UNITED STATES

DEPARTMENT OF THE INTERIOR

BUREAU OF RECLAMATION

OBSERVATION OF SEDIMENTATION

I

IN

PATHFINDER RESERVOIR, WYOMING

SEDIMENTATION SECTION

HYDROLOGY BRANCH

DIVISION OF PROJECT INVESTIGATIONS

DENVER, COLORADO

MAY 1959
OBSERVATION OF SEDIMENTATION

IN

PATHFINDER RESERVOIR, WYOMING

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CONTENTS

Page

Introduction 1

Purpose 1

Description 1

History 2

Sedim ent Gravity Flows 3

Sedimentation Pattern 4

Sedimentation at Pathfinder Dam 4

Sedimentation on Sweetwater Arm 5

Delta Area 5

Narrows Area 5

Sedimentation on North Platte Arm 6

Sedimentation in the Central Basin 6

Effect of Reservoir Evacuation 7

Summary 7

Acknowledgments 10

References 11

Cover Photo No. PP-2816
INTRODUCTION

Purpose

This report is a summary of information on reservoir sedimentation obtained from an inspection of Pathfinder Reservoir during

October 1958.

To facilitate construction of the power tunnel from Pathfinder

Reservoir to the Fremont Canyon Powerplant and sealing of the south

outlet work tunnels at Pathfinder Dam, Pathfinder Reservoir was completely evacuated in the fall of 1958. This evacuation provided an opportunity to sample, inspect, and photograph sediment deposits of up to

50 years of age in the reservoir. The information gained from the inspection will be helpful in the study, evaluation, and development of procedures for predicting sediment accumulation and behavior in existing

and planned reservoirs.

Description

Pathfinder Dam is a masonry, arch-gravity type dam situated

in a deep, narrow, granite canyon. The dam ha.s a structural height of

214 feet, a hydraulic height of 184 feet, a base width of 96.5 feet, and

a crest width of 10.9 feet. The crest of the dam is 432 feet long at an

elevation of 5858. 1 feet (USGS datum).

Pathfinder Reservoir extends 24 miles up to the North Platte

River and 19 miles up the Sweetwater River from Pathfinder Dam. The

maximum width of the reservoir is about 3 miles. At the spillway elevation (5850. 1 feet) the reservoir had an initial surface area of 22, 600

acres and an initial capacity of 1, 056, 300 acre-feet.

The lake beginning a.t the dam is confined to a narrow canyon

about 120 feet deep and less than 300 feet wide below elevation 5810.

Above this elevation the lake widens to 1/2-mile at spillway crest elevation. About one-third mile above the dam, the narrow canyon ends

and the whole lake broadens abruptly to a. width of one-half mile or more.

About 1-1/2 miles west of the dam, the lake is restricted by the base of

a. mountain and a rock point before opening into a large central ba.sin

formed by the confluence of Sand Creek, Canyon Creek, and the Sweetwa.ter River with the North Platte River. This basin is approximately 8

miles long and has a. maximum width of 3 miles. The North Platte River

arm enters this basin midway along the east side, 7-1/2 miles above the

dam, through a. relatively narrow rugged valley varying in width from 1/4

to 1/2 mile. About 4-1/2 miles above the North Platte entrance to the

basin, the lake na.rrows into a canyon section approximately 500 feet

wide for a distance of about 1-3/4 miles. Continuing upstream, the lake

gradually widens to about 1 mile a.nd stays this width to the upper end before narrowing to the back water area. 10 miles above the tnarrowst! or

24 miles above the darn.
The Sweetwater River arm enters the central basin from the

north through a. rugged canyon about 1/2 mile wide. About 3-1/2 miles

above its confluence with the North Platte River, the lake narrows to a

width of 800 feet and turns sharply west. Immediately above the "narrows" the lake widens to about 1 mile for a. distance of 3 miles and then

widens again to its maximum width of 1-1/2 miles where Horse Creek

enters the reservoir 13-1/2 miles above the dam. The lake narrows

again to a width of 300 feet a.t a distance of 18 miles from the dam.

The lake widens again slightly before tapering off in the back water area

19 miles above the dam.

A general map of the reservoir with established sedimentation

ranges is shown on Figure 1.

History

Approval was given for construction of Pathfinder Dam in 1904

and construction was started in 1905. The dam and associated works

were essentially completed in 1909 when closure was made. In 1939

Seminoe Dam was placed in operation 6 miles above the high water line

of Pathfinder Reservoir. In September 1950 Kortes Dam, constructed

between Pathfinder Reservoir and Seminoe Dam, was closed. Since

1939 little sediment has entered Pathfinder Reservoir from the North

Platte River flow.

A resurvey of Pathfinder Reservoir was made in 1950 11(1)

which resulted in a. revision of the original capa.city and a. correction of

the Pathfinder datum to the U. S. Geological Survey datum. The original capacity was revised to 1, 056, 300 acre-feet a.t spillway crest elevation 5850. 1 instead of 1, 070, 000 acre-feet at spiliway crest elevation

5852. The 1950 resurvey showed a sediment accumulation of 40, 400

acre-feet in the reservoir between 1909 and 1950, indicating an annual

unit yield rate of 0.117 acre-feet per square mile for 1909-1939 and

0.079 acre-feet per square mile for 1939-1950 for the drainage area

above the dam. Sediment deposits were distributed throughout the reservoir in 1950 and have not changed to any great degree since that. time.

The original and 1950 thalweg profiles and the area-capacity

curve for 1950 are shown in Figures 2 and 3 respectively. Figure 2

may be misleading in some respects since it is a thalweg profile. It

indicates no deposition is present in the upper North Platte arm of the

reservoir. Actually, a great deal of deposition has taken place but the

river has degraded through the deposits back to its original profile a.s

explained later in this report. Figure 2 also gives no information on

lateral extent of the sediment deposits.

1/ Numbers in parenthesis refer to articles listed as references l the end of this report.

2
SEDIMENT GRAVITY FLOWS

For purposes of clarification, this discussion is added to this

report to assist in the description of the existing sedimentation pattern

in Pathfinder Reservoir. The mode of sediment distribution within the

lower reservoir area. is explained to a large degree by the action of

sediment gravity flows.

A gravity flow can be described as the flow of any fluid caused

by ravitational forces. A density current has been described by Bell (2)

as A gravity flow of a fluid through, under, or over a fluid of approximately equal density. According to Lane (3) one class of density currents called turbidity currents usually involve the deposition of sediment

in reservoirs and is a current "with movement due to the suspensionof

sediment in the flowing water. Lane defines three types of turbidity

currents depending on their action when flowing through a reservoir:

(1) underflows, (2) interflows, and (3) overflows. In this Pathfinder

sedimentation report, the underflowing turbidity current is of main interest. Lane further states:

"There is reason to believe, however, that in many

reservoirs the inflowing water does not dive under the

clear water, but pushes it downstream and forms a considerable body of muddy water, in which the sediment is slowly

settled to the bottom . . . . When this sediment reaches the

bottom, some of it collects in the form of a dense fluid,

which flows down the banks to the main channel and then

flows down the main channel in an underflowing turbidity

current, . . . . This means of formation of a density current is not so easily detected . . and has not been directly

observed but its existence is inferred from the deep deposits

of fine material in the streambed near the upper ends of

some large reservoirs. These deposits are much deeper

compared with the deposits outside the channel, than would

occur if there were no movement of the settled sediment

after it reached the reservoir bottom."

Lane also states:

"The velocity of flow in an underflow type of turbidity current depends on the density of the turbid water,

the depth of flow, and the steepness of the channel. The

force per unit volume causing the flow increases with

an increase in the density between the two fluids. It increases with an increase in the bottom slope and with

an increase in the thickness of the heavier liquid."
SEDIMENTATION PATTERN

Sedimentation at Pathfinder Dam

At the time of observation, sediment deposits extending from

the dam through the narrow gorge section were being eroded to some

extent as the river cut a new channel through the deposits (Photograph

2817, Figure 4). Construction work had disturbed the sediment deposits near the dam and north outlet trashrack. The river as it approached the north outlet was flowing in its original bed of cobbles and

entered the tunnel a.t sill elevation. Drainage of bank storage in the

deposits was causing consolidation of the deposits in the gorge. A triangular area. of deposits next to the dam was virtually undisturbed by

construction activity or river action and Sample P-i was secured from

this area. (Photograph 2835, Figure 4, Table 1, and Figure 15). The

elevation of this deposit was determined as 5696. 7 feet or 28.6 feet

above the north outlet sill. The resurvey of 1950 showed a deposit eleva.tion of 5690. 1 at Range 1. The sediment deposited in the gorge section and immediately above is coarse silt to fine sand with about 15 percent clay (Sample P-i, Figure 15).

Above the gorge section where the valley widens abruptly,

the old channel and flood plain are covered with a light density sediment deposit of clay, silt, and fine sand. The deposited sediment is

level. Some consolidation has taken place within a. few feet of the river

where some drainage has occurred after reservoir evacuation. This

light-density depositwas in the lowest part of the channel and floodplain, but no deposition other than a thin scum was evident above the

deposit level in the channel flood plain. From this evidence, it seems

logical that particle settlement at this location is not responsible for

the deposits but that the deposits are the result of density flow. Photographs PP-2815 and 2816 (Figure 5) show parts of bench slopes with no

sediment deposits that run directly into the light density deposits. It

can be concluded that even the lightest particles of incoming sediment

have settled to the bottom of the reservoir by the time they travel from

the reservoir backwater reaches to the area adjacent to the dam. Further conclusions indicate that settlement to the reservoir bottom does

not stop the downstream movement of sediment of small particle sizes

in this large reservoir but the particles are carried by density flows

on down to the dam. In Pathfinder this movement includes particle

sizes up to 0. 5 mm with dry densities oi up to 80 pounds per cubic foot

or higher, depending on exposure, drainage, and length of time for compaction.

Small tributaries entering this area. had deposited a few feet of

sand along their channels in contrast to the light density deposits along

the river channel.

4
Sedimentation on Swe etwate r Arm

Delta Area.

The Sweetwa.ter River does not carry an appreciable sediment

load into Pathfinder Reservoir but at the present time is the major contributor of sediment since the North Platte River is controlled by

Kortes and Seminoe Reservoirs immediately upstream. Sediment deposits of depths of 6 to 7 feet have occurred on the upper part or the

reservoir in what could be called the delta area. The Sweetwater River

flows in the shallow channel 25 feet wide in a meandering pattern across

the deposits. Deposition has taken place over the flood plain of threefourth mile in width (Photograph PP-2818, Figure 6).

The river channel follows the original meander pattern except

for the reach between Ranges 39A and 40. In this reach the channel

has changed from a. meander pattern of about 1, 500 feet in width to a.

fairly straight channel with no indication of meander. This change occurredduringa period of decreasing slope. This probably is due to a.

change in channel regime from decreasing load and slope.

A sample secured from the bank near the river is shown in

Photogra.phs PP-2820, Figure 7. The top part of the sample, representing recent deposition, was jelly-like clay and silt. The deposited

material increased in particle size with depth and changed to fine sands

near the sample break in Photograph 2820. Below the fine sands the

material graduated to coarse sands and fine gra.vels of up to 9 mm in

size. Sample P-2 shown in PP-2822, Figure 7, was taken from the

edge of the river. The top two-thirds of the sample was a. jelly-like

suspension of silt and clay which at first appeared solid. After a few

hours, the suspension broke and the suspended materials settled to a.

depth of about 3 inches. This suspension had a. dry density of 16 pounds

per cubic foot (P-2T, Figure 15, and Table 1). The coarse sand and

fine gravels ha.d a. density of 118 pounds per cubic foot (P-2B, Figure

15, Table 1). The coarse sample was very similar to the bed material

of the Sweetwater River above the back water reaches of the reservoir.

Narrows Area.

The thalweg profile shown in the 1950 resurvey (Figure 2)

shows approximately 17 feet of deposits a.t Range 34A above the original

river bed. At the present time, the river in the Sweetwater arm of

Pathfinder Reservoir near Range 34Aappears to be flowing in its original cobble bed after cutting through only 3 ,to 4 feet of sand deposits

(Figure 8). The material deposited near the entrance to the "narrows"

section is 6 percent clay, 15 percent silt, 74 percent sand, and 5 percent gravel up to a. maximum size of 9 mm (Sample P-3, Figure 15, and

Table 1). The 'narrows" section appears to be an effective filter to settle out particles of a. wide range in size. Silt and clay particles that are

carried into the "narrows" section a.ppear to be carried on through it in

suspension.

5



Sedimentaüon on North Platte Arm

Since construction of Seminoe Dam in 1939 above Pathfinder

Reservoir, sediment deposition on the North Platte arm of Pathfinder

Reservoir has virtually stopped. Deposits made before 1939 are in

place and undisturbed except for a degraded channel which was cut by

clear water, fluctuating releases from Seminoe and Kortes Reservoirs.

The river is now flowing in its original alluvial gravels and cobbles after

cutting through a. maximum depth of about 15 feet of sediment deposits

and some original alluvial gravels near the "narrows" section (Figures

10, 11, and 12). The deposits vary in depth from 15 feet at the "narrows"

to a. few feet at the upper end of the reservoir. Since the river is flowing

in original alluvial material all the way down to the "narrows" the 1950

thalweg profile shows no difference in elevation between the 1950 profile

and the original profile.

Undoubtedly clear water releases from Seminoe and Kortes

have caused the degrada.tion of the river back to the original profile by

the reduction of spring flood flows with high sediment loads to a. higher

sustained clear wa.ter flow during the summer when the wa.ter surfa.ce

in Pathfinder Reservoir is lowered from releases a.t Pathfinder Dam.

It is apparent that only the deposits over the present channel have been

removed with the major deposition over the flood plain still undisturbed

by reservoir level or clear water releases upstream (Figure 10).

The deposits tha.t have been removed to form the present channel are not necessarily those that covered the original channel. Near

Range 17 just above the "narrows" the original river channel in 1903

followed the western hillside in a. semicircle before entering the "narrows." The channel was in this same location in 1931, but between 1931

and 1950, the channel shifted to the east and now runs a. fairly straight

line across the original semicircle (see Figure 1 and Figure 10).

The material deposited above the "narrows" shows the effect

of trapping action similar to the Sweetwater "narrows." The deposits

are a. sandy loam, with yearly layers topped by vegetation, composed

of 7 percent clay, 22 percent silt, and 71 percent sand below a maxi

mum particle size of 1 mm (Sample P-4, Figure 15 and Table 1).

Sedimentation in the Central Basin

The large central basin of Pathfinder Reservoir was not observed because of inaccessibility. An observation of this area would

have been most helpful in completing the pattern of deposition. However, various conclusions can be drawn from information and observations above and below the central basin. The basin, the largest part of

the reservoir, acts as a settling basin for sediment moving through the

two "narrows" section. A delta area of some type has likely developed

below the mouths of both "narrows."

6
There is a strong possibility that the density flows which carry

finer particle sizes to the downstream section of the reservoir may be

formed or certainly aided by the two "narrows" sections. The long narrow sections of each arm would effectively aid the collection of finer

particles together near the bottom of the thalweg channel into a current

or flow which, when reaching the central basin, would continue as a. density flow to the reservoir reaches near the dam.

In the lower part of the basin, complete settlement of almost

all particles is accomplished and deposits in the lower part and on

downstream are made by density flow action. The deposited material

should be a combination of sands, silts, and clays with a mean size of

about 0. 08 mm. Deposition likely is limited to a band along the original

river and flood plain whether deposited by particle settlement or density

flow action. The two rivers have probably not cut into the deposits any

more than is necessary to form a shallow channel since reservoir evacuation.

Effect of Reservoir Evacuation

Complete evacuation of Pathfinder Reservoir has had some

minor effects on sediment deposits. On the North Platte arm no effect

is apparent since the river moved into its cut channel and may have cut

only some small amount of material from the deposited banks. On the

upper Sweetwater arm, the river flows in a shallow channel and is not

cutting into the sediment deposits other than as it enters the "narrows"

section. Little consolidation is taking place as the upper area deposits

are composed mostly of larger sediment particles. In the central basin

some deposit cutting is likely taking place, providing some drainage

and some consolidation. In the area. adjacent to the dam and below the

central basin, some drainage is occurring next to the river channel and

therefore some consolidation. In the gorge section, deposits have been

cut by the river because the slope is increased and a moderate amount

of consolidation has occurred. Material that has been cut from the

lower reservoir area is carried through the outlet works and is deposited downstream from the dam (Figures 13 and 14). A sample analysis of this material showed 5 percent clay, 15 percent silt, and 80 percent sand below a maximum size of 2 mm (Sample P-5, Figure 15, and

Table 1). Silts and clays finer than Sample P-5, cut from the lower

reservoir deposits have been carried on downstream to Alcova Reservoir and, therefore, this size analysis is an indication of maximum size

material removed from Pathfinder. It was apparent that only a very

small amount of sediment has been removed from the reservoir since

evacuation.

SUMMARY

Three types of sediment action influence the pattern of deposition taking place in Pathfinder Reservoir: (1) deposition of bed material

7
particles, (2) settlement of suspended particles, and (3) particle movement by density flow action. Generally, these three steps account for

the deposits in order moving downstream. Overlapping of two actions

and deposits from the actions may occur within the reservoir depending

on inflow conditions, deposit slopes, channel and valley side slopes,

and reservoir level fluctuations. The uniform nature of most Pathfinder

deposits at any location and the uniform decrease in size of deposited

sediment particles with distance from the head of the reservoir shows

an orderly pattern of sediment action and deposition. At the head of

both arms the larger bed load material has deposited. From the head

on down through the middle of the reservoir, deposition is mostly the

result of suspended sediment particle settlement. In the reservoir area

upstream from the dam, evidence points to density flow action as the

cause of deposition.

The one unusual and nonuniform type of deposition in Pathfinder

Reservoir is the layer of fine material overlying the coarse bed load material in the delta. area. of the Sweetwater River area. According to the

evidence and discussion by Lane earlier presented in this report, it is

logical that the fine material is the result of an underfiowing turbidity

current formed by the settlement of suspended particles near the location of Sample P-2. By using the dry unit weight and the wet unit weight

of Sample P-2T, the maximum concentration of the suspended fine material would be 225, 900 ppm. This concentration is certainly not in excess of any concentrations ever measured in a. moving stream and could

possibly flow down the river channel to the dam while the reservoir was

full, although the concentration is higher than most direct measurements

of density currents have been in the past.

Evidence of a. possible recent density current or underflowing

turbidity current is present in the narrow gorge above the dam and at

the dam itself. In photographs PP-2817 and PP-2835 a discoloration of

the gorge walls approximately 10 feet above the sediment deposit level

is quite evident. This discolored zone could be the zone occupied by a.

recent density current upon reaching the dam. The two evidences at

extreme ends of the reservoir could be indications of an underflowing

turbidity current which passed through the entire reservoir. The most

plausible explanation of the discolored zone near the dam, however, is

the retention of a small pond of about 500 to 800 a.cre-feet for about 28

days during September 1958 before the reservoir was completely

drained. The small storage would allow the river to pick up fine sediment above the storage level and carry it to the dam. The pond would

therefor be quite muddy and could easily have discolored the gorge

rocks and dam face up to the water surface.

In the small valley immediately above the dam and gorge section, the light density, fluid mixture of fine particles, and water is not

an unusual reservoir deposit. The one fa.ct that stands out about this

type of deposit is the absence of any sediment deposit above the level in

the channel flood plain. This clearly indicates the deposit is the result

of a. flow of sediment particles rather than a. settlement of sediment

8
particles. The dam has made a eseo of sediment in the lower

area with inflow as a density flow of sediment. The two "narrows"

areas on both areas could be instrumentalin forming the density flows

in the lower reservoir by trapping larger sediment particles above and

aiding in the collection of the smaller particles into a current or flow.

These density flows in the lower reservoir area would appear to have

a much higher concentration and density than the usual density currents

or underflowing turbidity current.

A summary of the sediment samples is shown in Table 1 and

Figure 15 and their location in the reservoir is shown on Figure 1 and

Figure 16. Figure 16 shows the sample location with respect to eleva.-

tion and reservoir fluctuation over the last 10 years. The unit weight

computations(4) from particle size analysis (Table 2) are quite accurate

for deposited material in the sand, silt, and clay range but do not express an accurate unit weight for deposits containing an appreciable percent of material larger than the sand sizes. (Sample P-2T should be excluded since the laboratory unit weight is based on a suspended condition.)

The size analyses of samples obtained during this observation compare

favorably with samples obtained by the Corps of Engineers in 1931 (1)•

All deposits tend to be level at any cross section but may slope

to the channel from river action during drawdown periods as on the

Sweetwater arm or may be sharply cut as on the North Platte arm. The

two conditions on the reservoir arms make an interesting contrast in

deposit formation and river action.

Longitudinal slopes of deposits vary a. great deal throughout

the reservoir. The deposit slopes upstream from the narrow section

on the North Platte arm have decreased from an average of 7 feet per

mile to 5 feet per mile in a uniform pattern. The deposit slopes on the

Sweetwater arm are nonuniform, some slopes have increased and others

have decreased during deposition. A short slope in the Sweetwater delta

area was approximately 0.0003 while the general slope in the area. was

about 0.001. This short flat slope was in the area of deposition of the

clay and silt layer over sand and gravel (Sample P-2).

The deposition pattern in Pathfinder Reservoir is generally

typical of large reservoirs with an interesting contrast between the pattern in the two arms.

9
ACKNOWLEDGMENTS

The cooperation of Regional Director R. i. Walters, Project

Manager I. J. Matthews, and M. M. Starr, Chief, Engineering and

Project Investigations Division in arranging for the observation survey

of Pathfinder Reservoir deposits is gratefully acknowledged. The field

work, analysis of data., and preparation of this report were carried out

by Mr. John R. Sheppard of the Sedimentation Section, Hydrology

Branch, Denver. Messrs. J. B. Marshall and K. A. Grover of the

Casper Project Office assisted in the collection of the field data.

10
REFERENCES

(1) "Sedimentation Surveys of Pathfinder and Seminoe Reservoirs, North Platte River, Wyoming," Project Planning Division,

Denver, Colorado, May 1953.

(2) "Density Currents as Agents for Transporting Sediments," H. S. Bell, Journal of Geology, Vol. 50, No. 5 July-August

1942, pp. 512-547.

(3) "S me Hydraulic Engineering Aspects of Density Currents,

Hydraulic Laboratory Report No. Hyd. -373, Bureau of Reclamation,

August 31, 1954.

(4) "Determination of the Unit Weight of Sediment for Use in

Sediment Volume Computations" by Carl R. Miller, U. S. Bureau of

Reclamation, February 17, 1953.
Table 1

LABORATORY UNIT WEIGHT DETERMINATIONS

Pathfinder Reservoir

Sediment Deposit Samples

October 1958

Volume of Wet Dry

Field sample sample unit weight unit weight

No. (cu ft) (lb/cu ft) (lb/cu ft)

P-i .0236 108.9 78.4

P-2T .0329 72.6 16.4

P-2B .0195 135.7 118.0

P-3 .0269 106.3 86.2

P-4 .0430 113.6 90.7

P-5 - No unit weight test
Table 2

COMPUTED UNIT WEIGHTS

PATHFINDER RESERVOIR

SEDIMENT DEPOSIT SAMPLES

October 1958

Esti- :

:mated*:

condi- Esti- : : : LaboField : Size Analysis : tion of: mated IrLitial** : Computed: ratory

sample :(Percent by weight): depos-: age of : unit : unit : unit

No. ::Clay : Silt : Sand: its : deposits: weight : weight : weight

P-i : 15 :51 34 b : 2 68.3 : 76.7 78.4

P-2T 28 :40 32 : - 0 64.2 : - 16.4

P-2B : 1 : 2.5 : 96.5: c : 24 : 92.7 93.4 : 118.0

P-3 6 :15 :79 a 19 85.0 86.7 : 86.2

P-4 7 :23 :70 : b : 45 : 82.2 : 87.4 90.7

P-5 5 :15 :80 : - : 0 85.7 - : -

*a. Sediment always submerged or nearly submerged

b. Normally a. moderare drawdown

c. Normally considerable drawdown

d. Reservoir normally empty

**Baged on Condition a.
(P' N

PP-2817--Exposed sediment deposits between Ranges 1 and 2, Pathfinder Reservoir about one-third mile above the dam. These

clay, silt, and fine sand deposits in the narrow gorge section

have drained to some extent and are of medium density in

comparison to the saturated light density material shown in

PP-2816. The river has cut 3 to 4 feet into these deposits

since the reservoir was evacuated. 10-28-58

r2d' ii

j

I

t

-

S

-

'S

. 1 -.1

,

y

r- 3!P•'

PP-2835--View of upstream face of Pathfinder Dam after reservoir

5-

5..

evacuation. River is flowing into lower north outlet tunnel.

The triangular shaped lower right deposits of clay, silt, and

fine sand have been undisturbed by construction activity and

are at elevation 5696.7, approximately up to the top of the

north intake vertical trashracks, 1O-31-58

5-

Si

-

Figure 4
-

-:

.- -$

4__i.

-

-

-

¶

PP-2815---Exposed sediment deposits at Range 2, Pathfinder Reservoir

about one-half mile above the dam. Deposits are mostly

clay, silt, and fine sand of light to medium density. 10-28-58

PP-2816---Exposed sediment deposits between Ranges 2 and 3, Pathfinder Reservoir about three-fourths mile above the dam.

Deposits are mostly clay, silt, and fine sand of light

density that will not support the weight of a rock. Windblown sand is also apparent on the upper bench above the

river flood plain. 10-28-58 Figure 5

a-
-

. -,1-...

:',

-

•-' .. :

PP-2818- -Upper Sweetwater arm of Pathfinder Reservoir between

Ranges 38 and 39 looking upstream to the left. 10-29-58

PP-2821--Sweetwater River above Range 38, Pathfinder Reservoir,

looking downstream at delta area. Slope of deposits in this

area is approximately 0.0003. 10-29-58

Figure 6
PP-2820--Sample of bed material removed from Sweetwater River between Ranges 38 and 39, Pathfinder Reservoir. Top 6

inches of material is jelly-like clay and silt suspension.

Material below 6 inches is a coarse sand to medium gravel

deposit. 10-29-58

-

PP-2822- -Sample of deposited material from bed of Sweetwater River

between Ranges 38 and 39, Pathfinder Reservoir. The upper

two-thirds of this sample is jelly-like clay and silt which is

in a thick suspension. The suspension later broke and the

sample tube contained approximately 6 inches of clay-silt

and the rest of the upper tube was clear water. The lower

one-third of the tube is coarse sand to medium gravel.

10-29--58

Figure 7
- -

-.

.

-

:

. •L -a-. -. -

rn

PP-2823--View of "narrows" on Sweetwater arm of Pathfinder Reservoir. 10-28-58

-

....

-

PP-2824--Sweetwater River in "narrows"at Range 34A, Pathfinder

Reservoir. River has cut through 3 to 4 feet of coarse sand

deposits in this area and is in its original cobble bed.

10- 2 9-5 8

Figure 8
T

- . - .

. -

: --

- . ,- .-

PP-2825- -Sweetwater River just above Range 34A looking upstream

toward Range 35 before river enters the "narrows" Pathfinder Reservoir. The river has not cut the sediment deposits to any extent and has not reached its original cobble

bed as it has after entering the "narrows" section. 10-29-58

--

1.:

- - -' - -'
-

____ ___

- -

-

- -

- V

* -

r

- =

PP-2827- -Sediment deposits on North Platte River arm of Pathfinder

Reservoir about 1/4 mile upstream from Range 22. Deposits are about 7 feet high and were deposited before construction of Kortes arid Seminoe Dams upstream. 10-30-58

4

--

PP-2831--Flat deposition of sediment above I!narrowst of the North

Platte River arm, Pathfinder Reservoir, looking toward

Range 17. River has cut through deposits to alluvial sands

and gravels. 10-30-58

Figure 10

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PP-2828- -Sediment deposits on left bank near Range 17 above the North

Platte River Narrows, Pathfinder Reservoir. These deposits are approximately 15 feet high and composed of sandy

loam material. River has cut through the deposits to its

original alluvial sands and gravels. 10-30-58

-

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- •i

__J__

PP_2829__Narrowsu section of North Platte River arm of Pathfinder

Reservoir just below Range 17. High waterline (white) is

apparent. Lower dark line on rocks is the same elevation

as cut deposits shown on right side indicating about 20 feet

of sediment deposits were cut out of Unarrowst section by

the river. 10-30-58 Figure 11
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-

PP-2832- -Looking upstream from "narrowst' section of North Platte

arm of Pathfinder Reservoir toward Range 18. Level deposits shown on left. On right side of picture, sediment

deposits have been covered by wind-blown sand. 10-30-58

•

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PP-2833- -Left bank of cut deposits about 25 feet high near Range 17,

North Platte arm of Pathfinder Reservoir. The lower strata

is alluvial river gravels with the layered sediment deposits

above. Material above is sandy loam. 10-30-58

Figure 12
' •1

- .

PP-2830--Deposits of material washed from Pathfinder Reservoir and

deposited just below Pathfinder Dam. No deposits were

present in the river channel before the reservoir was evacuated. 10-31-58

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PP-2834- -Deposits of material washed from Pathfmder Reservoir and

deposited just below Pathfinder Dam. No deposits were

present in the river channel before the reservoir was evacuated. 10-31-58

Figure 13
;L1;

PP-2836-- -Deposits of coarse silt to fine sand in the North Platte River

about 1/2 mile below Pathfinder Dam. This material was

washed out of the reservoir and deposited in the river channel during and after the reservoir was evacuated. 10-31-58
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FIGURE 15