boundaries along the Jocko River define areas that

would be inundated by the 100- and 500-year flood discharges (Mapsegments 1, 2, and 3). The flood boundaries were delineated usingwater-surface elevations calculated at each cross section. Betweencross sections, the flood-profile graphs were used to determine water-surface elevations, which in turn were used to interpolate floodboundaries based on interpretation of aerial photography, limited onsitesurveys, and onsite visual inspection.

Along the Jocko River, the ground surface of small areaswithin the flood boundaries for the 100-year flood discharge might beabove the calculated water-surface elevation, but cannot be shownbecause of the limitations of the map scale or the lack of sufficientlydetailed topographic data. Likewise, small areas within the floodboundaries for the 500-year flood discharge might be above thecalculated water-surface elevation. Also, some locations in the studyarea might be subject to flooding from Valley Creek, Finley Creek, orother small tributaries to these streams or the Jocko River. In addition,other locations might also be subject to flooding from overflows fromirrigation ditches. Flood boundaries for these streams and irrigationditches are not shown.

A levee or bankline is shown on the left bank (south orwestern side of the Jocko River) in all split-flow reaches to indicatewhere the main channel and left-flood plain are delineated. Mainchannel areas on the right side of the bankline have calculated water-surface elevations for the 100- and 500-year flood discharges that aredifferent from those in the left-flood plain. Although berms, levees, andbanklines exist in numerous locations outside the split-flow reaches,they are not shown on Map segments 1, 2, or 3.

Boundaries of the FloodwayThe area within the boundaries of the 100-year flood discharge

is divided into two parts for flood-plain management purposes. Thefloodway, which is reserved for the passage of flood discharges,includes the channel and some adjoining flood plain. The flood fringe,which is allowed to be developed with structures elevated on suitablefill material, includes the outer parts of the area inundated by the 100-year flood discharge. The width of the floodway under MontanaAdministrative Rule 36.15.502 (1995) is determined by hydrauliccalculation such that the water-surface elevation of the 100-year flooddischarge is increased by no more than 0.5 ft by complete filling(encroachment) of the flood fringe. Montana Administrative Rule36.15.502 (1995) further indicates that flood-plain areas where flowdepths or velocities for the 100-year flood discharge exceed 3 ft or 3ft/s, respectively, need to be included in the floodway. Figure 10 (sheet(sheet 3) shows a theoretical cross section with a floodway and water-surface elevations for the 100-year flood discharge with and withoutencroachment in the flood fringe.

The HEC-RAS hydraulic model was used to delineatefloodway boundaries for existing conditions (1999-2001) byincrementally simulating encroachment on the 100-year flooded areafrom each side of each cross section and re-running the hydraulicmodel until calculated water-surface elevations for the 100-year flooddischarge were increased by a maximum of 0.5 ft at all sections. Areaswithin the 100-year flood plain, where high depths (greater than 3 ft)and high velocities (greater than 3 ft/s) occurred, were included in thefloodway even if the increase in the water-surface elevation was lessthan 0.5 ft. The floodway width was expanded to equal the flood-plainwidth at all cross sections in split-flow Reaches 2, 3, and 7. At a fewcross sections, the calculated water-surface elevation for the 100-yearflood discharge with a floodway was less than that without a floodway.The calculated lower water-surface elevation probably is the result ofmore efficient hydraulic transitions between adjacent cross sectionswith a delineated floodway than without. For those few sections withlower calculated water-surface elevations with a floodway, the lowerelevations were raised to match those without a floodway.

The HEC-RAS hydraulic model was used to delineate