Geology of Waterpower Sites 

on Scenery, Cascade, and 

Delta Creeks near 

Petersburg, Alaska

GEOLOGICAL SURVEY BULLETIN 1 0[3 1 - E

Geology of Waterpower Sites 

on Scenery, Cascade, and 

Delta Creeks near 

Petersburg, Alaska

By JOHN CHARLES MILLER

GEOLOGY OF WATERPOWER SITES IN ALASKA

GEOLOGICAL SURVEY BULLETIN 1031-E

A preliminary examination of the 

geologic feasibility of proposed 

dams it es

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D.G.
CONTENTS

Page

Abstract____________._.______________.________ 103

Introduction___________________________________-_______________.__ 103

Purpose and scope_____________________________________________ 103

Maps and photographs_________________________________________ 104

Acknowledgments. __________________________________________ 104

Geography______________________________________________________ 104

Location.__________________________________________________ 104

Climate and vegetation________________________________________ 104

Accessibility ____________________________________________________ 106

Physiography. _ _____________________________________________________ 106

General geology._____--____-___-__-___-_________________________ 107

Igneous rocks___________________________________________________ 108

Metamorphic rocks ____________________________________________ 108

Unconsolidated rocks.__________________________________________ 109

Structure_____________________________________ 109

Seismic activity.____________________________________________ 110

Geology of possible powersites applied to waterpower development_____ 111

Scenery Lake powersite. _ _ _____________________________________ 112

Swan Lake powersite________________________________________ 117

Ruth Lake powersite__________________________________________ 122

References cited______________________________..__.__-____-_______ 124

Index-.----..---.-_-________________...___________ 125

ILLUSTRATIONS

[Plates in pocket]

PLATE 8. Geologic reconnaissance map.

9. Maps and sections of powersites.

Page

FIGURE 23. Index map showing location of powersites______.__.__.___ 105

ill

GEOLOGY OF WATERPOWER SITES IN ALASKA

GEOLOGY OF WATERPOWER SITES ON SCENERY,

CASCADE, AND DELTA CREEKS NEAR

PETERSBURG, ALASKA

By JOHN CHAKLES MILLER

ABSTRACT

The Scenery Lake damsite is partly in quartz diorite and partly in hornblendeplagioclase gneiss. The Swan Lake and Ruth Lake damsites are in quartz 

diorite. The bedrock at these damsites is suitable for dams of the height that 

would be required to achieve the full development of the potential power of the 

streams. No appreciable leakage from the reservoirs is anticipated.

The tunnel routes from Scenery and Swan Lakes would be in quartz diorite. 

The tunnel route from Ruth Lake would pass through quartz diorite and diorite 

gneiss.

The geologic examinations described in this report indicate that the development of the potential power of the streams under consideration is feasible as 

far as geologic conditions are concerned.

INTRODUCTION

PURPOSE AND SCOPE

This reconnaissance geologic report was made to evaluate the 

geologic feasibility of the sites for the various structures that would be 

required for the development of hydroelectric power from Scenery, 

Swan, and Euth Lakes. These lakes are on Scenery, Cascade, and 

Delta Creeks, respectively. The field examinations included the damsites at the outlets of the three lakes, the reservoir sites above these 

damsites, and tentative tunnel routes from these lakes to powerhouse 

sites at or near tidewater. The geologic feasibility of the sites under 

consideration is one of the determining factors in evaluating the waterpower potential of these streams.

The fieldwork on which this report is based was conducted during 

July and August 1951. The examination of the proposed sites of 

reservoirs, dams, and tunnels required a total of 29 days on the Scenery 

Lake and Scenery Creek areas; 21 days on the Swan Lake and Cascade

103
104 GEOLOGY OF WATERPOWER SITES IN ALASKA

Creek areas; and 1 day on the Euth Lake and Delta Creek areas. The 

author was assisted by J. L. Colbert, hydraulic engineer, who had 

worked in the area the previous season obtaining control data for the 

compilation of a topographic map.

This report in essentially its present form and under the same title, 

was released to open file by press notice No. 90568 dated December 6, 

1955.

MAPS AND PHOTOGRAPHS

A special topographic map (U.S. Geological Survey, 1952) of the 

area served as a base map for the geologic mapping. This map includes an area that extends from Scenery Lake on the north to the 

Patterson Eiver on the south, and inland 4 to 5 miles from the east 

shore of Thomas Bay. The map is on a scale of 1:24,000 with a 40- 

foot contour interval. Included on the same sheet are detailed maps 

of the Scenery Lake damsite, scale 1:4,800, and Swan Lake damsite, 

scale 1:2,400; both detailed maps have 10-foot contour intervals. Underwater contours are shown on Scenery, Swan, and Euth Lakes and 

on the damsite maps above mentioned. Profiles are shown for Scenery, 

Cascade, and Delta Creeks. Aerial photographs taken in 1948 were 

also used in the fieldwork.

ACKNOWLEDGMENTS

Acknowledgment is due to the personnel of the Alaska Communication System for their friendly cooperation and assistance in maintaining radio communications with the field party.

GEOGRAPHY

LOCATION

The area investigated is on the mainland about 20 miles northeast 

of Petersburg on the east side of Thomas Bay, an arm of Frederick 

Sound, and is about 100 miles southeast of Juneau. Scenery, Cascade, and Delta Creeks, which flow westward into Thomas Bay, are 

effluents of Scenery, Swan, and Ruth Lakes, respectively. The location of these lakes and streams is shown on figure 23.

CLIMATE AND VEGETATION

The climate is characterized by moderate temperatures the year 

round, including mild winters and cool summers, and by heavy precipitation. The mean annual precipitation at Petersburg for the 

15 years 1927, 1931-32 and 1938-49 was 111 inches. The runoff records for Scenery and Cascade Creeks indicate that the precipitation 

in the area of study is greater than at Petersburg.
WATERPOWER SITES NEAR PETERSBURG, ALASKA 105

57°

FIGURE 23. Index map showing location of powersites.

The area is within the Tongass National Forest and, except for the 

steep slopes, is covered with a dense growth of timber consisting of 

western hemlock, Sitka spruce, western redcedar, and Alaska cedar. 

From sea level to about 2,000 feet the forests have a dense undergrowth of bushes, mainly salmonberry and huckleberry, and other 

vegetation including devilsclub. Where there have been slides and 

on some of the talus slopes, willow and alder form an almost impassable barrier. The best timber is in the well-drained areas on the steep 

hillsides, but there is a large quantity of poorer grade timber suitable 

for paper pulp within the drainage areas of the three creeks and in 

the area bordering Thomas Bay.
106 GEOLOGY OP WATERPOWER SITES IN ALASKA

ACCESSIBILITY

The three damsites investigated are at the outlets of Scenery, Swan, 

and Ruth Lakes. Scenery Lake is 3 miles east of the shore of Thomas 

Bay and 957 feet above sea level; Swan Lake is about 21/£ miles east 

and 1,514 feet above sea level; and Ruth Lake is less than 2 miles 

east and 1,353 feet above sea level. A development at tidewater on 

Scenery Cove would be accessible to ocean-going tugs and large vessels. U.S. Coast and Geodetic Survey chart 8210 of Thomas, Farragut, and Portage Bays shows that soundings in Scenery Cove range 

from 9 to 21 fathoms at mean lower low water. A powerhouse for a 

development on Swan Lake, if built near the mouth of Cascade Creek 

and near sea level, would be accessible from Thomas Bay. Also 

accessible from Thomas Bay would be a powerhouse built at the mouth 

of Delta Creek. There are no roads in the report area and any road 

construction would be complicated by the extremely rugged terrain 

and the consequent occurrence of rockslides or snowslides. The trail 

up Cascade Creek is well marked to Falls Lake, and it is discontinued 

at the lake. The trail from Falls Lake to Swan Lake is very steep 

and indefinite, being obscured in places by snowslides. The U.S. Forest Service maintains a skiff on Scenery and Swan Lakes, but none is 

available on Ruth Lake. A steep trail is available near Delta Creek 

for access to Ruth Lake.

PHYSIOGRAPHY

The rugged mountainous character of the area provides excellent 

topographic locations for the three damsites examined. In the 

higher altitudes valley glaciers and waterfalls descend from the 

icecap and the melt water forms streams which enter the lakes. The 

western slope of the Coast Range has been "* * * deeply dissected by 

river erosion, modified by the great Pleistocene ice sheet, and sculptured by alpine glaciers of Pleistocene and Recent age." (Buddington 

and Chapin, 1929, p. 23.) The maximum height of the surface of the 

Pleistocene ice was probably less than 5,000 feet, as evidenced by the 

sharp ridges and lack of roundness above altitudes of 4,200 and 4,500 

feet. Locally, valley glaciation in Recent and Wisconsin time has resulted in alpine sculpture, but probably the most pronounced glacial 

erosion was due to the Cordilleran glacier complex in Wisconsin time.

Evidence of the great ice flood of Pleistocene time is found in the 

fiords such as Scenery Cove, in the three lake valleys, in the occurrence 

of hanging valleys such as the creek valley on the south side of Scenery Lake and about 1.2 miles from the head of the lake, a similar valley at the head of the drainage southeast of Swan Lake, and others in 

the region under consideration.
WATERPOWER SITES NEAR PETERSBURG, ALASKA 107

At the mouths of the hanging valleys on the south side of Scenery 

Lake and southeast of Swan Lake are low rounded transverse ridges, 

called lips, that roughly span each valley except where the streams 

flow through the postglacial notches.

There are many cirques in various stages of preservation. The 

U-shaped tributary valleys contain lateral, medial, and end moraines 

of preexisting alpine glaciers. The three lakes investigated were 

formed by glacial action which gouged deep troughlike basins out of 

the rock. The sharp knife-edged or comb ridges between these glacial 

lakes are the result of frost wedging, extensive alpine glaciation, and, 

to some degree, difference in character of the rock that is, the occurrence of less resistant gneisses within the diorite mass.

In the main creek valleys the cross section is not a simple U and 

the valleys are not everywhere broad and flat. In some places they 

show evidence of former successively narrower valleys produced by 

more than one period of erosion. The loosely consolidated material 

left by glaciation in the valleys has been modified by normal stream 

erosion, and gorges have been cut in the rock floor of the valley. The 

streams emanating from the lakes follow tortuous and precipitous 

paths to tidewater, and rapids and falls are numerous. The falls on 

Cascade Creek at the head of Falls Lake are between 60 and 70 feet 

high. Similar falls were observed near the head of the broad, flat 

valley of the stream which enters the northeast side of Swan Lake. 

These falls were probably formed as rock steps produced by the 

quarrying action of a glacier. The steep walls of the three main 

creeks are intersected by glacial troughs or by deeply incised valleys of 

tributary creeks. Most of the topographic features described must be 

considered when any conduit is planned from the lakes to powerplants 

near tidewater.

GENERAL GEOLOGY

The limits of the mapping by Buddington and Chapin (1929, pi. 1) 

extend about 3 miles east of the bay, but their map does not include 

Scenery Lake and its catchment area. Swan and Kuth Lakes are near 

the edge of the area previously mapped, but parts of their catchment 

basins are beyond the area mapped by Buddington and Chapin.

The geology of the area under consideration, shown on plate 8, is 

based on this field investigation and on the previously published work 

of Buddington and Chapin. Samples of rock at the damsites, tunnel 

routes, and reservoir areas were selected for petrographic examination. The engineering geology of each site will be discussed following the description of the geology of the area as a whole.

620666 62 2
108 GEOLOGY OF WATERPOWER SITES IN ALASKA

IGNEOUS ROCKS

The predominant rocks of the mainland of southeastern Alaska and 

of the area involved are hornblende-quartz diorite of the compositeCoast Kange batholith, the Coast Mountains geanticline of Payne 

(1955), and gneisses and schists of the metamorphic-complex belt 

lying adjacent to the batholith. The intrusive quartz diorite is reported by Buddington and Chapin (1929, p. 61) to be contaminated 

in varying degrees with inclusions, remnants, or assimilation products 

resulting from the breaking up or disintegration of schist. In the 

area under consideration, particularly along the valley of Scenery 

Creek, the intrusive sheets of quartz diorite appear to contain very 

few assimilation products. Some increase in the mafic mineral content, mainly hornblende, was observed along Cascade Creek and a 

marked increase was evident along Delta Creek farther south. This 

rock is referred to in this report as diorite.

Dikes of aplite or felsite are more common in the Swan Lake basin 

than in the Scenery Lake basin. The dikes range in width from a few 

inches to tens of feet.

The texture of the diorite ranges from fine to coarse and the fabric 

is equigranular, although locally it has a slightly porphyritic appearance. From north to south across the area an increase in the content 

of f erromagnesian minerals in the diorite was observed and locally the 

country rock resembles gabbro. The area of quartz diorite which 

comprises the western part of the Coast Range batholith is 3 to 15 miles 

wide in southeastern Alaska and is considered to be of Late Jurassic 

or Early Cretaceous age.

METAMORPHIC ROCKS

The metamorphic rocks are crystalline schists and injection gneisses, 

and the main belt of these rocks, 13 miles wide in the Thomas Bay 

region, roughly parallels on the southwest the Coast Kange batholith. 

In places the linear flow structure and the roughly parallel orientation of hornblende, give the diorite a more or less gneissic appearance. 

This type of structure in the intrusive bodies is more prevalent in the 

southern part of the area. The gneisses are predominantly hornblende-plagioclase gneiss, but mica and garnet occur in some of them. 

These gneisses and schists in general show various stages of injection 

and occur as blocks, stringers, or as infolded masses locally intercalated 

with beds of limestone or marble, which are in places as much as 10 

feet thick. The gneisses and schists, however, may be regarded as 

members or integral parts of the larger and more common occurrences 

of hornblende gneisses.
WATERPOWER SITES NEAR PETERSBURG, ALASKA 109

In several localities it was difficult to determine the boundary between diorite and gneiss because the exposures vary from feebly metamorphosed diorite gneiss to highly metamorphosed hornblende-plagioclase gneiss and garnetiferous mica schist for example, on the northeast side of Swan Lake where the limits of the metamorphic rock are 

in doubt, in part owing to forest cover and topographic relief. The 

boundary shown between diorite and gneiss is to be regarded as indefinite, but it delimits the area of predominantly metamorphic rocks 

which, however, may not be of the same age or origin in different 

parts of the area mapped. On plate 8 the rocks are classified broadly 

under four types of interest to construction engineers. The age of 

these metamorphic rocks probably ranges from Ordovician to Jurassic 

or younger.

UNCONSOLIDATED BOCKS

In the broader valleys and in the valleys that enter Thomas Bay, 

deposits of glacial boulders and gravel are exposed. The valleys of 

the streams entering Swan and Scenery Lakes contain deposits of 

sand in limited amounts. The streams have carried glacial debris 

toward Thomas Bay, although a large part of the deposits in these 

valleys must have been left by recession of the valley glaciers of Quaternary age. The glacial debris has been reworked locally and Recent 

stream deposits consist of a large amount of reworked morainal material. Some forested areas in the broader valleys entering Thomas 

Bay are underlain by outwash from glaciers which occupied the area. 

Proglacial streams from the broad lobe of Baird Glacier deposit a 

large volume of sand and gravel which is added to morainal material 

left by recession of the glacier and by older glacial streams. These 

streams also carry rock flour and silt which is discernible for several 

miles out in Thomas Bay.

STRUCTURE

The damsites investigated are near the western edge of one of the 

major geologic features of southeastern Alaska, the Coast Range 

batholith of the mainland. This batholith is included in the tectonic 

element designated by Payne (1955) as the Coast Mountains geanticline. The Prince of Wales geanticline of Payne (1955) is 50 miles 

or more west of the mainland, and between this anticlinorium and 

the mainland lies the Juneau synclinorium (Buddington and Chapin, 

1929, pi. 22), or the Seymour geosyncline of Payne (1955), striking 

northwestward roughly parallel to other major structural elements. 

The beds in this synclinorium are overturned to the southwest. On 

the mainland, however, in the area between Delta and Scenery Creeks, 

the strike of the foliation, which is generally north to northwest 

parallel to the western border of the Coast Range batholith, is locally
110 GEOLOGY OF WATERPOWER SITES IN ALASKA

interrupted by beds of gneiss or schist that cut across the regional 

strike. The structures observed are probably residual from sedimentary rocks existing prior to intrusion of the magma.

Many shear zones are found within the area, but any appreciable 

displacement which might have occurred is masked by forest growth 

and rockslides. Jointing is commonly steep (75° to vertical), generally in a northeast and northwest direction, rather widely spaced, 

and generally cutting across the lineation. Among the many structural patterns in evidence on aerial photographs, a well-defined line 

extends from a point near the triangulation stations Boulder and Land 

as shown on the special topographic map of the area (U.S. Geological 

Survey, 1952), to a point about 2 miles to the east near the headwaters 

of the South Fork of Scenery Creek. A most pronounced system of 

joints or fractures that was observed on aerial photographs extends 

northwestward parallel to the synclinorium. This lineation is intersected by another one trending northeastward. Several lineaments 

shown on plate 8 were located after an examination of aerial photographs by George Plafker of the Geological Survey. Overturned 

isoclinal folds in the gneiss occur sporadically, but in general the 

structure near the western margin of the batholith is not complex. 

Several main divides or sharp ridges consist of gneiss. Whether 

their occurence is due to the tectonics of the region, resistance to 

erosion, or to glaciation was not determined.

SEISMIC ACTIVITY

According to a seismic probability map of the United States (U.S. 

Coast and Geodetic Survey, 1950), these damsites are on the edge of 

what is designated as zone 2, an area in which structures are reported 

to have been moderately damaged one or two times. The adjoining 

zone 1 to the east is a region of minor damage by earthquakes but no 

structural change. In the explanatory note accompanying the map 

cited, it is stated that:

A compilation of records shows that between 70 and 80 percent of earthquakes 

occur in the same general regions, although not necessarily at the same epicenters, where previous earthquakes have occurred and that these regions are 

generally within fairly well-defined zones or belts. * * * At present there are not 

sufficient data available to prepare a general seismic probability map of the 

country based on surface geology. The zone limits shown on this map are based 

almost entirely on earthquake history and represent the general combined 

opinions of a number of scientists.

Even though the prospective damsites, tunnel routes, and powerhouse sites are located in an area of only moderate to minor probable 

damage from earthquakes, the possibility of more severe earthquakes
WATERPOWER SITES NEAR PETERSBURG, ALASKA 111

and greater damage should be considered in the location and design 

of structures.

GEOLOGY OF POSSIBLE POWERSITES APPLIED TO 

WATERPOWER DEVELOPMENT

The general plans or methods by which power would be developed 

in this area are described by F. A. Johnson (1962). Any plans for 

power development will require the creation of storage to equalize the 

pronounced irregularities in streamflow, and conveying the water from 

the storage sites to powerhouse sites at or near tidewater. Opportunity for storage exists in three lakes Scenery Lake on Scenery Creek, 

Swan Lake on Cascade Creek, and Ruth Lake on Delta Creek. Storage development could be accomplished on Scenery and Swan Lakes 

by the construction, in each instance, of a dam at the lake outlet to 

raise the lake surface above its natural level, or by drawing the 

lake below its natural level, or by a combination of the two methods. 

Ruth Lake is comparatively shallow and has a small capacity below 

lake level so any storage development would require the construction of a dam at the lake outlet. To utilize the full potential 

head, the powerhouse sites would be as near tidewater as practicable 

and, in general, would be near the mouths of the creeks under consideration. It would also be possible to convey the water from Swan 

Lake to a powerhouse site at the mouth of Scenery Creek, thus using 

the water from both Scenery and Swan Lakes in a single powerhouse. 

The fieldwork was directed toward the examination of damsites at the 

lake outlets and the suitability of the lakes as reservoir sites, and examination of tunnel routes from each of the three lakes to possible 

powerhouse sites near the creek mouths and from Swan Lake to a 

powerhouse site near the mouth of Scenery Creek. Only tunnels were 

considered as a means of conveying the water from the storage sites 

to the head of the penstocks leading to the powerhouse sites. Owing 

to terrain as well as climatic conditions, the use of canals or pipelines 

was considered impractical. The water-surface elevation in the reservoirs may range from 100 to 150 feet. Consequently, any tunnels 

considered will have to withstand the hydraulic pressure equivalent to 

the maximum range in the elevation of the reservoir surface.

The character of the rock beneath the surface is more or less inferred on the basis of field investigation and samples obtained on or 

near these tunnel courses, and because the dioritic intrusions are recognized as part of the Coast Range batholith. The rock to be penetrated is competent to maintain the tunnel openings and to withstand 

the hydraulic pressures expected. The surface profiles along the tunnel courses were drawn from the topographic map. The locations of
112 GEOLOGY OF WATERPOWER SITES IN ALASKA

the penstocks were determined by inspection of the topography, and 

their courses were selected to provide a minimum cover of 100 feet of 

rock in place.

The engineering geology of each damsite and tunnel site discussed 

in the following pages is the result of reconnaissance work in these 

areas. More detailed examination, aided by drill-hole data, will be 

necessary prior to the planning of any construction. The three reservoir sites are in relatively impervious rock, and no structural or other 

geologic conditions were observed which might cause appreciable 

leakage from the reservoirs. Rock-fill or concrete dams of either gravity or arch type could be constructed at all sites.

SCENERY LAKE POWERSITE 

TOPOGRAPHY

The catchment basin above Scenery Lake which receives the melt 

water from two valley glaciers terminating near the unnamed lake 

east of Scenery Lake (pi. 8), has steep walls probably carved by a more 

extensive glacier in Pleistocene time. Scenery Lake is a glacial lake 

and its steeper slopes are barren of vegetation, but most of the rockslides and alluvial fans at the foot of some of the slopes are covered 

with alder and brush. The floor of the valley above Scenery Lake 

has a minor amount of relief, and is partly covered by trees and partly 

by alder and brush. Some spots are poorly drained, and swamps 

caused by beaver dams or by snowslides that dam drainage courses 

with rock and debris are common.

The valley bottom of Scenery Creek for the first mile upstream 

from the head of Scenery Cove is about half a mile wide and of relatively low relief. Fallen timber and large blocks of rock are common 

near the edges and on the adjoining slopes.

GEOLOGY

Rock types. The basin of Scenery Lake has been gouged out of 

solid rock which is almost entirely hornblende-quartz diorite. The 

basin walls both above and below the lake outlet were sampled at 

intervals of about a quarter of a mile in order to discover any changes 

in the wallrock or country rock that might be obscured by forest 

growth. Occasional samples were also taken at intervals in the 

upper valley of Scenery Creek as far as the lake and about l1/^ miles 

eastward. Changes in the content of the ferromagnesian minerals, 

principally hornblende, were noted in a few localities where the 

diorite became gneissic.

At the outlet of the lake a contact between hornblende-plagioclase 

gneiss on the southwest and quartz diorite on the northeast crosses
WATERPOWER SITES NEAR PETERSBURG, ALASKA 113

the damsite area diagnonally, as shown on plate 8. Both the diorite 

and the gneiss are firm rocks which do not weather deeply and 

possess low porosity. The jointing in the diorite is about vertical, 

rhomboidal, and widely spaced. Individual layers of gneiss are as 

much as 2 to 4 feet thick. About 200 feet downstream from the 

lake outlet in the canyon wall on the right bank the ferromagnesian 

minerals in the gneiss appear to increase, probably owing to a segregation. However, this was not examined closely because it was difficult to reach. The lake outlet is a V-shaped notch caused by plucking of gneiss. Aplitic dikes and sills are common in the diorite.

Structure. Evidence of geologic structure is provided by the occurrence of infolded crystalline schist or gneiss which has been included as bands, blocks, and stringers in the hornblende-quartz diorite. The foliation or the separate layers dip northward at 57° and 

strike about 1ST. 60° W. at the damsite. Although it was not followed 

for the entire length of the outcrop, the gneiss zone apparently extends in the line of strike northwestward to a point about 1,500 feet 

north of Scenery Creek and about 1 mile northwest of the outlet of 

Scenery Lake. It is not known whether the gneiss was completely 

assimilated in the diorite northwest of this point.

A well-defined shear or fault zone appears on the aerial photographs beginning at a point 2.7 miles north of the mouth of Cascade 

Creek on Thomas Bay and extends more than 2 miles in an eastward 

direction. In the absence of good exposures the direction of movement was not determined, but the trace of this zone on the aerial 

photographs indicates a southward dip of the plane of fracture. As 

this line of weakness does not extend through the Scenery Lake 

damsite or reservoir site and probably becomes nonexistent east of 

the South Fork of Scenery Creek, it was not traced beyond this point. 

A tunnel from Swan Lake to Scenery Creek, however, would cross 

this zone about 2 miles south of the point where South Fork flows 

into Scenery Creek. In addition to this shear zone, several lineaments are shown on plate 8 which were located by examination of 

aerial photographs.

DAM SITES

In this report two possible locations for a dam below the outlet 

of Scenery Lake are considered. These are shown on plate 9 as 

A-A' and B-B'. Johnson (1962) indicates that a dam of a height 

of more than 90 feet above the lake surface would be required to 

develop the requisite storage for equalization of the streamflow.

The first site, A-A', is located about 50 feet downstream from the 

actual outlet and the geology at this site is shown in map view and 

section on plate 9. The left abutment and the section across the
114 GEOLOGY OF WATERPOWER SITES IN ALASKA

channel would be in a hornblende-plagioclase gneiss. The remainder 

of the section and the right abutment would be in a hornblendequartz diorite. The gneiss forms a natural weir through which the 

lake water flows; consequently, no waterborne deposits exist at that 

point.

The slopes of both abutments are stable. On the right or north 

wall the abutment would be in the nearly vertical wall of diorite 

which rises toward a knoll about 200 feet above the lake. This slope 

presents no serious problem. The slope of the abutment on the 

south side, however, is largely bare gneiss with a minor amount of 

brush and timber, particularly in the upper slopes. The slope is as 

much as 60° over short distances but averages about 45°, and rock 

loosened by ice and frost action could slide toward the dam. Snow 

building up on the upper slopes of this hill could slide toward this 

abutment, carrying rock and timber with it.

A dam at the first site would have a crest length of about 900 feet 

and a maximum height of about 130 feet above the channel in order 

to raise the lake level about 90 feet. The bedrock at this site is suitable for a flexible fill or masonry type of dam of the height required.

The second site considered is designated as B-B' on plate 9. A section along this line is also shown on plate 9. This will require a main 

dam (lower northward-trending solid line, from B] across the channel and an auxiliary dam (upper northward-trending solid line to 

B'} across the saddle to the north. The main dam would be located 

about 300 feet downstream from the lake outlet. This site would be 

entirely in the metamorphic rock or hornblende-plagioclase gneiss, the 

individual layers of which range in thickness from 2 to 4 feet and 

in general dip at an angle of 57° N. and strike N. 60° W. This foliation persists between the site and the lake outlet and beyond, probably 

for several miles to the southeast. This crystalline gneiss offers a 

satisfactory foundation for a dam of the height considered, although 

probably as much as 10 feet of rock might have to be scaled from the 

steep high wall on the south side of Scenery Creek to reach firm rock 

for the abutment in that bank. It is doubtful whether any grouting 

between layers of gneiss would be required. This dam would be about 

140 feet in height above the stream channel and would have a crest 

length of slightly less than 300 feet. The main dam (lower solid line, 

from B on fig. 3) would not be subject to slides on the right bank, but 

the left bank would be subject to the same slide conditions described 

for site A-A'.

The auxiliary dam (upper solid line, to B' on pi. 9) mentioned in 

the preceding paragraph, is located about 900 feet west of the lake 

outlet across the saddle on the north bank. A dam at this site would 

have about the left one-third of its length and the south abutment in
WATERPOWER SITES NEAR PETERSBURG, ALASKA 115

gneiss and the right two-thirds of its length and the north abutment 

in quartz diorite. It would have a height of about 35 feet above 

present ground level and a crest length of about 250 feet. Foundation 

and abutment conditions are satisfactory for any type of dam that 

might be selected for this site.

The area between the main dam in the channel and the auxiliary 

dam in the saddle, represented by the westward- and northwestwardtrending lines (pi. 9), is well above the maximum assumed flow line 

of 1,050 feet and is all in plagioclase gneiss. This barrier is relatively impervious and would serve as a natural dam. The area along the 

auxiliary damsite is covered with brush, timber, and muskeg. The 

slope at the south abutment is gentle and no slide problems would be 

experienced. The slope at the north abutment (B') of the auxiliary 

dam, however, consists of massive quartz diorite boulders which may 

be the remnants of a lateral moraine of the valley glacier, or which 

may have been moved down from higher altitudes north of these cliffs, 

by normal erosion processes.

Whether the dam is the overflow or the nonoverflow type, erosion of 

the channel of Scenery Creek by water from the spillway or spillway 

apron would be a minimum as the gneisses in the area are firmly 

crystallized and probably sufficiently resistant to dissipate the effects 

of hydraulic friction and cavitation.

RESERVOIR

The reservoir is almost entirely in hornblende-quartz diorite but a 

minor area of hornblende-plagioclase gneiss is exposed near the damsite. Both types of rock are relatively impermeable, and in the 

absence of pronounced faulting or continuous openings which might 

provide channels for escape of water impounded by the dam, no appreciable leakage from the reservoir is anticipated.

The steep slopes on the borders of the lake show evidence of snowslides at several points. The principal creeks entering the north and 

the south sides of Scenery Lake thread through debris cones, and 

during flood stages they carry some detrital material to the lake. A 

negligible amount of silt may be carried into the lake on such 

occasions, but it is clear at all times. Moreover, this debris is relatively stable, and even if rockslides occurred they would constitute no 

serious impediment in the reservoir.

Timber and brush cover the swampy area drained by the upper part 

of Scenery Creek at the east end of the lake, but some of the steep to 

nearly vertical slopes around the lake support no timber. The damsite area north of the creek, however, is timber covered.
116 GEOLOGY OF WATERPOWER SITES IN ALASKA

TUNNEL

A tunnel for conveying water from Scenery Lake to a powerhouse 

at or near the head of Scenery Cove could be located on either side 

of the Scenery Creek valley. The field investigations were directed 

primarily toward a south bank location but the valley walls on the 

north side of the creek were also examined at intervals. With the 

exception of the occurrence of plagioclase gneiss, the wallrock on the 

north side was essentially the same as on the south side of the Scenery 

Creek valley.

For illustrative purposes in this report a tunnel route was assumed 

on the south side of the valley, shown in map view as G-G' on plate 8 

and in section on plate 9. As shown on plate 9 the tunnel diverts 

from the lake at an altitude of 900 feet. In actual development this 

may range from as low as 800 feet to as high as 950 feet. The geologic 

conditions, as nearly as can be inferred from the available information 

are essentially the same within this range in altitude. The tunnel 

alinement, as shown on plate 8, is based on an assumed altitude of 900 

feet at the diversion from the lake and an adequate cover of rock, 

about 100 feet, where the alinement crosses the several creek valleys.

As shown on plate 8, a tunnel route on the south side of the valley 

would have a total length of nearly 20,000 feet and would be in hornblende-plagioclase gneiss for about the first 1,200 to 2,000 feet from 

the lake with the remainder in quartz diorite. A location on the 

north side of the valley would be in quartz diorite for about the first 

half mile from the lake, then would pass through a zone of hornblendeplagioclase gneiss for 1,500 to 2,000 feet with the entire remaining part 

in quartz diorite.

The character of the rock to be penetrated, whether on the south or 

north bank, assures stability in openings of the tunnel throughout 

most of its length. However, one or more zones of weakness appear 

on the aerial photographs along the proposed tunnel route near the 

South Fork of Scenery Creek and these zones should be drilled prior 

to location of the tunnel or the penstock. No other unfavorable geologic conditions are believed to exist along the tunnel routes that would 

most likely be considered from Scenery Lake to Scenery Cove.

CONSTRUCTION MATERIALS

A concrete or fill type of dam could be built at the Scenery Lake 

damsite. Coarse aggregate for concrete could be obtained by quarrying and crushing rock in the vicinity of the site. There are scattered 

gravel bars along Scenery Creek, particulary near mile 1.0. An inexhaustible supply of relatively clean fine to coarse sand and gravel is 

available at the north end of Thomas Bay in the broad, outwash plain
WATERPOWER SITES NEAR PETERSBURG, ALASKA 117

below the Baird Glacier. This deposit is 1% miles north of the entrance to Scenery Cove and is accessible to boats of shallow draft. 

Small deposits of sand and gravel occur in the valley of Scenery 

Creek above the lake and in creeks entering the lake. There is some 

doubt as to the availability of the necessary fine materials for a fill 

type of dam.

SWAN LAKE POWERSITE 

TOPOGRAPHY

The north and south boundaries of the Cascade Creek drainage 

basin are marked by comb ridges. The catchment basin above Swan 

Lake is characterized by evidence of both past and present glacial 

action. Streams descending from valley glaciers and from the icecap 

flow into the lake. The cirques or minor basins which contribute their 

drainage to the lake directly are probably the work of alpine glaciers. 

The creek which enters the northeast part of the lake is fed by the melt 

water of the icecap and two valley glaciers several miles east of the 

lake. This creek flows through a broad relatively flat valley about 

1 mile long, from the upper end of which falls descend about 60 feet 

to the valley floor.

Cascade Creek, which flows out of Swan Lake, descends about 350 

feet in the first half mile. A falls at the head of Falls Lake is about 

80 feet high. After leaving Falls Lake through a narrow gorge the 

stream gradient is steeper, and in the next mile the creek follows a 

straight course and descends about 850 feet. In the last half mile 

to the coast it falls about 250 feet and flows through a narrow rockwalled valley. The steplike character of the upper streambed is shown 

on the profile of Cascade Creek on the topographic map. As its name 

implies, there are many rapids and cascades throughout the length of 

Cascade Creek.

The side slopes of the lake in general are not quite as precipitous 

as those of Scenery Lake and are covered in large part by forest 

growth.

GEOLOGY

Rock types. The rock in the reservoir area is generally gneissic in 

character, particularly on the south side of the lake, and in comparison with the Scenery Lake area, it exhibits a definite increase in 

content of mafic minerals. There may be two sheets of intruded 

batholithic material in this area, one of which crosses the outlet of 

the lake. One of these intrusive sheets may be classed locally as 

gabbro and the other as hornblende-quartz diorite. These rocks have 

not been mapped separately and in detail because they are part of the 

same batholithic intrusion in which quartz diorite predominates.
118 GEOLOGY OF WATERPOWER SITES IN ALASKA

The massive gneiss layers on the south side of the lake consist of 

hornblende and plagioclase with quartz, garnet, biotite, and epidote 

as accessory minerals. The hornblende crystals are parallel to the 

foliation. Aplitic dikes 6 inches to 3 or 4 feet in thickness were 

observed, and although southeast of the lake the aplitic zone may be 

very much thicker, it is probably of lenticular shape. Small lightcolored dikes are numerous and conspicuous around the lake and they 

intersect each other at various angles. Beds of marble or crystalline 

limestone ranging from a few inches to 4 or 5 feet in thickness were 

observed in the gneiss north of the lake outlet, also southeast of the 

lake, and between this point and the falls on the creek which enters 

the lake on the northeast side. The marble is pure white and in places 

medium to coarsely crystalline; some thin stringers are pure calcite.

Deposits of glacial sand and boulders occur in the valleys of the 

creeks on the east, end of the lake. Angular boulders and reworked 

glacial debris occur at the mouths of drainage channels at or near lake 

level. Cascade Creek below the gorge at the outlet of Swan Lake 

flows through cobbles and boulders before dropping over the falls at 

the head of Falls Lake.

Around Falls Lake the country rock and the ledge which forms the 

falls are hornblende-quartz diorite. Sampling at intervals along Cascade Creek to the coast revealed no abrupt change in the composition 

of the bedrock, although the content of hornblende appears to increase westward and the rock becomes definitely gneissic in character.

Structure. Near the east end of the lake the layers of hornblendeplagioclase gneiss in a small exposure strike N. 55° W., and dip 47° 

NE. At the contact with the hornblende-quartz diorite near the west 

end of the lake the gneiss contact trends northwestward. However, 

along the creek entering the northwest side of the lake near the lake 

outlet and about 500 feet higher than the lake, calcareous gneiss is exposed in a small sharp isoclinal fold about 75 feet across. This calcareous gneiss zone extends over the divide beyond the headwaters of 

the South Fork of Scenery Creek and trends slightly west of north. 

There appears to be a shear zone on both sides of Swan Lake roughly 

following the creeks that enter the lake near the outlet. The creek on 

the south side of Swan Lake east of the lake outlet, however, roughly 

follows the contact of the hornblende-plagioclase gneiss and the 

quartz diorite. In the lower part of the South Fork of Scenery Creek 

the calcareous gneiss was absent but was observed about 1 mile above 

the main creek.

The slightly incised valleys near each abutment site and transverse 

to the axis of the proposed dam, between 800 and 1,000 feet to the 

north and south of the lake outlet, so far as could be determined by 

field examination, are not faults but probably are joint fractures which
WATERPOWER SITES NEAR PETERSBURG, ALASKA 119

eroded more easily than the neighboring rock. These localities should 

be drilled to verify this supposition as it was not possible to inspect 

these points closely because of forest growth.

The straight course of Cascade Creek southwestward from Falls 

Lake for a distance of about 4,000 feet, its abrupt turn at that point 

southeast for about 1,000 feet, and another right-angle turn southwestward toward the coast are evidently the result of the structural pattern 

of the area. This might affect the stability of a tunnel from Swan 

Lake to a point near the mouth of Cascade Creek.

DAMSITB

The narrow gorge immediately downstream from the outlet of Swan 

Lake presents favorable topography for the location of a dam. The 

bedrock in this gorge consists entirely of hornblende-quartz diorite. 

The south or left wall of the gorge is practically bare for more than 100 

feet above the altitude of Swan Lake. The north or right side of the 

gorge is covered with thick brush, trees, and large boulders and blocks 

of diorite. Within the narrower part of the gorge large blocks of 

diorite, 10 to 20 feet in maximum dimension, line the stream channel 

and to a lesser extent form the outlet of the lower part of the channel. 

There are two small incised valleys, one on each side of the gorge, 

roughly parallel to Cascade Creek and several hundred feet higher 

than the altitude of Swan Lake. Examination of these localities 

furnished no evidence of faulting, either active or dormant, and it was 

concluded that jointing was solely responsible for these incised valleys. 

Exploratory drilling to verify this should be included as part of any 

detailed investigation of this site.

The right or north side of the gorge, because of the cover of trees, 

and brush and the large irregular boulders, should be reasonably free 

from slides. The left or south wall might be susceptible to slides,, 

however, as it is practicaly bare and is steeper.

The most likely location for a dam would be at the narrowest part 

of the gorge (D-D'', pi. 9) which is about 500 feet downstream from 

the lake outlet. A dam at this site should have a crest length of 300 

feet. The waterpower report by Johnson (1962) shows that a dam 

that would raise the lake level about 100 feet would be required at this 

site to attain the desired regulation of outflow from the lake. The 

foundation and abutment conditions are satisfactory for the construction of either a masonry or rockfill type of dam of the height indicated. The rock conditions in the channel downstream from the dam 

would effectively withstand the erosion from any spillway discharge.

RESERVOIR

The rock surrounding Swan Lake includes, in addition to hornblende-quartz diorite, a large amount of hornblende gneiss, diorite-
120 GEOLOGY OF WATERPOWER SITES IN ALASKA

gneiss, and calcareous gneiss that contains beds of marble. With the 

exception of the gneiss containing marble, near the west end of the 

lake, none of the rocks observed would be susceptible to rapid weathering. The location of these calcareous gneisses near the lake would 

not promote leakage, and permeability is lacking in other rocks in the 

reservoir. No faults, joints, or openings were observed that might; 

cause water losses. However, the two notches on each side of the lake 

outlet previously described should be core drilled.

The reservoir rock supports a good growth of timber over much of 

the lake basin and, with the exception of a few areas subject to snowslides, the slopes can be considered stable.

An increase in elevation of the lake surface would flood some timbered areas. The valleys of the two creeks that enter the upper part 

of the lake are covered with alder and brush.

Water from Swan Lake could be diverted to a powerhouse site at 

or near the head of Scenery Cove (C-E, pi. 8) or to a site at or near 

the mouth of the Cascade Creek (F-Ff, pi. 8). Sections along these 

routes are shown on plate 9. In this figure the tunnels have been 

shown as diverting at lake level. If the storage were utilized in the 

absence of a dam by drawing the lake below its natural level, these 

tunnels could be about 125 feet lower than indicated. Judging from 

the information available, geologic conditions would be virtually the 

same at either altitude considered, lake level or 125 feet lower.

TUNNELS

Swan Lake-Scenery Cove tunnel. The tunnel route from Swan 

Lake to Scenery Cove, C-E as considered in this report, would follow 

a course roughly parallel to the creek that enters the northwest end 

of the lake and parallel to the South Fork of Scenery Creek (pi. 8). 

Reconnaissance of these creek valleys discloses that gneiss containing 

white marble or crystalline limestone bands and stringers is continuously exposed from a point about 150 to 200 feet above Swan Lake 

to the divide between the Scenery Creek and Swan Lake drainage. 

Although detailed information is lacking, this gneiss zone apparently 

is continuous northward beneath the snow and icecap to a point on 

the South Fork of Scenery Creek about l1/^ miles below the divide. 

Whether these infolded or included blocks and stringers of gneiss 

exist at the depth of the probable tunnel grade below an altitude of 

1,500 or possibly 1,375 feet is not known, but this can be determined 

by diamond drilling.

Reconnaissance along this tunnel route discloses that the first sizable outcrop of marble occurs about 600 feet above lake level at an 

altitude of about 2,100 feet. A conclusion based solely on this observa-
WATERPOWER SITES NEAR PETERSBURG, ALASKA 121

tion is that a tunnel grade at an altitude of 1,500 feet or lower would 

probably not find the calcareous gneiss, particularly if the tunnel were 

driven west from the outlet of Swan Lake west of the South Fork of 

Scenery Creek through the quartz diorite along a course approximating that indicated by the line on plate 8. This tunnel would intersect a shear zone, which apparently extends eastward from the bay 

about 1,000 feet south of triangulation station "Land" to a point about 

2 miles south of Scenery Cove, as shown on plate 8. In addition to 

this zone, six lineaments are shown which may indicate fracture zones 

that could affect tunneling operations along the route indicated.

The total length of this tunnel would be about S1/^ miles, and the 

length of the penstock would be about 2,500 feet.

Cascade Creek tunnel. The tunnel route from Swan Lake to a 

powerhouse site at or near the mouth of Cascade Creek was selected 

for consideration in this report on the north side of the Cascade Creek 

valley, and is designated as F-F' on plate 8. The alinement indicated 

was based on topographic considerations so that there would be an 

adequate rock cover over the tunnel at all points. A course on the 

south side of the valley was not considered as desirable because of 

the presence of several reentrants resulting from Kecent and Pleistocene erosion and glaciation. Sampling at intervals between the lake 

and the bay along Cascade Creek showed that the tunnel route as 

indicated would be in hornblende-quartz diorite throughout its entire 

length. Four shear zones may be intersected by the proposed tunnel. 

A section along the route is shown on plate 9. The tunnel would be 

about 2^4 miles long. If the powerhouse site were at the mouth of 

Cascade Creek on Thomas Bay a penstock over 4,000 feet long would 

be required. If a satisfactory powerhouse site could be had at a point 

along the shore about 3,500 feet north of the mouth of Cascade Creek 

the penstock length could be reduced to about 2,000 feet or less.

CONSTRUCTION MATERIALS

Because of the scarcity or absence of appropriate fine materials, a, 

fill type of dam would not likely be considered at the Swan Lake 

damsite. Coarse aggregate for a concrete dam could be obtained by 

quarrying and crushing rock in the vicinity of the site. A limited 

supply of sand and gravel may be available in the small creek valley 

1,000 feet southeast from the outlet of Swan Lake. A substantial 

quantity is available in the creek valley upstream from Swan Lake. 

If this source is unsatisfactory or inadequate the outwash plain below 

the Baird Glacier (p. 116) could be considered.
122 GEOLOGY OF WATERPOWER SITES IN ALASKA 

RUTH LAKE POWERSITE 

TOPOGRAPHY

The north wall of Ruth Lake is nearly vertical, but the south wall 

is less steep and covered with trees. The channel out of the lake is 2 

to 5 feet deep, probably owing in part to the accumulation of boulders,, 

after which Delta Creek enters a narrower channel. The north bank, 

less than 200 feet from the outlet, is almost vertical to a height of 

60 to 70 feet above stream level but the south bank slopes more gently,, 

and large blocks of quartz diorite have accumulated from the outcrop 

of bedrock about 50 feet above the stream. Delta Creek is slightly 

more than 2 miles long and follows a more circuitous course than 

Scenery Creek or Cascade Creek.

GEOLOGY

Rock types. The exposed north wall of the lake contains manjr 

dikes that have penetrated the hornblende-quartz diorite in the same 

manner as at Scenery and Swan Lakes to the north, and the diorite 

forms a similar reservoir of smaller dimensions. The joints in the 

intrusive masses also govern minor drainage to some extent. Samples 

were taken on the north and south banks. The lower 50 feet of the 

north wall was identified as hornblendite and apparently is a segregation, but the quantity of hornblende diminishes in the upper part of 

this wall and the rock seems to change to a gabbro. Farther downstream the right bank is hornblende-quartz diorite. Rock of the 

south bank is virtually the same type where exposed. In general,, 

the bedrock along Delta Creek toward the bay contains increasing 

amounts of ferromagnesian minerals and is gneissic in several localities. Within :y2 to % mile of Thomas Bay the bedrock is entirely 

hornblende gneiss.

Structure. The foliation of the schist and massive gneiss within 

half a mile of the coast dips generally eastward, although this foliation may be from overturned beds of the Juneau synclinorium (Seymour geosyncline of Payne). In the gneiss zone near the coast dips 

as high as 70° E. were noted. The foliation becomes less distinct eastward and the bedrock is essentially a part of the diorite mass. Without a boat little opportunity was afforded to examine the structure of 

the reservoir, but so far as could be determined from the outlet of th& 

lake and from aerial photographs no faulting or folding has affected 

the reservoir area; at least no unfavorable conditions were apparent.. 

The reservoir area seems to be a part of the main mass of hornblendequartz diorite.
WATERPOWER SITES NEAR PETERSBURG, ALASKA 123 

DAMSITE

The Ruth Lake damsite shown on. plate 9 was not surveyed but is 

an enlargement of that part of the special topographic map of the area 

(U.S. Geol. Survey, 1952).

Delta Creek, immediately downstream from Ruth Lake, flows 

through a very narow canyon section about 600 feet in length. About 

200 feet downstream from the lake outlet the right or north bank is 

very steep and the first 60 or 70 feet above the stream is practically 

vertical. The left or south bank is also very steep but not quite as 

steep as the north bank. The bedrock through the canyon section 

consists mainly of hornblende-quartz diorite although there is an 

increase in mafic mineral content as compared with the area examined 

along Cascade and Scenery Creeks to the north. The slopes on both 

sides of the canyon, at the most probable site for a dam, seem to be 

stable.

Waterpower studies (Johnson, 1962) indicate that it would be necessary to raise the level of Ruth Lake as much as 215 feet to obtain 

the desired regulation of the outflow. The required dam would be 

about 245 feet high and have a crest length of 400 feet. The rock 

conditions in the canyon are satisfactory for the construction of 

either a masonry or rockfill dam. Rock conditions in the channel 

downstream from the dam location would withstand the erosion from 

spillway discharges.

RESERVOIR

The country rock at the lake outlet is hornblende-quartz diarite; 

locally, however, it approaches the composition of a gabbro. From 

the brief field observation made and from examination of aerial photographs, it is concluded that the entire reservoir is this type of rock. 

No faults or continuous openings which might permit escape of impounded water were observed, and the lack of permeability of the bedrock indicates that the probability of any appreciable leakage from the 

reservoir is^remote. A small area of unconsolidated sediments is present in the stream valley at the upper end of the lake.

The slopes of the reservoir seem to be stable throughout, although 

minor snowslides and rockslides might occur on the north side of the 

lake from the bare slopes and in the stream valleys.

The north wall is steep to vertical for a height of more than 500 feet 

above the lake and supports little timber. The south border of the 

lake supports a good growth of timber.

TUNNEIjS

Plans for the utilization of the water from Ruth Lake would require 

diversion by tunnel along one of three different routes: (a) from Ruth
124 GEOLOGY OF WATERPOWER SITES IN ALASKA

Lake to a point at or near the mouth of Delta Creek on Thomas Bay; 

(b) from Ruth Lake to a point at or near the mouth of Cascade Creek 

on Thomas Bay; and (c) from Ruth Lake south westward to some 

point in the Patterson River valley. The first two are designated as 

H-H' and F-I on plate 8; the third route is not shown.

Observations along the foot trail in the Delta Creek valley indicate 

that the quartz diorite rock in the valley is locally gneissic in character and becomes entirely gneiss as the coast is approached that is, 

toward the west. A tunnel from Ruth Lake to the mouth of Delta 

Creek would be mainly in the quartz diorite, except in the area from 

one-half to three-quarters of a mile from the coast where it would be 

in gneiss.

Rock conditions along the route to Cascade Creek were observed 

only in the vicinity of Ruth Lake and at the mouth of Cascade Creek. 

The rock in both these areas was quartz diorite and it is believed that 

the entire route would be in the diorite.

No observations were made in the Patterson River valley, but it is 

assumed, based on aerial photographs, that a tunnel route from Ruth 

Lake that terminated in this valley would find essentially the same 

rock conditions as were observed at Ruth Lake; that is, quartz diorite 

and gneiss.

CONSTRUCTION MATERIALS

Conditions at the Ruth Lake damsite are about the same as at the 

Scenery Lake and Swan Lake damsites, in that the necessary fine materials for a fill type of dam are not available. Coarse aggregate 

could be obtained by quarrying and crushing rock in the vicinity. A 

limited supply of sand and gravel may be available in the valley upstream from the lake. Other sources for sand and gravel would be the 

Patterson River delta or the outwash plain below the Baird Glacier.

REFERENCES CITED

Buddington, A. F., and Chapin, Theodore, 1929, Geology and mineral deposits

of southeastern Alaska : U.S. Geol. Survey Bull. 800. 

Federal Power Commission and U.S. Dept. Agriculture, Forest Service, 1947,

Water powers of southeast Alaska: Federal Power Commission, Washington, B.C.; U.S. Forest Service, Juneau, Alaska. 

Johnson, F. A., 1962, Waterpower resources, southeastern Alaska mainland,

vicinity of Petersburg and Juneau: U.S. Geol. Survey Water-Supply Paper

1529. 

Payne, T. G., 1955, Mesozoic and Cenozoic tectonic elements of Alaska: U.S.

Geol. Survey Misc. Geol. Inv. Map 1-84. 

U.S. Coast and Geodetic Survey, 1950, Seismic probability map of the United

States, with explanatory note. 

U.S. Geological Survey, 1952, Plan and profile of Cascade Creek and vicinity,

Alaska, including Scenery Lake and Swan Lake and damsite: U.S.

Survey River Survey Map.
INDEX

Page 

Abstract.. --.__..._.....___.___ 103

Acknowledgments..______________ 104

Baird Glacier, proglacial streams_______ 109

sand and gravel deposits___ .. 109,121,124

Buddington and Cbapin, quoted_______ 106

Cascade Creek, proposed powerhouse on_ 106,120 

Climate of area__ ______________ 104 

Coast Mountains geanticline..__._____ 108 

Coast Range batholith----___....__... 108,109

Delta Creek, proposed powerhouse..____ 106 

Development of powersites, proposed plans. Ill, 112 

Dikes, aplite.........__...._____.... 108,118

felsite.... . . 108

Fractures_.___.__....___._ 110,118,121

Gabbro, Ruth Lake powersite_______ 122,123

Swan Lake powersite__________. 117

Qlaciation.......................... 106,107,112,117

Gneiss, beltof __.___.._ 108,109

isoclinal folds in... . .............. 110,118

Ruth Lake powersite__________ 122,124 

Scenery Lake powersite- _ 112,113,114,115,116 

Swan Lake powersite________ 118,119,120

Hanging valleys_____ ____..__.. 106,107 

Hornblendite, Ruth Lake powersite___. 122

Jointing.__..................___.._._ 110

Ruth Lake powersite__________. 122

Swan Lake powersite_____ _... 118,119

Scenery Lake powersite--- -.. 113

Juneau synclinorium._.._.__..___. 109,122

Location of area..__ -.. .. 104

Marble, in gneiss..-.-.---.._......... 108,118,120

Metamorphic rocks, age. . 109 

structure in......___.. .. ....... 108,109

Patterson River delta. __ 124

vaUey.----------------------------------- 124

Pleistocene ice flood, evidence for 106

Precipitation, mean annual at Petersburg-..- 104

Prince of Wales geanticline. 109

Quartz diorite, age.__ -- _- 108 

aplite dikes and sills in..... .-_. . 113

inclusions in_._--..._----...----.--- 108'

petrology of-- . 108 

relation to Coast Range batholith. _ 108 

Ruth Lake powersite---._._ ------ 122,123

Scenery Lake powersite---- 112,113,114,115,116

Swan Lake powersite___ 117,118,119,120,121

PageReferences cited____.___________ 124 

Rock steps..__________________ 107 

Ruth Lake, altitude______________ 106 

location and accessibility..-_______, 106 

storage possibilities__________ __ 111 

Ruth Lake powersite, available construction

materials_____________ 124 

damsite__..._______________ 123 

petrography_________________ 122 

physical features...____._______ 122 

proposed tunnel routes... ___ 123,124 

reservoir.__ __ _ .. 123 

structure.__________________ 122

Sand and gravel deposits... 109,116,117,118,121,124 

Scenery Cove development, accessibility..... 106

site for powerhouse___....____ 116,120

Scenery Lake, altitude..____________ 106 

location and accessibility _________. 106 

storage possibilities____ ___ .. Ill 

Scenery Lake powersite, auxiliary damsite.- 114,115 

available construction materials..____ 116 

damsite A-A'............................ 113

B-B'................................. 114

petrography.- . __ 112,113 

physical features-...________ _ 112 

proposed tunnel route.. 116 

reservoir__ __ .... 115

structure-_ . . -. 113 

Schist.-.-.---__........._._----- 108,109,122

Seismic activity.- - - -- 110, 111 

Seymour geosyncline.-._________-> 109,122 

Shear zones ...................... 110,113,118,121

Structure in area. - ______________ 109,110

Swan Lake, altitude. . . 106

location and accessibility._____ _ 106

metamorphic rocks.--..- .. .- 109

storage possibilities. . 111

Swan Lake basin, dikes in 108

Swan Lake powersite, aplitic dikes in_..._ 118

available construction materials... _ 121

damsite____..___.__- -.- 119

petrography- - 117,118

physical features------- . - 117

proposed tunnel routes ------ __ . 120,121

reservoir.... .-- . -. 119,120

structure...-------..--- ------------ 118,119

Thomas Bay, glacial material in... .. 109

Tunnels, Cascade Creek tunnel--....- _ 121

Swan Lake-Scenery Cove tunnel.. . 120,121

Unconsolidated rocks...- 109 

Vegetation in area..-___------___----- 104,105

Water falls. 107

125

o