Geologic Investigations 

of Proposed Powersites at 

Baranof and Carbon Lakes 

Baranof Island, Alaska

GEOLOGICAL SURVEY BULLETIN 1031-B

Geologic Investigations 

of Proposed Powersites at 

Baranof and Carbon Lakes 

Baranof Island, Alaska

By KENNETH S. SOWARD

GEOLOGY OF WATERPOWER SITES IN ALASKA

GEOLOGICAL SURVEY BULLETIN 1031-B

A description of the proposed dam, 

tunnel, and reservoir sites on each lake, 

with conclusions and recommendations

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961
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.C.
CONTENTS

Page

Abstract _______________________________________________________ 25

Introduction._ _____________________________ ______^________________ 25

General geology___________________________________________________ 27

Earthquakes._____________________________________________________ 28

Baranof Lake powersite.___________________________________________ 29

Location and accessibility ______________________________________ 29

Topography and vegetation___________________________________ 29

Geology----------_-_-_-____-__-_______-___-_-_-_-___-_-_-____ 30

Bedrock___.____.______________________.___.__________ 30

Structural features..-.____________________________________ 30

Unconsolidated deposits__----_______-_____-_---___-__---___ 33

Hot springs.______________________________________________ 33

Development schemes________________________________________ 34

Damsite_ ___________________________________________________ 35

Appurtenant works ____________________________________________ 36

Conduit routes to powerhouse sites__ ________________________ 36

Powerhouse sites--_____-_-_________-_-___-_---_-_-------__ 36

Exploration.__ ________________________________________________ 36

Construction materials-_-_____--____-__-__-_-__-_----_-___-____ 37

Reservoir site.___________________________--____-_-__--_--__-_. 37

Carbon Lake powersite-------------_-_-----_-----_----------------- 38

Location and accessibility. _-______________.__-___---____-------- 38

Topography __________________________________________________ 38

Geology._____________________________________________________ 40

Bedrock._________________________________________________ 40

Structural features--__-_-----_____----_____---__---__---__- 40

Unconsolidated deposits-___________________________________ 41

Development scheme._-____.-_-_-____.__-._____--_--____-__-_-_ 41

Damsite_ __-__-___-_____-____________-.__________-_____-----__ 41

Saddle damsite________________________________________________ 43

Appurtenant works-___________________________________________ 44

Conduit routes to powerhouse sites_ _________________________ 44

Geology.-_----______-___j___________-_---__--------__ 44

Anticipated rock conditions.____________________________ 45

Powerhouse site.__________________________________________ 45

Reservoir site________--_________________________--___----_---_ 45

References cited-_____-_-_-_____,____________-_. ,______________--_-_ 46

m
IV CONTENTS

ILLUSTRATIONS

Page 

PLATE 5. Geologic maps and sections of proposed powersites at Baranof

and Carbon Lakes-__---------------_----------_---- In pocket

FIGURE 2. Index map showing location of Baranof and Carbon Lakes.-__ 26

3. East end of Baranof Lake___-__-___________-____-_____-- 31

4. View south westward across lower end of Baranof Lake.------ 31

5. Shore of Warm Spring Bay south from mouth of Baranof

River._______________________________________________ 34

6. Rock ridge at east end of Carbon Lake.____________________ 39

7. Carbon Lake outlet._____________________________________ 42
GEOLOGY OF WATERPOWER SITES IN ALASKA

GEOLOGIC INVESTIGATIONS OF PROPOSED POWERSITES 

AT BARANOF AND CARBON LAKES, BARANOF ISLAND,

ALASKA

By KENNETH S. SOWARD

ABSTRACT

The dam, conduit, and powerhouse locations that would be considered in the 

development of the Baranof Lake powersite are all on or in biotite-quartz diorlte that is sound, strong, and suitable as a foundation for such structures. No 

large scale structural defects are exposed, but on aerial photographs1 linear 

features, which are interpreted as faults, can be projected through the damsite 

and across the adjacent ridge at points 400 and 3,200 feet south of the Baranof 

Lake outlet. Overburden 1 to 15 feet thick covers much of the area in which a 

dam would be located, and detailed exploration by drilling will be necessary. Hot 

springs occur 500 to 1,000 feet east of the lake outlet along the left bank of the 

Baranof River, between the altitudes of 120 to 154 feet.

The damsite and probable tunnel route of the Carbon Lake powersite are on or 

in migmatite that is strong and suitable as a foundation for such structures. The 

penstock and powerhouse sites are on a massive biotite-quartz diorite dike that 

forms a cliff along the inner shore of Cascade Bay. Overburden covers bedrock on the left (west) abutment and in the stream channel at a possible dam 

location. A large rock fall is at the toe of the right (south) abutment. Detailed exploration by drilling is necessary in these areas.

INTRODUCTION

The U.S. Geological Survey, as part of its program of classifying 

public lands with respect to mineral and water resources, is currently 

making a systematic study and evaluation of the potential waterpower sites in Alaska. This report describes geologic conditions at 

two proposed powersites on the east coast of Baranof Island, Alaska 

(fig. 2). The sites were examined to determine their limiting geologic features and to outline the next steps for an orderly and complete geologic investigation. The general term "powersite" is used 

in this report to include all elements, such as the dam, conduit, powerhouse, and reservoir sites of power development.

25
26 GEOLOGY OF WATERPOWER SITES IN ALASKA

136°

FIGDEE 2. Index map showing location of Baranof and Carbon Lakes.

The geology of Baranof Island has not been investigated in detail. 

The regional geology is described by Knopf (1912) and Overbeck 

(1917), and it is shown on the "Geologic Map of Alaska" (Dutro and 

Payne, 1957). The geology of the island is referred to briefly in 

the summary report on the geology of southeastern Alaska by Buddington and Chapin (1929). Detailed mineral investigations of small 

parts of the island, mainly on the west coast, have been made (Guild
BARANOF AND CARBON LAKES, BARANOF ISLAND 27

and Balsley, 1942). The waters from the hot springs at Baranof 

have been reported on by Waring (1917). Power potential and possible means of development of the lakes were first investigated by Dort 

(1924, p. 110-116). The sites are also described in a report by the 

Federal Power Commission and the U.S. Forest Service (1947).

The topographic surveys and the geologic investigations of the 

Baranof Lake and Carbon Lake powersites were made during July 

and August 1956.

Baranof Lake and Carbon Lake are shown on the U.S. Geological 

Survey Sitka quadrangle of the Alaska reconnaissance topographic 

series, scale 1: 250,000 (1 inch equals approximately 4 miles), contour 

intervals 200, 250, and 1,000 feet; and on the Sitka A-3 quadrangle 

of the 1: 63,360 (1 inch equals 1 mile) topographic series, contour interval 100 feet. The lakes and adjacent areas are shown on a river 

survey map, "Plan and Profile, Baranof Lake and Carbon Lake, 

Alaska, Dam Sites." On this map the lakes and adjacent areas are 

shown at a scale of 1: 24,000 (1 inch equals 2,000 feet) with 20-foot 

contour interval. Underwater contours are shown for the lakes to 

depths of 60 to 100 feet below lake level with a few spot soundings indicating maximum depth. Maps of the prospective Baranof Lake and 

Carbon Lake damsites are included. The former is on a scale of 

1: 2,400 (1 inch equals 200 feet) and the latter on a scale of 1: 4,800 

(1 inch equals 400 feet). Both damsite maps have a 10-foot contour 

interval. A profile of the mapped reaches of each stream, including 

the surface and approximate bottom profile of each lake along its 

centerline, is shown.

Baranof Island is rough and mountainous. Along the east shore 

the ground surface rises steeply from sea level to altitudes ranging 

from 1,000 to 2,900 feet. Above these altitudes the slope flattens, 

but the ground surface continues to rise to altitudes of 5,000 to 6,000 

feet in the mountainous interior.

GENERAL GEOLOGY

Baranof Island has been extensively glaciated, and the present 

topography is the result of ice erosion only slightly modified by later 

stream erosion. Alpine glaciers flowed down from the central mountains and eroded spectacular glaciated valleys. These valleys often 

terminate in small fiords that indent the coastline for distances of 

0.8 to 6 miles, and they are spaced 2 to 13 miles apart along the 

straight eastern shoreline of the island. Glacial lakes occupy some 

of the lower sections of the valleys. The streams in the valleys have 

high gradients, and many of them cascade into tidewater at the 

head of the fiords.
28 GEOLOGY OF WATERPOWER SITES IN ALASKA

The rocks of Chichagof and Baranof Islands lie in broad belts that 

strike northwest-southeast and conform with the prevailing structural 

trend of southeastern Alaska (Knopf, 1912, p. 11). The core of the 

island is made up largely of quartz diorite intruded parallel to the 

stratified rocks. A large batholith that crosses the northern end of 

the island is exposed over a distance of 34 miles and a width of 5 to 

10 miles. Bordering the batholith on the southwest is a northwestward-trending belt of metamorphic rocks. In turn, this belt is 

bounded by another composed of graywacke and slate' that forms 

the bedrock on the west side of the islands.

Baranof Lake powersite is in biotite-quartz diorite about 1 mile 

north of the southern edge of the batholith, as shown on the "Geologic 

Map of Alaska" (Dutro and Payne, 1957). Carbon Lake powersite 

is about three-fourths of a mile south of the batholith in an area of 

metamorphic rock. Bedrock is a migmatite.

An inferred major northward-striking fault lies in Chatham Strait 

about 9 miles east of Baranof Lake outlet (Wright and Wright, 1908; 

Martin and Williams, 1924). The estimated length of the fault is 

250 miles, extending north from a point about 100 miles south of 

Warm Spring Bay to Lynn Canal. The Chatham Strait fault is a 

split or extension of the Denali fault, a great arcuate fault that can 

be traced from Lynn Canal to Bristol Bay (St. Amand, 1957). Twenhofel and Sainsbury (1958, p. 1435) believe that the Denali fault 

and probably the Chatham Strait fault are active along their entire 

length and have been active, at least since early Tertiary.

A large northwestward-striking fault that lies in Peril Strait intersects the Chatham Strait fault about 11 miles northeast of Baranof 

Lake outlet. Twenhofel and Sainsbury (1958, p. 1435) include this 

fault as a segment of a large fault, which they call the Clarence Strait 

lineament and which extends from Clarence Strait to Cross Sound. 

The Clarence Strait fault may intersect the active Fairweather fault 

that lies on the southwest side of the Fairweather Range. However, 

they believe this interpretation may not be correct as the. Fairweather 

fault cuts Tertiary (Eocene) rocks, whereas the Clarence Strait 

lineament on Kuiu, Kupreanof, and Admiralty Islands is covered 

by Tertiary (Eocene) rocks and presumably has not been active 

since that time.

EARTHQUAKES

Although the structural settings, within and adjacent to a major 

batholith, provide favorable locations for dams and related structures, 

the earthquake history of southeastern Alaska shows that the area is 

still seismically active. In the Sitka-Juneau-Skagway area, 16 earth-
BARANOF AND CARBON LAKES, BARANOF ISLAND 29

quakes with an estimated intensity of 5 or more on the Modified Mercalli scale have been reported between 1843 and 1956 (Heck and' 

Eppley, 1958, p. 72-78). Between Cape Spencer and Yakutat Bay, 

120 to 250 miles northwest of Baranof and Carbon Lakes, 9 earthquakes with intensities between 5 and 10 have been recorded between 

1896 and 1946. The Cape Spencer-Yakutat Bay area lies along or 

adjacent to the Fairweather fault. The relation of nearby active and 

inactive faults with areas of recorded strong seismic activity makes 

it prudent that maximum protection against earthquakes be considered 

in the design of any structure built at the sites. Also, if a powerhouse 

should front on tidewater, consideration should be given to tsunamis in its design.

BARANOF LAKE POWERSITE

LOCATION AND ACCESSIBILITY

Baranof Lake damsite is at the outlet of Baranof Lake, 0.3 mile 

from the point where Baranof River discharges into Warm Spring 

Bay. (See pi. 5.) Baranof is a small settlement at the upper end of 

the bay. It is a popular port of call for fishing boats and yachts because of its protected harbor and nearby hot springs. Waters from 

the springs are used in one small public bathhouse and a number of 

tubs in cabins. In 1956, there was weekly airmail service and weekly 

boat service for mail and freight between Baranof and Juneau. The 

proposed dam and powerhouse sites are easily accessible by trail from 

Baranof.

TOPOGRAPHY AND VEGETATION

Baranof River and Lake are in a deep, narrow, U-shaped glacial 

valley. The lower end is now occupied by a fiord, Warm Spring Bay, 

which is 2.4 miles long, and Baranof Lake, which is 3.0 miles long. 

The lake, with a water surface at altitude 145 feet, is retained by a 

low rock ridge that extends north for 3,600 feet across the valley. The 

highest point on the crest of the ridge is at an altitude of about 325 

feet. The lake outlet is within 1,500 feet of tidewater as is also a 

point on the lakeshore 1,400 feet south of the outlet. (See pi. 5.) 

A small hill west of the south end of the ridge rises to an altitude of 

500 feet. A saddle crosses the ridge about 3,200 feet south of the lake 

outlet at the point where the ridge joins the main valley wall. The 

maximum altitude of the ground surface in the saddle is about 240 

feet above sea level, or 95 feet above the lake level. Another smaller 

saddle with ground surface near altitude 235 feet is along the north 

valley wall about 450 feet north of the lake outlet.
30 GEOLOGY OF WATERPOWER SITES IN ALASKA

Baranof River crosses the ridge about 600 feet south of the north 

' valley wall and in one-third mile descends 145 feet to the bay. From 

the lake outlet to a point TOO feet downstream, through the proposed 

damsite, the river falls 8 feet.

The damsite is immediately downstream from the lake outlet. (See 

pi. 5 and figs. 3 and 4.) Near the possible axis line of the dam shown 

in section A-A' (pi. 5) the valley is 110 feet wide at the river surface 

at altitude 145 feet; 150 feet wide at 150 feet; 790 feet wide at 200 

feet; 1,040 feet wide at 240 feet; and about 2,300 feet wide at 300 feet.

A heavy growth of vegetation covers much of the damsite area. 

The left (north) abutment has been burned over and now supports 

a thick growth of lodgepole pine, hemlock, and grass. On the right 

(south) abutment the soil layer on top of bedrock is thin and tree 

growth is sparse. However, bedrock is concealed in many places by 

a heavy growth of grass and light brush, and small peat bogs have 

developed in the undrained depressions on bedrock. Natural terraces 

with bogs have been built on some slopes. Wherever the soil layer 

is thick enough, there is a heavy stand of timber.

GEOLOGY 

BEDROCK

All rock exposed at the damsite is classified from megascopic examination as biotite-quartz diorite or a closely related rock type. The 

rock is light to dark gray and medium to fine grained and has crystals 

that range from 1 to 4 mm in length. The biotite crystals are wavy 

and many of the quartz and plagioclase crystals are slightly crushed 

or granulated. Occasional garnet crystals or masses 0.5 to 2.0 mm 

in diameter occur. Rocks of more dioritic composition occur in small 

exposures at the footbridge, 100 feet south of the rapids between altitudes of 135 and 116 feet, and near the mouth of Baranof River.

STRUCTURAL FEATURES

The only structural features exposed at the site are a few small 

faults and joints. The faults range from partings with slickenslides 

to thin seams with as much as 2 inches of broken rock and gouge 

along them. Their general strike is to the north, and 3 of 4 fractures 

with crushed rock along them are in a set that strikes N. 12°-30° W. 

and dips 80°-88° SW. The other faults fall into sets with the following attitudes: strike N. 8° E. and N. 7° W., dip 68° and 79° E.; N. 

27°^5° E., 26°^0° SE.; and N. 25°-27° W., 41°-63° NE.

Two major joint sets include 56 percent of the joints on which 

observations were taken. Of the remainder, 23 percent fall into 5 

sets that have 2 to 4 joints per set, and 21 percent are fractures with
BARANOF AND CARBON LAKES, BARANOF ISLAND 31

%f "V; -, : - «+-*«***r *_ zwBHBOP^^wr*-" »

FIGURE 3. East end of Baranof Lake. Outlet of lake is slightly left of center of view.

FIGURE 4. View southwestward across lower end of Baranof Lake. Lake outlet in lower

left center of view.
32 GEOLOGY OF WATERPOWER SITES IN ALASKA

random orientations or have strikes and dips that are close to those 

of the sets. The two major sets have the following attitudes: strike 

N. 20°-45° E., dip 16°-40° SE., and N. 20° W.-N. 19° E., 66°-89° E. 

The minor sets have the following attitudes: strike N. 27°-43° W., 

dip 36°-57° E.; N. 25°-28° W., 84°-89° SW.; N. 50°-55° W., 82°- 

85° NE.; and N. 72°-82° E., 80° SE. to 85° NW.

On the aerial photographs covering Warm Spring Bay and Baranof 

Lake, three sets of strong linear features stand out prominently. 

Linear features are straight or gently curving physiographic features, 

and these are interpreted as faults or fault zones. The bearing of 

one set is N. 16° W. to N. 15° E. This direction correlates with one 

of the major joint sets and one of the minor fault sets. A second 

set strikes N. 37°-65° E. and partly overlaps the other major joint 

set and a minor fault set. The third set strikes N. 31°-60° W. and 

correlates roughly with two of the minor joint sets.

The linear features may be related to structural features of greater 

size and extent than any exposed at the damsite. In the set that 

bears N. 37°-65° E. they are continuous and pronounced. One of 

the linear features can be projected along the draw that is 400 feet 

south-southeast of the lake outlet, and its extension to the east-northeast would lie in the river channel. However, no structural feature 

that can be related to it is exposed in the area. Any fault or shear 

zone striking through the draw would have to be less than 2 or 3 

feet wide, because closely spaced bedrock outcrops near the river about 

250 feet northeast from the point where section A-A' crosses the 

draw would expose a wider fault zone. Another linear feature that 

falls in this set extends through the narrow saddle at the south end 

of the ridge. No structural features are exposed, but a sharp small 

reentrant is present in the east face of the ridge northeast of the 

saddle. At altitude 190 feet, rock outcrops are 100 feet apart on 

either side of the reentrant, and a fault or shear zone as much as 20 

to 30 feet wide could be concealed.

A prominent line on the photographs that bears N. 6° W. crosses 

the damsite area. Any structural features related to it should crop 

out in the south bank of Baranof River 300 to 400 feet upstream from 

the footbridge. Apparently the line reflects only minor structural 

features, because in this area two small faults and some joints are 

exposed. The faults, which have thin zones of broken rock 1 to 2 

inches thick along them, strike N. 17° and 30° W. and dip 84° SW. 

The joints that parallel the line strike N. 10°-15° W. and dip 

77°-84° NE.

A prominent linear feature that bears N. 35° W. extends through a 

small indentation in the shoreline 1,500 feet south of the lake outlet. 

There are no exposures of any structural feature related to it.
BARANOF AND CARBON LAKES, BARANOF ISLAND 33 

TJNCONSOLJDATED DEPOSITS

Throughout most of the damsite area, a thin layer of unconsolidated material overlies bedrock. Alluvium consisting of boulders, 

gravel, and sand is exposed in a few places along the left river bank 

from the lake outlet downstream for 600 feet. Sand and silt are exposed on the lake bottom near the outlet and along the shore. In the 

draw north of the lake outlet from 200 to 800 feet east of the rock outcrop at the bend in section A-A' (pi. 5), and in the small draw to 

the west of the outcrop, there are boulder-strewn areas with subangular boulders 2 to 3 feet and a few as much as 7 or 8 feet in diameter. Soil, gravel, and silt are exposed between the boulders. Over 

much of the remainder of the site, topsoil and a heavy growth of 

grass, brush, and timber conceal the underlying deposits.

Although the ridge alined across the lake outlet is a glacial feature, 

no till is exposed in it. A thin layer of soil and rocks that may have 

been derived from till is exposed along the edge of the lake in the 

small cove about 1,500 feet south of the outlet. In the draw northeast of the lake outlet, till may be at depth, but no exposures were 

found. At Baranof, impervious boulder clay is exposed in a small 

drainage ditch around one of the cabins at the foot of the north valley 

wall.

HOT SPRINGS

Baranof hot springs are on the north side of the Baranof River 500 

to 1,000 feet east (downstream) from the lake outlet. The springs 

are between altitudes of 120 and 154 feet or from 25 feet below to 9 

feet above the surface altitude of Baranof Lake. Four of the ten 

springs found are 2 to 9 feet above the lake level. All the springs 

issue from unconsolidated deposits of quartz diorite boulders, gravel, 

and silt, but spring 3 (see pi. 5) is only a short distance from the bedrock outcrop.

The hot springs have been discussed by Waring (1917, p. 27-32). 

As closely as possible, the numbering of the springs shown on plate 

5 follows that given on his sketch map. Spring 9 in the river channel was not noted in this survey, and its location is from Waring's 

map. Springs 10 and 11 are minor seeps not shown on his map.

Waring classified the water from the springs as sodium carbonate 

type, of moderate total mineral content, with silica making up an 

unusually large proportion of the total. He concluded that the springs 

appear to be in a locality of some crustal movement, and may possibly 

be ascribed to fault fissures that allow the upward escape of deepseated waters.

The analyses of Baranof hot springs, are comparable to those of 

siliceous geyser waters given by Clarke (1924, p. 197). However, the

588295 O 61 2
34 GEOLOGY OF WATERPOWER SITES-IN ALASKA

mineral content of the geyser waters is much greater, and the percentage of chloride ions in all but one analysis is 4 to 12 times greater 

than that of the Baranof waters. Although Waring attributes the 

waters to a deep-seated source, the low mineral content and small 

percentage of chloride ions indicates that the magmatic waters are 

being diluted by metoric water.

A number of the springs are along two lines that bear N. 52° E. and 

N. 82° E. These bearings fall in or close to a major set of linear 

features observed on the aerial photographs and may reflect fractures 

along which the water could rise. To the southwest these lines would 

intersect the lake basin, but if they do, the openings must now be 

sealed.

DEVELOPMENT SCHEMES

Development of the Baranof Lake powersite could be accomplished 

by the construction of a dam at the lake outlet of sufficient height to 

obtain the required storage for regulation of the streamflow. The 

water could be conveyed by a conduit from the resulting reservoir 

to a powerhouse located either on the shore of Warm Spring Bay about 

250 feet south of the mouth of Baranof River or on a tidal lagoon 

2,100 feet south of the river mouth (fig. 5). The conduit would be 

either a pipeline or tunnel, or a combination of both. If the powerhouse were located at a point about 250 feet from the mouth of

FIGURE 5. Shore of "Warm Spring Bay south from mouth of Baranof River. Note lagoon

in upper left part of view.
BARANOF AND CARBON LAKES, BARANOF ISLAND 35

Baranof River the conduit (pipeline) would cross the bench in the 

cleared area above the river on the right side o^ the view in figure 5. 

If the powerhouse were located on the lagoon, water would be conveyed to it from the lake by a tunnel through the ridge shown in the 

upper right part of the view.

DAMSITE

The most favorable location for a dam, based on topographic considerations, is immediately downstream from, the lake outlet. This 

location is indicated by section A-A' (pi. 5). Along this section, 

bedrock crops out occasionally or is covered by unconsolidated deposits, estimated to be as much as 10 to 15 feet in depth. However, 

at two places the depth to bedrock may be greater. A narrow, deep 

channel or slot could be concealed between the rock outcrop on the 

right (southeast) bank 50 feet downstream from section A-A' and 

the next rock outcrop 265 feet to the northwest. Similarly, between 

the north end of the section and the first outcrop 150 feet to the south, 

the depth to bedrock may be more than the estimated 10 to 15 feet. 

Immediately north of the section, the valley wall rises at a steep angle 

for 200 feet, and bedrock is at shallow depths.

A fault or shear zone may be concealed in the low area about 400 

feet south of the outlet where the linear feature that strikes N. 57° E. 

crosses section A-A'. About 660 feet south of the outlet the ground 

surface rises to above altitude 250 feet, and from that point to the 

south end of the section, scattered bedrock outcrops extend through 

a thin cover of topsoil, some alluvium, and a small amount of talus.

The saddle about 3,200 feet south-southeast from the lake outlet 

must be considered in any plan of development which would raise 

the maximum lake altitude an appreciable amount. The ground 

surface in this saddle is at an altitude of 240 feet, which is 95 feet 

above normal lake level. The saddle area is covered by soil and a 

heavy growth of grass and some brush. The bedrock surface is 

estimated to be as much as 30 to 40 feet or more below the ground 

surface. There are a number of large angular blocks of talus, some 

of which are 15 feet in diameter, at the foot of the south valley wall. 

A strong linear feature extending through the saddle probably indicates a fault or shear zone. Corrective measures such as grouting 

or a cutoff wall may be required to prevent serious leakage through 

the fault or shear zone.

The exposed rock in the damsite area is biotite-quartz diorite that 

is light to dark gray and medium to fine grained and has crystals 1 

to 4 mm in length. This rock has a high crushing strength and low 

permeability. It is suitable as a foundation for a concrete dam of 

any height that would be considered for this site.
36 GEOLOGY OF WATERPOWER SITES IN ALASKA

APPURTENANT WORKS 

CONDUIT ROUTES TO POWERHOUSE SITES

A conduit to a powerhouse site on the lagoon would include a tunnel 

and penstock. The tunnel would be in biotite-quartz diorite for its 

entire length (B-B', pi. 5). No major defects are exposed along 

the route, but two linear features previously mentioned, cross the 

line. The linear feature that bears N. 35° W. and lies along the 

northeast side of the high rock knob west of the ridge might intersect a tunnel driven at lake level near the portal and would intersect a tunnel driven at a lower altitude within 150 to 200 feet of the 

portal. The fault or shear zone related to the linear is not exposed, 

and detailed exploration is required to determine its size and effect 

upon a tunnel. The second linear feature bears N. 6° W. and crosses 

section B-B' 1,000 feet east of the lake. Only small faults and some 

joints are exposed where the north extension of the linear feature 

crops out in the south bank of Baranof River. These small structural features would intersect B-B' at angles of 50° to 70° and would 

cause no trouble during excavation of a tunnel. The rock along 

section B-B' is strong and sound, and will probably not require 

support during tunnel excavation except possibly at the intake portal.

A pipeline could be used to carry the water to an alternative powerhouse site 250 feet south of the river mouth. The first 400 feet 

of the pipeline could be placed in a shallow opencut excavated along 

the draw 400 feet south-southeast of the lake outlet. Downstream 

from the intersection of the draw and the river, the pipeline could 

be carried for about 750 feet on a bench excavated into the river bank. 

Geologic conditions will cause no construction difficulties along the 

opencut and bench. If part of the lake basin were to be used for 

live storage, a tunnel might be necessary. If so, two possible defects 

would have to be avoided: the tunnel line should be moved southeast 

of the draw away from the fault or shear zone that strikes along it, 

and any fractures that contain hot water would have to be located 

and sealed off from the tunnel.

At both powerhouse sites the penstocks could be placed either on 

the surface or in tunnels in strong, sound biotite-quartz diorite.

POWERHOUSE SITES

Both of the powerhouse sites considered are in strong, sound biotitequartz diorite.

EXPLORATION

Adequate exploration of the site should be possible by drill holes. 

To insure that no narrow slots eroded along faults or shear zones
BARANOF AND CARBON LAKES, BARANOF ISLAND 37

are missed, angle drill holes should overlap in rock beneath the unconsolidated deposits along the proposed axis. If any zones of hydrothermally altered rock are found additional exploration will be 

necessary, because extensive correctional work might be required. To 

determine if the fissures along which the hot water rises extend to 

the damsite, a comparison could be made between the flow of the 

springs under normal conditions and during water-pressure tests in 

the holes drilled along the axis of the dam.

At the south end of the ridge, angle drill holes are required to explore the saddle for a narrow slot eroded along the fault that might 

be concealed there.

CONSTRUCTION MATERIALS

Sand and gravel are lacking in the vicinity of the damsite. A limited amount of aggregate for concrete may be available on the left 

abutement, in the river channel, and on the lake bottom near the 

outlet. However, the main source of aggregate is the outwash deposit 

in the valley upstream from the lake. At the point where Baranof 

River flows into the lake, there is a large exposure of subround to 

tabular gravel that has a maximum diameter of 6 inches and an average of 3.

The composition of the gravel is 50 to 60 percent biotite-quartz 

diorite, 35 to 45 percent fine-grained quartzite, 5 percent argillite, and 

a small amount of chert. In the biotite-quartz diorite the biotite 

has worn or weathered out, so that most of these rocks now have a 

pitted surface.

A large amount of fine sand and silt is exposed on the shore north 

of the mouth of the river.

RESERVOIR SITE

Baranof Lake is 3 miles long and 1,000 to 3,800 feet wide. The 

lake completely fills the U-shaped valley bottom, and from the shoreline the valley walls rise in steep slopes. Baranof Eiver valley extends for about 4 miles upstream from the lake, gradually narrowing 

from 2,000 feet to 600 feet. At the upper end the steep walls of the 

central mountain range rise to altitudes above 5,000 feet.

The lake bottom descends on a moderate slope, and 600 feet south 

and 300 feet west of the outlet, it is at altitude 30 feet or 115 feet 

below the lake surface. About 1,900 feet southwest of the outlet, 

the lake bottom is 135 feet below sea level, or 280 feet below the lake 

surface. At the deepest point, 2.1 miles from the outlet, the lake 

has a depth of 303 feet.
38 GEOLOGY OF WATERPOWER SITES IN ALASKA

The entire lake basin is in igneous rock. Most of the rock is biotitequartz diorite similar to that at the damsite, but some diorite was 

noted at the upper end. Glacial outwash covers the floor of the 

valley upstream from the lake.

The rock in the walls of the reservoir is sufficiently impermeable 

that only minor water losses will occur.

CARBON LAKE POWERSITE 

LOCATION AND ACCESSIBILITY

Carbon Lake powersite is on the first large drainage south of Warm 

Spring Bay and Baranof River (fig. 2). The damsite i& on Coal 

Creek, at the outlet from Carbon Lake, about 4.5 miles south-southeast of Baranof, and 1 mile west of Cascade Bay. The powerhouse 

site, at the head of Cascade Bay, is easily accessible by boat, but the 

damsite and conveyance route are accessible only on foot. For the 

first 3,000 feet from the bay, the easiest route to the damsite is along 

the south side of the valley. For the last 2,000 feet, the easiest route 

is along the valley wall between the altitudes of 250 and 400 feet.

TOPOGRAPHY

Coal Creek, with a reported flow slightly larger than that of Baranof River, heads in the high mountains and glaciers near the midsection of Baranof Island. The creek, like Baranof River, falls into 

tidewater at the head of a bay or small fiord that makes a reentrant of 

0.9 mile in the relatively straight east coast of the island.

Coal Creek valley has been glaciated, but erosion has not been as 

great as in Baranof River valley, as is shown by the narrower, more 

irregular valley, and the smaller fiord. The downstream 3 miles of 

Coal Creek valley is 400 to 1,000 feet wide and fairly irregular. The 

upper 3 miles is 2,000 to 3,000 feet wide and quite straight.

Carbon Lake fills the valley bottom from a point 1 mile west of the 

bay to 3.5 miles upvalley. The lake ranges from 250 to 2,100 feet 

in width and has a surface area of about 400 acres. On August 16, 

1956, the altitude of the lake surface was 211.4 feet. The lake is impounded on the east by a northward-trending ridge that extends 

1,200 feet north from the south valley wall. The ridge is 400 to 550 

feet wide, and on the east side drops off in an almost vertical cliff 75 

to 100 feet high (fig. 6). On the west or lake side, the slope 

is gradual, and the 150-foot contour, which is under water, is 350 

feet west of the lakeshore. There are two high points on the ridge: 

one at altitude 331 feet 700 feet north of the south valley wall, and 

the other at altitude 284 feet 200 feet from the south wall (pi. 5).
BARANOF AND CARBON LAKES, BARANOF ISLAND 39

FIGURE 6. Rock ridge at east end of Carbon Lake. Lower end of Carbon Lake in left 

edge of view. Unnamed lake in upper right part of view drains into Coal Creek 650 

feet downstream from Carbon Lake outlet.

Between the two knobs, the ground surface descends to within a few 

feet of the lake surface, and during periods of high water, a small 

stream flows through the saddle. Between the lower knob and the 

south valley wall, the ground surface is 5 to 6 feet above lake level. 

Water has not flowed through this saddle recently.

About 3,500 feet west of the lake outlet a saddle at an altitude of 

295 feet separates Carbon Lake from two small unnamed interconnected lakes about one-half mile north of the lower part of Carbon 

Lake. These lakes, which are at an altitude of 246 feet, are drained 

by a small stream that flows into Coal Creek, 650 feet downstream 

from the outlet of Carbon Lake.

About 1,500 feet downstream from Carbon Lake a small, rounded, 

glaciated point, which rises to altitude 300 feet, extends out from the 

south valley wall. The point produces a narrows in the valley and a 

fall of 25 feet in the river. East of the point the valley is 600 to 1,000 

feet wide. The fairly level floor, which is near altitude 100 feet, 

extends to within 300 feet or closer to the bay before it drops off in a 

steep cliff to tidewater.

Coal Creek has a fall of 12 feet in a distance of 450 feet downstream 

from the outlet of Carbon Lake. In the next TOO feet, the stream 

descends 75 feet to altitude 125 feet in a series of rapids and small 

falls. For 1,000 feet below these rapids, the stream is wide and slow
40 GEOLOGY OF WATERPOWER SITES IN ALASKA

flowing. Below this it falls 25 feet to another wide, lakelike reach 

0.7 mile long at an altitude of 98 feet. From this section the river 

cascades over a rock ledge into the bay.

GEOLOGY 

BEDROCK

The entire powersite area is underlain by metamorphic rocks that 

are migmatites or composite gneisses, resulting from intimate penetration of magma into country rocks. From a megascopic examination it appears that sedimentary rocks, originally either impure 

quartzite or a graywacke high in quartz, have been intensely metamorphosed and intruded on a large scale by biotite-quartz diorite. 

The intruding biotite-quartz diorite is in sheets or dikes that at the 

damsite are as much as 10 feet thick, and in the reservoir and downstream from the damsite are as much as 150 feet thick. Most of the 

rock is foliated, slightly schistose, with layers of fine- to mediumgrained granulated quartz grains and thin layers of wavy biotite. 

The texture of the intruding biotite-quartz diorite is generally 

granitoid, but in some places the crystals have been granulated and 

some small masses of the rock have been drawn out slightly. The 

quartz diorite probably was intruded over a considerable range of 

temperature, pressure, and time, and the earlier intrusions have been 

dynamically modified.

Although the petrologic history is quite complicated, the resulting 

gneiss is strong and very capable of supporting a concrete dam and 

related structures and tunnels.

STRUCTURAL FEATURES

Throughout the powersite area the attitude of the foliation varies 

widely, often within short distances. The strikes range from east to 

K 60° W., and the dips range from 20° to 70° north and west. At 

the damsite the strike of the foliation ranges from N. 30° E. to N. 

45° W., and the dips range from 29° to 70° W.

Only minor faults were found in the dam, reservoir, and tunnel 

areas. However, on the aerial photographs there are strong, extensive linear features, some of which may be topographic expressions 

of large structural features. At the damsite the low saddle across 

the ridge (C-C*', pi. 5) is controlled by a narrow fault that strikes 

N. 34° E. and dips 34° SE. The fault has a zone of broken and 

crushed rock y2 inch to 4 inches thick. A related slip that strikes 

N. 23° E. and dips 34° NW. also has a zone of crushed rock a few 

inches thick along it. Two other minor faults, with attitudes of N.
BARANOF AND CARBON LAKES, BARANOF ISLAND 41

2° E., 55° NW., and K 37° W., 36° SW., have zones of crushed rock 

1/2 inch to 3 inches thick along them.

On the aerial photographs a strong linear feature follows the small 

draw 1,000 feet east of the damsite. A careful search of the draw 

disclosed a fault with a 21/£-foot zone of closely jointed rock with a 

few thin seams of crushed rock. The attitude of the fault is N. 30° 

W., 38° SW., and slickensides show that the hanging wall has moved 

to the southeast. Near altitude 325 feet a 4-foot-thick quartz diorite 

dike lies along the fault.

A few minor faults undoubtedly are concealed by the surficial 

deposits, but no major fault is suspected in the dam or tunnel areas.

The bedrock is cut by a few long, continuous joints.

UNCONSOI.rDATEI> DEPOSITS

Surficial deposits at the damsite and on the valley walls include 

thin, patchy layers of organic material, soil, silt, sand, gravel and 

talus. Upstream from Carbon Lake the valley bottom is covered by 

an extensive deposit of outwash material, but downstream from the 

lake only thin deposits of alluvium overlie bedrock.

DEVELOPMENT SCHEME

Development of Carbon Lake powersite could be accomplished by 

construction of a dam at the lake outlet of sufficient height to obtain 

the required storage for regulation of streamflow. The water would 

be conveyed by a pressure conduit from the resulting reservoir to a 

powerhouse located on the shore of Cascade Bay. An auxiliary dam 

would be required in the saddle west of the lake outlet if the flow 

line is above altitude 295 feet.

DAMSITE

The control of Carbon Lake would in addition to a main dam on Coal 

Creek require two auxiliary dams in the saddles described on page 58. 

If the lake is raised to an altitude of more than about 280 feet, these 

three dams would be connected and become a single structure with an 

overall length of about 950 feet. The main dam would be located 

300 feet downstream from the lake outlet. The geologic conditions 

along the axis of the main dam as well as of the two additional dams 

are shown by section C-C' (pi. 5). The left (west) abutment is 

formed by a hill that rises to altitude 1,095 feet. The right (east) 

abutment for the main dam would be formed by the rock knob that 

rises to altitude 331 feet. (See fig. 7.)

The foundation and abutments are believed to be suitable for a 

concrete dam to a height that would develop the required reservoir
42 GEOLOGY OF WATERPOWER SITES IN ALASKA

FIGURE 7. Carbon Lake outlet. Probable axis for dam would cross channel at upper end 

of rapids. Unnamed lake In upper center part of view drains into Coal Creek 650 feet 

downstream from Carbon Lake outlet.

capacity. Along section C-C', bedrock is exposed or is covered by a 

thin layer of overburden, except at two places where there is an estimated 20 to 30 feet of unconsolidated material. One of these areas 

extends from the right (east) bank of Coal Creek to the left (west) 

abutment up to about altitude 280 feet. There is an estimated 10 

feet of alluvium in the river channel and as much as 20 feet of mixed 

talus, topsoil, and silt on the left bank. The second area extends from 

the toe of the south valley wall near altitude 215 to altitude 270 feet. 

A rock slide has occurred here, and large blocks as much as 20 feet 

in diameter are stacked at the base of the slope. The depth to bedrock may be as much as 30 feet, and possibly more if the toe of the 

rock slope has been undercut. On the valley wall above the slide from 

an altitude of 270 to 360 feet, there is a thin layer of overburden and 

talus. Bedrock is exposed from an altitude of 360 to 400 feet. The 

rock face on the south valley wall and the rock face on the right (east) 

side of Coal Creek at section C-G' are controlled by the dip of the 

foliation.

Permeability of the rock appears to be low. Water losses will occur mainly along fractures such as the minor faults, joints, or partings 

parallel to the foliation. Although the grains of the rock have been 

granulated, the minor fractures probably are limited in extent and
BARANOF AND CARBON LAKES, BARANOF ISLAND 43

are not interconnected. Metamorphism has greatly reduced the original porosity in the rock, and small or no water losses are expected 

because of percolation through the pores.

If a limited exploration program is undertaken, the first work 

should be concentrated in the stream channel, the lower part of the 

left abutment, and on the lower part of the right (south) abutment.

SADDLE DAMSITE

This site is in a saddle that trends north-northeast from Carbon 

Lake. The low point is at an approximate altitude of 295 feet, and 

between the 300-foot contours the saddle is 80 to 100 feet wide over 

most of its length. The right abutment rises at a gradual slope of 

about 20°, and the left abutment rises in a series of steps at an average slope of 35°.

The right (east) abutment has a thin, patchy cover of topsoil, 

silt, and sand. On the left (west) abutment the steps between the 

rock cliffs are covered by an estimated 10 to 15 feet of talus mixed 

with topsoil, silt, and sand.

Bedrock at this site is similar to that at the Carbon Lake outlet. 

West of the lowest part of the draw the strike of the foliation ranges 

from N. 20° to 80° E. and dip ranges from 32° to 55°NW. Southeast 

of the low point along section D-D' (pi. 5), the strike of the foliation ranges from N. 65° to 70° W., and the dip ranges from 64° to 

88° SW.

A small fault, with an approximate strike of N. 30° E., probably 

cuts through the low spot in the saddle. Another fault, with a somewhat more northerly trend, also cuts through the saddle and probably 

joins the first one in the north part of the map area. Although there 

are no exposures of these faults, the latter one apparently controls 

the rock slope at the edge of the lake at the south end of the map 

area. If so, the approximate strike is N. 10° E., and the dip is 78° 

NW. The estimated width of crushed rock along the fault is 6 to 

18 inches. Along the lake shore about 100 feet west of the map area, 

another fault, with about 12 inches of crushed rock along it, crops 

out. The strike is N. 21° E. and the dip is 74° NW. Similar faults 

combined with the foliation probably produce the steps on the west 

abutment.

The majority of the rock exposed in the saddle is sound and strong, 

and the site is suitable for a concrete dam. An overflow spillway 

could be constructed on a dam if the broken rock along the narrow 

fault zones were protected from scour.
44 GEOLOGY OF WATERPOWER SITES IN ALASKA

APPURTENANT WORKS 

CONDUIT ROUTES TO POWERHOUSE SITES

A conduit from the reservoir site to a powerhouse site on Cascade 

Bay could be either a pipeline, part pipeline and tunnel, or all tunnel. 

Dort (1924) suggested a part tunnel and part pipeline. In this plan 

the tunnel would extend for 2,000 feet from the reservoir to the east 

side of the rounded point that is 1,500 feet downstream from the 

damsite (E-E'', pi. 5). From here a pipeline would follow the hillside to a point above the small cove 1,700 feet south of the mouth of 

Coal Creek and then join the penstock leading to the powerhouse. 

An alternative route is possible if a low pressure pipeline can be 

placed on the relatively flat valley bottom. From the end of tunnel 

line E-E' the line would descend to the valley floor and run to a 

powerhouse site in the steep cliff 500 to 600 feet south of the mouth 

of Coal Creek. The line would be slightly shorter than one to the 

cove and much easier to build.

Line E-F (pi. 5) shows a suggested all-tunnel route about 1 mile 

long leading to the powerhouse site in the cove (Arthur Johnson, 

written communication, Oct. 13, 1958).

GEOLOGY

The rock along line E-E' is a migmatite similar to that at the 

damsite. Biotite-quartz diorite has been intruded into a slightly 

schistose impure quartzite or a graywacke high in quartz to produce 

a massive gneiss. A dike 150 feet thick of medium-grained biotitequartz diorite is exposed on the west side of the downstream point. 

In other places near the possible tunnel route, thin veins and sheets 

of biotite-quartz diorite have been intruded generally parallel to the 

foliation.

The attitude of the foliation differs considerably from place to 

place. The strikes range from N. 55° E. to N. 609 W. and the dips 

range from 21° to 70° W.

No large faults are known or suspected along line E-E'. A small 

fault, described previously, is exposed near altitude 250 feet in the 

draw 1,000 feet east of the lake. The rock point downstream from 

the damsite is separated from the main valley wall by a small saddle 

that may reflect some structural defect, but overburden covers the 

area.
BARANOF AND CARBON LAKES, BARANOF ISLAND 45 

ANTICIPATED ROCK CONDITIONS

The rock along a possible tunnel line at section E-E' (pi. 5), is 

sound and of high strength. It probably would stand without support 

both during and after construction of a tunnel. From the intake 

area to the dike or for approximately the first TO percent of its length, 

a tunnel would be in migmatite; the next 10 percent would be in a dike 

of massive biotite-quartz diorite, and the remaining 20 percent in 

migmatite. Along the last part of the tunnel line, the strike of the 

foliation intersects the line at a small angle, and the dip is at a low 

angle. As a result, a small amount of overbreak can be expected along 

this reach. The tunnel probably would need a concrete lining in order 

to prevent water and pressure losses along joints and fractures in the 

rock. However, if the rock cover overlying the tunnel is thick 

enough or the distance from the tunnel to the valley wall is great 

enough, a concrete lining might not be needed.

Along line E-F (pi. 5), the geology of the first 1,150 feet is similar 

to that along E-E'. The remainder of the line was not mapped, but 

the rock probably is a migmatite. Aerial photographs show that a 

linear feature lies about 100 to 200 feet south of the last 1,600 feet of 

the line.

POWERHOUSE SITE

Along the inner part of Cascade Bay, a steep rock cliff rises from 

the water surface to altitude 100 feet and extends from the mouth of 

Coal Creek 1,700 feet south-southeast to a small cove. The cliff is 

formed by a thick, massive dike of quartz diorite that offers an excellent rock foundation for a powerhouse and tunnel penstock. The 

small cove has been suggested as a powerhouse site, as it offers a small 

amount of protection against the storms that sweep into the bay (Dort, 

1924).

BESEEVOIR SITE

The reservoir site is surrounded by steep rock walls except at the 

upper end where it widens out to a U-shaped gravel-filled valley. The 

rock is a migmatite similar to that at the damsite.

Outwash deposits at the upper end of Carbon Lake are the best 

source of aggregate in the area. Extensive deposits of silt and fine 

sand occur, but along the river channel above the lake, there is some 

gravel that may be suitable for aggregate. The gravel is composed of 

about 70 percent biotite-quartz diorite, 20 percent quartzite, and 10 

percent miscellaneous rock types that include pyroxenite and some 

fine-grained igneous rock high in pyrite.

No leakage can occur from the reservoir outside of the damsite area.
46 GEOLOGY OF WATERPOWER SITES IN ALASKA 

REFERENCES CITED

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

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

Clarke, F. W., 1924, The data of geochemistry, fifth edition: U.S. Geol. Survey

Bull. 770. 

Dort, J. C., 1924, Water powers of southeastern Alaska: Federal Power

Commission. 

Dutro, J. T., Jr., and Payne, T. G., 1957, Geologic map of Alaska: U.S. Geol.

Survey.

Federal Power Commission and U.S. Forest Service, 1947, Water powers, southeast Alaska. 

Guild, P. W., and Balsley, J. R., Jr., 1942, Chromite deposits of Red Bluff Bay

and vicinity, Baranof Island, Alaska: U.S. Geol. Survey Bull. 936-G, p.

171-187. 

Heck, N. H., and Eppley, R. A., 1958, Earthquake history of the United States:

U.S. Coast and Geodetic Survey, no. 41-1, revised edition (through 1956). 

Knopf, Adolph, 1912, The Sitka mining district, Alaska: U.S. Geol. Survey

Bull. 504. 

Martin, Lawrence, and Williams, F. E., 1924, An ice-eroded fiord; the mode of

origin of Lynn Canal, Alaska: Geog. Rev., v. 14, p. 576-596: 

Overbeck, R. M., 1917, Geology and mineral resources of the west coast of

Chichagof Island: U.S. Geol. Survey Bull. 692-B, p. 91-136. 

St. Amand, Pierre, 1957, Geological and geophysical synthesis of the tectonics

of portions of British Columbia, the Yukon territory, and Alaska: Geol.

Soc. America Bull., v. 68, p. 1343-1370. 

Twenhofel, W. S., and Sainsbury, C. L., 1958, Fault patterns in southeastern

Alaska: Geol. Soc. America Bull., v. 69, no. 11, p. 1431-1442. 

Warning, G. A., 1917, Mineral springs of Alaska: U.S. Geol. Survey WaterSupply Paper 418, p. 26-32. 

Wright, F. E., and Wright, C. W., 1908, The Ketchikan and Wrangell mining

districts, Alaska: U.S. Geol. Survey Bull. 347.

U. S. GOVERNMENT PRINTING OFFICE : 1961 O -588295