water-surfaceelevationsand hydrologicanalysis o … · 2006-07-10 · v372 ents dori dain n445...

V 372

ents

dori

dain

N 445

eomer

bilbo

bifur

beorn

balin

arwen

eomund

elrond

dwalin

deagol

cotton

cirdan

bombur

barrow

balrog

entings

dunadan

dernhem

crebain

aragorn

denethor

bregalad

butterbur

bracegirdle

U 372

bofur

elessar

boromer

entwives

beachbone

dunlending

black riders

1

2

5

4

3

6

7

89

10

1112

13

14

15

17

17.01

16

18

17.02

17.03

17.04

19

20

2122

23

24

25

26

27

2829

3031

32

33

34

35

36

37

38

39

MAP SEGMENT 1

N

A

A'

200

200

200

Flathead

River

Dixon0.125 mile

Agen

cy0.

25m

ile

12388200

JockoRiver

Jocko

River

Railroad bridge

Railroad bridge

Highway bridge

Railroadbridge

S-212

Looking downstream from levee on left bank near cross section 21.

Looking downstream from left bank near cross section 13.

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

Figure 3. Cross section 32, which is typical of channel and flood-plain conditions in Reach 5 along the Jocko River, western Montana.

ELEV

ATIO

N,I

NFE

ET(N

AVD

88)

0 200 400 600 800 1,000100 300 500 700 900

DISTANCE FROM LEFT EDGE, IN FEET

Ground surface

0 200 400 600 800 1,000100 300 500 700 900


2,630

ELEV

ATIO

N,I

NFE

ET(N

AVD

88)

Ground surface

2,650

For this simulation, the left bank is assumed to extend vertically to 500-yearwater-surface elevation in HEC-RAS model (flow actually spills from mainchannel to left-flood plain)

Mai

nch

anne

l

Mai

nch

anne

l

Flood plain

Flood plain

Main channel (existing condition)

Left-flood plain (existing condition)

Hypothetical (no-levee) condition

CALCULATED WATER SURFACE FOR 500-YEAR FLOOD DISCHARGE





Leve

e

Leve

e

Ground surface used forhypothetical (no-levee) condition


2,644

2,648

2,642

2,646

2,632

2,634

2,640

2,636

2,638

2,630

2,650

2,644

2,648

2,642

2,646

2,632

2,634

2,640

2,636

2,638

35

EXPLANATION

34

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

AREA INUNDATED BY THE 500-YEAR ESTIMATED FLOOD DISCHARGE--Includesthe area inundated by the 100-year flood

FLOODWAY--Existing conditions (1999-2001)

CHANNEL CENTERLINE

CROSS SECTION AND NUMBER--Red cross-sectionsymbol represents cross section in split-flow reach

NATIONAL GEODETIC SURVEY BENCH MARK AND NAME

APPROXIMATE LOCATION OF LEVEE OR BANKLINE--Shownonly in split-flow reaches

N 445

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

butterbur

Aerial-photographic base (scanned) from Water Consulting, Inc.,for the Confederated Salish and Kootenai Tribes of the FlatheadNation, 1:3,600, 2002. Imagery flown by MAP, Inc. Lambert ConformalConic Projection State Plane Coordinate System (SPCS) Standardparallels 45 00' and 49 00'; central meridian -109 30'. North AmericanDatum of 1983 (NAD 83)

00 0

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

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

SCIENTIFIC INVESTIGATIONS MAP 2912(Sheet 1 of 3)

Prepared in cooperation withTHE CONFEDERATED SALISH AND KOOTENAI TRIBES

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

ByKatherine J. Chase and Charles Parrett

93

200

20093

0

0 1

1 2 3 4 5 MILES

5 KILOMETERS432

Base modified from U. S. Geological SurveyPlains and Seeley Lake, Montana, 30' x 60'quadrangles, 1979 and 1980, 1:100,000.North American Datum of 1927 (NAD 27)

47 11'o

47 15'o

114 15'o 114oR.19W.R.20W.R.21W.

T.18N.

T.17N.

T.16N.

1 6

36 31

1 6

36 31

1 6

36

31

1 6

36 31

LAKE

CO

Arlee

Dixon

FLATHEAD

INDIAN

RESERVATION

MISSION

RANGE

St.Ignatius

Flathead

Rive

r

CSKT12384500

FishHatchery

Jocko

River

Jocko

River

Spring

Creek

K

Canal

Jocko

River KnifeCreek

BigFinley

Creek

Vall

ey

Creek

Fork

E.

RESERVATION

DIVIDE

NATIONAL

BISON

RANGEMAP SEGMENT 1

MAP SEGMENT 2

MAP SEGMENT 3

Figure 1. Location of the Jocko River study area, western Montana.

A

A'

B

Agency

S-212

Ravalli

B'

SAN

DERS

CO

Mission ReservoirInset

below

12388000

12388200

R 1

R 4

R 6

R 7

R 8

R 5

R 2, 3

61

3136

1R.21W.R.22W.

R.23W.R.19W.

T.18N.

Flathead

River

200Dixon

Perma

12388700

R.24W. Inset0

0 1 KILOMETER

1 MILE

114 30'o

47 20'o

JOCKO RIVER STUDY AREA

REACH AND NUMBER

STREAMFLOW-GAGING STATIONAND NUMBER

U.S. Geological Survey

Confederated Salishand Kootenai Tribesof the Flathead Nation

CSKT

12384500

R 6R 5

MATCHLINE--Line separating Mapsegments 1-3

A A'

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

Reach 1

Reach 2, 3

Reach 4

Reach 5

Reach 6

Figure 2. Cross section 18, which is typical of split-flow conditions in Reach 3 along the Jocko River, western Montana.

0 200 400 600 800 1,000100 300 500 700 900 1,100






Ground surface

0 200 400 600 800 1,000100 300 500 700 900 1,100


ELEV

ATIO

N,I

NFE

ET(N

AVD

88)





Ground surface

No effective conveyence of floodwatersbeyond left edge of surveyed section

Mai

nch

anne

l

Mea

nder

chan

nel

Mea

nder

chan

nel

Mai

nch

anne

l

No effective conveyence of floodwatersin abandoned meander channel (ponded)

No effective conveyence of floodwatersin abandoned meander channel (ponded)

No effective conveyence of floodwatersbeyond left edge of surveyed section

Flood plain Flood plain

Leve

e

Leve

e



2,570

2,590

2,584

2,588

2,582

2,586

2,572

2,574

2,580

2,576

2,578

2,566

2,568

ELEV

ATIO

N,I

NFE

ET(N

AVD

88)

2,570

2,590

2,584

2,588

2,582

2,586

2,572

2,574

2,580

2,576

2,578

2,566

2,568

Looking upstream from railroad bridge at cross section 6.

122 DS 2

2 US 2

34566 DS 2

6 US 2

7899 DS 2

9 US 2

1011121314151616 DS 2

16 US 2

1717.01

0.00.42.43.45.63

1.682.052.442.452.472.693.013.293.293.323.594.886.737.768.91

10.0010.3510.3510.3710.9511.36

2,513.22,511.02,511.02,511.02,514.72,514.52,519.42,520.42,520.42,520.42,519.92,523.02,524.32,524.32,524.32,525.62,533.52,545.92,550.62,553.02,554.92,559.42,559.42,559.42,561.42,562.9

2,518.02,519.42,520.22,520.42,521.82,523.02,524.52,528.22,528.12,528.32,529.22,531.52,531.92,533.12,533.52,535.72,539.32,553.82,556.82,559.42,563.82,565.02,565.42,565.42,568.72,569.3

2,518.02,519.42,520.22,520.42,521.82,523.02,524.52,528.22,528.12,528.32,529.22,531.52,531.92,533.12,533.52,535.72,539.42,553.82,556.82,559.42,563.82,565.02,565.42,565.42,568.72,569.3

2,518.72,520.42,521.12,521.72,523.72,524.52,525.72,529.52,529.32,529.82,531.12,533.02,533.82,533.82,537.62,540.92,541.22,555.52,558.32,560.32,564.52,566.12,566.72,566.72,570.42,570.8

2,518.72,520.42,521.12,521.72,523.72,524.52,525.72,529.52,529.32,529.72,531.12,533.02,533.82,533.82,537.62,540.92,541.22,555.52,558.32,560.32,564.52,566.12,566.72,566.72,570.42,570.8

17.0217.0317.04

11.4612.1712.57

2,563.42,566.02,567.5

32,571.032,572.832,574.9

32,569.732,571.432,572.8

32,572.632,573.232,574.9

32,570.832,572.332,573.6

181920212223

12.9814.5115.9517.1418.0218.78

2,569.02,576.52,580.02,586.62,589.02,594.4

32,576.632,584.632,591.032,594.632,597.532,601.0

32,574.132,582.732,590.532,594.032,598.632,600.3

32,577.032,585.832,592.032,595.932,598.832,602.1

32,574.832,584.232,591.232,596.032,599.532,600.9

24252627

20.9022.2623.3325.06

2,600.32,607.52,614.52,619.2

2,608.32,613.92,620.52,627.9

2,608.32,613.02,620.62,627.9

2,609.02,614.92,621.82,628.6

2,609.02,614.12,621.82,628.6

28293031323334

26.2327.1227.8428.1628.6029.5030.91

2,622.22,627.12,630.82,629.02,632.22,634.82,641.5

32,630.732,633.832,637.732,638.532,641.232,644.332,650.5

32,630.432,633.032,638.232,639.232,640.532,644.632,650.7

32,632.132,635.432,639.132,639.632,643.732,645.932,652.0

32,631.732,634.432,639.732,641.132,641.632,645.732,651.8

3536373839

32.2233.5934.4536.0437.18

2,648.62,651.72,658.02,664.02,666.4

2,655.62,661.42,664.52,671.02,673.5

2,655.82,661.42,664.52,671.02,673.3

2,657.42,663.02,665.22,671.82,674.2

2,657.32,662.12,665.72,671.72,674.1

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

[Table 4 continued on sheets 2 and 3. 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). Abbreviations: DS, downstream side ofbridge; US, upstream side of bridge]

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)


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

Reach 1 (Map segment 1)

—Reach 2 Split-Flow 1 (Map segment 1)





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.

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

[Table 5 continued on sheet 3. Distance upstream from cross section 1 measured along channel centerline shown on U.S. GeologicalSurvey 7.5-minute quadrangle maps. Vertical coordinate information referenced to North American Vertical Datum of 1988 (NAVD 88)]

2,571.02,572.82,574.9

2,576.62,584.62,591.02,594.62,597.52,601.0

2,630.72,633.82,637.72,638.52,641.22,644.32,650.5

2,569.72,571.42,572.8

2,574.12,582.72,590.52,594.02,598.62,600.3

2,630.42,633.02,638.22,639.22,640.52,644.62,650.7

2,571.02,571.32,571.7

2,573.32,583.82,591.32,595.72,598.72,599.3

2,630.82,634.12,640.02,640.12,640.22,644.42,652.5

2,569.72,571.42,572.8

2,574.12,582.72,590.52,594.02,598.62,600.3

2,630.42,633.02,638.22,639.22,640.52,644.62,650.7

17.0217.0317.04

181920212223

28293031323334

Crosssection

11.4612.1712.57

12.9814.5115.9517.1418.0218.78

26.2327.1227.8428.1628.6029.5030.91

Distanceupstream

from crosssection 1,

inthousands

of feet

2,563.42,566.02,567.5

2,569.02,576.52,580.02,586.62,589.02,594.4

2,622.22,627.12,630.82,629.02,632.22,634.82,641.5

Minimumstreambedelevation,

in feet

Main channel Left-flood plain

Existingcondition

(with levee)


Existingcondition

(with levee)


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

indicated condition




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

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

Reach 1 (Map Segment 1)

12

2 DS2

2 US2

3456

6 DS2

6 US2

789

9 DS2

9 US2

10111213141516

16 DS 2

16 US 2

1717.01

1,460150

140

140550

1,000250130

110

110

190350

70

70

70

350280170750820

1,090130

13 0

130

820900

1,460150

140

140400410150130

110

110

180200

70

70

70

1508090

490400740130

130

130

540750

2,518.02,519.4

2,520.2

2,520.42,521.82,523.02,524.52,528.2

2,528.1

2,528.3

2,529.22,531.52,531.9

2,533.1

2,533.5

2,535.72,539.32,553.82,556.82,559.42,563.82,565.0

2,565.4

2,565.4

2,568.72,569.3

2,518.02,519.4

2,520.2

2,520.42,521.82,523.22,524.72,528.2

2,528.1

2,528.3

2,529.22,531.52,531.9

2,533.1

2,533.5

2,535.72,539.32,554.12,557.22,559.42,563.92,565.0

2,565.4

2,565.4

2,568.72,569.3

0.0.0

.0

.0

.0

.2

.2

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.3

.4

.0

.1

.0

.0

.0

.0

.0


24 430 430 2,608.3 2,608.3 .025 850 580 2,613.9 2,613.9 .026 1,000 400 2,620.5 2,620.5 .027 950 540 2,627.9 2,628.0 .1

28 100 100 2,630.7 2,630.9 .229 150 150 2,633.8 2,634.1 .330 110 110 2,637.7 2,638.0 .331 60 60 2,638.5 2,638.7 .232 125 125 2,641.2 2,641.7 .533 100 100 2,644.3 2,644.6 .334 330 330 2,650.5 2,650.8 .3

35 140 140 2,655.6 2,655.6 .036 650 310 2,661.4 2,661.5 .137 700 180 2,664.5 2,664.5 .038 1,060 870 2,671.0 2,671.4 .439 960 610 2,673.5 2,673.5 .0

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

3

Crosssection

(Mapsegments

1-3)

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

Reach 5 (Map Segment 1)3


Cross sections not shown on map segments.

Calculated water-surface elevations in Reach 5 are for main channel only.

Year

1907

1907

1908

1909

1910

1916

1948

1997

5 Data for Confederated Salish and Kootenai Tribes gaging station below the intake to K Canal(Seth V. Makepeace, Confederated Salish and Kootenai Tribes of the Flathead Nation, writtencommun., 2003).

4 Data for streamflow-gaging station 12388200 from U.S. Geological Survey (2004).

Table 1. Largest estimated and recorded annual-peak discharges for theJocko River, western Montana.

[Abbreviation: e, estimated. Symbol: --, no data]

1 Data for streamflow-gaging station 12388000 from U.S. Geological Survey (1955).2 Data for streamflow-gaging station 12384500 from U.S. Geological Survey (1955).3 Data for streamflow-gaging station 12388000 from Rantz and Riggs (1949).

Discharge of Jocko River, in cubic feet per second

At Dixon11,860

11,540

17,500e

11,540

11,590

--33,700e

42,710

Near Arlee

--

--26,200e

21,480

--21,630

22,660e

52,300

Stream reach(fig. 1)

Drainagearea,

in squaremiles

Crosssectionnumbers

100 500 May 1997

Mouth of Jocko River to Valley Creek(includes reaches 1-5 and part of reach 6)

1380

21-71 4,790 7,510 2,710

Valley Creek to Finley Creek (includes partof reach 6, reach 7, and part of reach 8) 236 72-101 4,400 6,760 2,500

Finley Creek to upstream study areaboundary (includes part of reach 8)

5157 102-112 4,090 6,170 2,300

5 Drainage area and peak discharges are the same as those determined for streamflow-gaging station Jocko River below Big Knife Creek, near Jocko(station 12384500).

1Drainage area and peak discharges are the same as those determined for streamflow-gaging station Jocko River at Dixon (station 12388200).

2 Valley Creek channel joins the Jocko River channel between cross sections 68 and 69. Flood discharges from Valley Creek mix with flood dischargeson Jocko River upstream to about cross section 71.

Table 2. Drainage-area, flood-frequency, and May 1997 peak flow data for selected stream reaches on theJocko River, western Montana.

Peak discharge, in cubic feet persecond, for indicated recurrenceinterval in years or specified date

3 3

3 3 5

4

1

44

Estimated by applying the log Pearson Type III probability distribution to the recorded and estimated peak discharges at U.S. Geological Survey andConfederated Salish and Kootenai Tribes of the Flathead Nation gaging stations.

3

Estimated based on linear interpolation between logarithms of flood discharges at Jocko River at Dixon and logarithms of flood discharges at JockoRiver at Arlee using logarithms of drainage area as the basis for interpolation (Parrett and Johnson, 2004).

4

Table 3. Estimated distribution of flood discharge between the main channel and the left part of the flood plain for split-flow reaches on theJocko River, western Montana.

Peak discharge, in cubic feet per second

100-year recurrenceinterval

500-yearrecurrence interval May 1997Stream

reach

Crosssectionnumbers Main

channelLeft-flood

plainMain

channelLeft-flood

plainMain

channelLeft-flood

plain

Reach 2—split-flow 1

Subreach 17.02

Subreach 17.03

Subreach 17.04




17.02-17.04

17.02

17.03

17.04

18-23

28-34

92-93

Varies 1

2,910

3,710

3,890

3,890

4,390

2,940

Varies1

1,880

1,080

900

900

400

1,460

Varies1

2,910

3,710

4,410

5,070

6,590

3,300

Varies1

4,600

3,800

3,100

2,440

920

3,460

2,710

2,710

2,710

2,710

2,710

2,710

2,380

0

0

0

0

0

0

120

1Distribution of discharge between the main channel and left-flood plain changes throughout the reach.

Maparea

M O N T A N A

Hydrologic AnalysisThe annual-peak discharge that has a 1-percent chance of

exceedance in any year was selected by the CSKT for analysis to beconsistent with other flood studies in Montana. This peak discharge isexceeded, on average, once in any 100-year period and commonly isreferred to as the 100-year flood discharge. However, larger floods canoccur at shorter intervals. To indicate the flood limits associated with

an even rarer flood discharge than the 100-year flood, the CSKT alsoselected the 500-year flood discharge (0.2-percent chance ofexceedance) for analysis.

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 usingmethods described by the Interagency Advisory Committee on WaterData (1982). The peak discharges for 1908 (7,500 ft3/s) and 1948(3,700 ft3/s) are considered to be the largest during the entire periodfrom 1907 to 2001; thus, these values were treated as historic peakdischarges (largest peak discharges in 94 years) as described by theInteragency Advisory Committee on Water Data (1982).

The 100- and 500-year flood discharges for Jocko River nearArlee were estimated by applying the log Pearson Type III probabilitydistribution to the recorded and estimated peak discharges at the USGSand CSKT gaging stations for the period of record from 1908, 1916,1948, and 1990 to 2001. The peak discharges for 1908 (6,200 ft3/s) and1948 (2,660 ft3/s) also were treated as historic peak discharges (largestpeak discharges in 94 years).

Flood discharges on the Jocko River change between Dixonand Arlee as Valley Creek and Finley Creek join the river. From themouth of the Jocko River to Valley Creek (fig. 1), the 100- and 500-year flood discharges were assumed to be the same as those determinedfrom the gaged data at Jocko River at Dixon. Upstream from ValleyCreek to Finley Creek, the 100-year and 500-year flood dischargeswere estimated based on a linear interpolation between logarithms offlood discharges at Jocko River at Dixon and logarithms of flooddischarges at Jocko River near Arlee using logarithms of drainage areaas the basis for interpolation (Parrett and Johnson, 2004). The 100- and500-year flood discharges upstream from Finley Creek to the upstreamlimits of the study area were assumed to be the same as thosedetermined from the gaged data near Arlee. Drainage area and selectedflood-frequency data for the Jocko River are summarized in table 2(sheet 1). Recorded and estimated peak-flow discharges for May 18,1997, also are included in table 2.

Hydraulic Analysis

MethodsHEC-RAS, version 3.1.1, a one dimensional, hydraulic-flow

model developed by the U.S. Army Corps of Engineers (2001a,b,c),was used to calculate water-surface elevations at cross sections for the100-year and 500-year flood discharges and for the 1997 peakdischarge. Channel and flood-plain geometry for 113 cross sectionswere surveyed between 1999 and 2001 for use in the model. Bridgegeometries at 9 of the 113 sections located just downstream frombridge crossings were surveyed. Data from surveyed cross sectionsdownstream from the bridges were used to estimate channel and flood-plain geometry at the upstream and downstream edges of the bridgesfor use in the HEC-RAS model. For clarity, only the surveyed crosssection immediately downstream from each bridge is shown on the mapsegments. Data from interpolated cross sections were used at 15 morelocations where the hydraulic-model results indicated that additionalchannel and flood-plain geometry were required. For clarity, only 5 ofthe 15 interpolated sections are shown on the map segments. Overall, atotal of 146 cross sections (113 surveyed cross sections, 15 interpolatedcross sections, and 18 cross sections at the upstream and downstreambridge edges) were used for hydraulic analyses. Data for the crosssections are available in files at the USGS Montana Water ScienceCenter office in Helena, Mont.

Cross sections are numbered in increasing order upstream.The interpolated cross sections are designated on the maps and tableswith a whole number and two digits following the decimal point. Thewhole number corresponds to that of the closest downstream surveyedcross section, and the digits after the decimal point are numbered inupstream order. For example, four cross sections were interpolatedbetween cross sections 17 and 18 (Map segment 1). The cross sectionfarthest downstream was designated as 17.01, while the cross sectionfarthest upstream was designated as 17.04. To describe the hydraulicanalysis, the Jocko River was divided into eight reaches (fig. 1). Allstudy reaches, except the first, begin halfway between the first crosssection in the reach and the next downstream cross section outside the

reach. Likewise, all study reaches, except the last, end halfwaybetween the last cross section in the reach and the next upstream crosssection outside the reach. The first study reach begins at the first(farthest downstream) cross section in the study area, and the last reachends at the last cross section (farthest upstream) in the study area.Study reaches are described in upstream order in subsequent sections ofthe report.

Manning’s roughness coefficients (Henderson, 1966) werenoted in the field at the time of the survey. The roughness coefficientswere subsequently checked and adjusted for consistency during thehydraulic analysis. The elevations of high-water marks, such as debrisdeposits, from the 1997 and earlier floods were surveyed where markscould be found. These high-water mark elevations were compared withwater-surface elevations initially calculated by the model. Thecomparisons were used to verify the roughness coefficients, whichranged from 0.035 to 0.130 for the flood plain and from 0.035 to 0.050for the main channel. Both ranges are similar to values for Manning’sroughness coefficients that were calculated from measured dischargeand channel-geometry and flood-plain data for similar streams (Barnes,1967; Arcement and Schneider, 1989; and Coon, 1998).

Field surveys and elevations were referenced to NationalGeodetic Survey bench marks along the Jocko River valley (Mapsegments 1-3). Information about the bench marks can be obtainedfrom the National Geodetic Survey at http://www.ngs.noaa.gov(accessed July 2005). Additional elevation-control points wereobtained from the CSKT (Map segments 1-3).

Split-Flow EstimationAt many locations along the Jocko River, the 100- and 500-

year flood discharges can overtop channel banks. At most of theselocations, water on the flood plain flows in the same direction as that inthe channel, and the water-surface elevation is the same on both sides ofthe flood plain, as measured on a line perpendicular to the flowdirection. However, along Reaches 2, 3, 5, and 7 (fig. 1), floodwater onthe left-flood plain (left and right defined by looking downstream; forexample, the left bank is on the south or western side of the JockoRiver) can move independently from floodwater in the main channel fora substantial distance along the channel, creating a split-flow condition.Consequently, the water-surface elevations can differ substantially fromone side of the flood plain to the other. In these split-flow reaches,discharge in the main channel and discharge on the left-flood plainwere analyzed separately.

The methods used to estimate the distribution of flooddischarges in the split-flow reaches (table 3, sheet 1) are described inthe following sections. In Reaches 2, 3, and 5, the split-flow conditionswere, at least partly, the result of berms and levees on the left bank(Map segment 1). Although berms and levees are present in all reachesof the Jocko River, only in Reaches 2, 3, and 5 are they long enoughand high enough to create split-flow conditions. The berms and levees

section in the reach and the next downstream cross section outside thereach. Likewise, all study reaches, except the last, end halfway

occur at shorter intervals. To indicate the flood limits associated withan even rarer flood discharge than the 100-year flood, the CSKT also

Water-Surface Elevations andBoundaries of the 100-Year and500-Year Floods and Floodwayfor the Jocko River, WesternMontanaBy Katherine J. Chase and Charles Parrett

IntroductionThe 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. The Jocko River also is an important trout fishery and atributary to the Flathead River. Much of the agricultural land along theJocko River is being considered for residential and commercialdevelopment. This development has the potential to affect the JockoRiver and the fishery. In addition, proposed levee modifications andstream-restoration work could further affect the Jocko River channeland its aquatic habitat.

The Confederated Salish and Kootenai Tribes of the FlatheadNation (CSKT) needed information for assessing the effects ofproposed development and stream-restoration work on the Jocko Riverchannel and fishery. The potential for increased development andstream-restoration work highlighted the need for a better understandingof the boundaries of the 100-year and 500-year floods. To that end, theU.S. Geological Survey (USGS), in cooperation with CSKT, conductedhydrologic and hydraulic analyses of the Jocko River to identify areasalong the river that are subject to flooding.

This report presents water-surface elevation data for flooddischarges having recurrence intervals of 100 and 500 years and depictsmapped flood and floodway boundaries. The flood boundaries showthe extent of flooding under existing (1999-2001) conditions from flooddischarges having recurrence intervals of 100 and 500 years. Themapped floodway boundaries show the area within the boundaries ofthe 100-year flood discharge that is reserved for the passage of floodflows under Montana Administrative Rule 36.15.502 (1995). Thehydrologic analysis is based on recorded annual-peak discharge data forvarious years from 1907 to 1998 at four streamflow-gaging stationswithin or near the study area that extends from the fish hatchery nearArlee to the mouth of the Jocko River near Dixon. The hydraulicanalysis is based on channel- and bridge-geometry data collectedbetween 1999 and 2001.

Description of Study AreaThe Jocko River and its major tributaries, Finley Creek, Spring

Creek, and Valley Creek, are perennial streams. The Jocko Riveroriginates in the Mission Mountains east and north of Arlee and flowsthrough a relatively narrow valley corridor in Lake and SandersCounties before joining the Flathead River near Dixon (fig. 1, sheet 1).The Jocko River valley is bounded by the Mission Range on the eastand mountains along the Reservation Divide on the south and west.Elevations in the basin range from more than 7,700 ft (NGVD 29) inthe mountains to 2,800 ft (NGVD 29) along the valley floor (Kendy andTresch, 1996).

The river channel is incised in some reaches and braidedacross the valley floor in other reaches within the study area. Severalabandoned meander channels form depressions along the river in thebraided reaches. In addition, irrigation-canal berms and flood-protection levees have been constructed on one or both sides of the riveralong much of the study area. The study area was divided into eightreaches, based on hydraulic conditions and the influence of the bermsand levees (fig. 1).

The climate in the Jocko River basin is characterized by warmsummers, cold winters, and moderate precipitation. Based on climate

data from 1896 to 2003 at St. Ignatius, about 5 mi northeast of Ravalli,mean monthly temperatures range from a minimum of 17.1°F inJanuary to a maximum of 84.3°F in July (Western Regional ClimateCenter, 2003a). The average annual precipitation at St. Ignatius is 15.915.9 in. However, the average annual precipitation increasesdramatically with altitude in the basin and probably reaches a value ofabout 80 in. in the Mission Mountains (Western Regional ClimateCenter, 2003b).

Rangeland and irrigated hay and grain production are thepredominant land uses throughout the study area. The Flathead AgencyIrrigation Division manages an elaborate system of canals that divertand distribute water from the Jocko River and its tributaries (Kendy andTresch, 1996). Only the largest canal (the K Canal) is shown on figure1. The only incorporated towns in the valley are Arlee (population 602)and Dixon (population 216) (U.S. Census Bureau, 2005).

Flood Conditions and HistoryMost flood runoff in the Jocko River basin results from spring

snowmelt, spring snowmelt mixed with rain, or spring rains. Recordedflood-discharge data for the Jocko River are available for sites nearArlee, Ravalli, and Dixon.

Flood-discharge data are available for two streamflow-gagingstations in the study area and for two located upstream from the studyarea. A streamflow-gaging station on the Jocko River at Dixon (fig. 1,station 12388200) has been operated by the USGS since 1990. TheCSKT operated a peak-discharge gaging station (crest-stage gage) atthe same location from 1986 to 1989. The USGS also operated astreamflow-gaging station at Jocko River at Ravalli (station 12388000)from 1906-1914. Annual-peak discharge data for this site are available

from 1907-1910 and 1948 and are considered to be equivalent to peak-discharge data at the current site at Dixon. Annual-peak discharge datafor stations 12388200, 12388000, and the CSKT crest-stage gage werecombined for analysis and the combined data will hereafter be referredto as annual-peak discharges for the Jocko River at Dixon.

Upstream from the study area, the CSKT have operated astreamflow-gaging station on the Jocko River below the intake to KCanal near Arlee since 1990 (fig. 1). Also upstream from the studyarea, the USGS operated a streamflow-gaging station, Jocko Riverbelow Big Knife Creek, near Jocko (station 12384500), from 1909-1916. Annual-peak discharge data for 1909, 1916, and 1948 areavailable for this station. Annual-peak discharge data for station12384500 are considered to be equivalent to those at the CSKT sitenear Arlee. Annual-peak discharge data for station 12384500 and theCSKT site below the intake to K Canal also were combined for analysisand the combined data will hereinafter be referred to as annual-peakdischarges for the Jocko River near Arlee.

Large floods on the Jocko River are known to have occurred in1908, 1948, 1964, and 1997. The largest known peak discharge on theJocko River was estimated to be 7,500 ft3/s in 1908 at Dixon (table 1,sheet 1). Peak discharge upstream near Arlee on the same date wasestimated to be 6,200 ft3/s. During the 1948 flood, peak discharges atthe same sites at Dixon and near Arlee were estimated to be 3,700 ft3/sand 2,660 ft3/s, respectively. Although peak-discharge data are notavailable for the Jocko River for 1964, the flood of June 7-9, 1964, isremembered by long-time residents in the area as one of the largest(Seth V. Makepeace, Confederated Salish and Kootenai Tribes of theFlathead Nation, written commun., 2005). The most recent flood onthe Jocko River produced a peak discharge of 2,710 ft3/s (the station atDixon, station 12388200) on May 18, 1997. The peak discharge on thisdate upstream near Arlee was 2,300 ft3/s.

summers, cold winters, and moderate precipitation. Based on climatedata from 1896 to 2003 at St. Ignatius, about 5 mi northeast of Ravalli,

from 1907-1910 and 1948 and are considered to be equivalent to peak-

U.S. GEOLOGICAL SURVEY STREAMFLOW-GAGING STATION AND NUMBER12388200

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