Geology of Waterpower Sites 

on Crater Lake, Long Lake 

and Speel River near Juneau 

Alaska

GEOLOGICAL SURVEY BULLETIN 1031-D

Geology of Waterpower Sites 

on Crater Lake, Long Lake 

and Speel River near Juneau 

Alaska

By JOHN CHARLES MILLER

GEOLOGY OF WATERPOWER SITES IN ALASKA

GEOLOGICAL SURVEY BULLETIN 1031-D

A preliminary examination of the 

geologic feasibility of proposed 

damsites

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

STEW ART 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 __ -_-_-_----__-_-_-__________________--____---__-_----__ 71

Introduction-_____________________________________________________ 71

Geography.______________________________________________________ 72

Location _____________________________________________________ 72

Climate and vegetation....__.__-___.__-__-_____.__.__________- 73

Population and industry___-____________-__-_--_--_--__-___--_ 73

Accessibility. _________________________________________________ 73

Physiography _____________________________________________________ 74

General geology___________________________________________________ 77

Igneous rocks._______.__--____-______.-___-____-___-_---___-_- 77

Metamorphic rocks-__________-_______--___________-__----____- 77

Unconsolidated deposits.-_______________-____--__..___--_-___--- 78

Structure__________________________-_-_-----_-_-_-_-_--- 78

Seismic activity__-_-__-______________-_____-_-_-___---_-_-___- 78

Geology of probable power sites----__-_--__-----_-_---_-__--_------- 79

Crater Lake powersite____-_______-___-_____---------___---_- 81

Long Lake powersite________________-_____---______--___-_-_- 87

Speel River powersites_______________________________________ 92

References cited___________________________________________________ 100

Index_______________________________________. 101

ILLUSTRATIONS

Page 

PLATE 7. Geologic reconnaissance map of powersites-__________ In pocket

FIGUBE 14. Index map showing location of powersites-_-.___________-_-_ 72

15. Geologic map of Crater Lake powersite-___________________ 84

16. Geologic sections of Crater Lake powersite_______________ 85

17. Geologic map of Long Lake powersite_____________________ 88

18. Geologic sections of Long Lake powersite__-_-__---_-_-_--_ 89

19. Geologic map of Speel River powersite________-____-_--- 96

20. Geologic section of Speel River powersite__________________ 97

21. Geologic map of Saddle damsite and Speel River powersite-__ 98

22. Geologic sections of Saddle damsite_______________________ 99

m

GEOLOGY OF WATERPOWER SITES IN ALASKA

GEOLOGY OF WATERPOWER SITES ON CRATER LAKE, 

LONG LAKE, AND SPEEL RIVER NEAR JUNEAU, ALASKA

By JOHN CHARLES MILLER

ABSTRACT

The rock type at the sites for hydraulic structures considered in this report 

is mostly quartz diorite containing varying proportions of hornblende and 

biotite.

The Crater Lake damsite is traversed by minor joints and tight fractures 

but is not close enough to any major line of structural weakness to be adversely 

affected and no leakage from the reservoir is anticipated.

The Long Lake damsite is fairly close to a major lineation and the exposed 

bedrock is somewhat fractured. Exploratory drilling will be required to determine the magnitude of fracturing and whether or not foundation treatment 

will be necessary.

The Speel River damsite presents no apparently unfavorable geologic conditions for construction of a dam, although the bedrock is foliated in places. The 

large amount of sediment carried by the Speel River, however, raises a question 

as to the probable useful life of a reservoir on this stream and consequently 

a question as to feasibility of development. Development of this site would 

require the construction of a dam in the saddle between Indian Lake and the 

Speel River, referred to as the Saddle damsite. The Saddle damsite may be 

underlain by an appreciable thickness of morainal material. The valley walls 

are of quartz diorite.

On the basis of the preliminary examination the sites investigated seem to be 

geologically feasible for the proposed structures needed for power development. 

The amount of sediment carried by the Speel River and the depth to bedrock 

in the Saddle damsite area may be deterrents to development of the Speel River 

site.

INTRODUCTION

This report describes the geologic conditions at the location of proposed structures needed for the development of hydroelectric power 

from Crater Lake, Long Lake, and the Speel River in the Speel Arm 

area. The fieldwork on which this report is based was conducted from 

June 20 to August 27,1952.

The geology of southeastern Alaska is described by Spencer and 

Wright (1906); and Buddington andChapin (1929).

71
72 GEOLOGY OF WATERPOWER SITES IN ALASKA

GEOGRAPHY 

LOCATION

The areas investigated are within the Tongass National Forest on 

the Alaska mainland about 42 miles by water southeast of Juneau, at 

the head of Speel Arm of Port Snettisham at about lat. 58° 10' N., and 

long. 133°40/ W. The location of these areas is shown on figure 14. 

These sites are briefly described in a report prepared jointly by the 

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

134°

FIGUBB 14. Index map showing the location of powersites in the Speel Arm area.
WATERPOWER SITES NEAR JUNEAU, ALASKA 73 

CLIMATE AND VEGETATION

The climate of southeastern Alaska is characterized by moderate 

temperatures at sea level, mild winters, cool summers, and heavy 

precipitation mostly in the form of rain. The annual rainfall averages about 142 inches at Speel River. Zero temperatures occur infrequently at or near sea level, and snow rarely accumulates to great 

depths.

The forest cover extends from sea level to an altitude of about 2,500 

feet; most of the commercial timber is within 2% miles of tidewater. 

Except for some of the very steep slopes, the area is covered with a 

dense growth of timber consisting mainly of hemlock and spruce. 

A dense undergrowth of shrubs, blueberry bushes, devilsclub, and 

shade-resistant small bushy trees occurs in the forests. On talus 

slopes and along the streams, tangled alder in some places forms an 

almost impassable barrier. The timbered area is broken by large 

muskegs which are not conducive to tree growth.

POPULATION AND INDUSTRY

The nearest town is Juneau with a population in 1950 of 7,500. 

There were no inhabitants in the area investigated. The principal 

industries in southeastern Alaska are sea fishing, lumbering, and 

allied services. The potential waterpower and the vast timber resources provide this area with unusual possibilities for pulpmills and 

papermills.

ACCESSIBILITY

There are no roads in the entire area of this report. Crater Lake, 

Long Lake, and Speel Arm. can readily be reached by float planes. 

The lakes, however, are not free of ice until late July or early August 

and remain open for only 3 or 4 months.

Crater Lake is 1,022 feet above sea level and may be reached by a 

steep trail about iy2 miles long from the head of Speel Arm. Another trail, west of the point where Glacier Creek enters Speel Arm, 

extends to the outlet of Long Lake, which is 814 feet above sea level. 

This trail also traverses the west side of First Lake and that of Second Lake, which are 234 feet and 193 feet, respectively, above sea 

level. The trail formerly branched at the Long River, but the part 

extending to Indian Lake is largely obliterated. A poor trail follows 

the right bank of the Speel River for a short distance below its junction with the Long River.

A powerplant at tidewater at the head of Speel Arm west of Glacier Creek could utilize water from Crater Lake, Long Lake, and the 

Speel River. This plant site is accessible at high tide by boats of 

shallow draft.
74 GEOLOGY OF WATERPOWER SITES IN ALASKA

PHYSIOGRAPHY

The area is rugged and mountainous and at Speel Arm the forest 

cover extends from sea level to the icecap. This dense forest growth 

is interrupted in places by muskeg where the bedrock presumably does 

not provide good drainage. In the higher altitudes snow and ice 

persist the year round and valley glaciers are common. It was noted 

that streams entering the lakes were clear at the time of the field 

investigations.

The walls of the larger valleys have been grooved by glaciers, and 

the ridges below altitudes of 4,500 feet have been rounded by mountain-ice-sheet glaciation during the Pleistocene epoch. Crater and 

Long Lakes occupy hanging valleys, probably left by the work of the 

Cordilleran glacier complex. It is difficult to tell whether the larger 

valleys were formed as a result of a single glacier moving northward 

toward the Speel Kiver or by two arms, one moving southeastward 

and the other northward. The presence of a glacial lake during or 

after the Pleistocene in the valley of the First and Second Lakes is 

indicated by varved-clay deposits at two points: one near the head 

of First Lake and the other northwest of it. A glacier moving down 

the Speel River valley toward Port Snettisham may have formed a 

lake which provided the relatively quiet water necessary for deposition of the varved beds.

The Speel River is a braided stream both above and below the 

canyon section downstream from the mouth of the Long River, and 

it carries a considerable amount of sediment derived from glaciers at 

its source. The stream is about 100 feet wide at the first gorge downstream from the mouth of the Long River, then it widens to about 

700 feet or more before plunging through a second gorge which is 

about 25 feet wide at the upper end. The damsite considered on the 

Speel River is in the first gorge. About a quarter of a mile farther 

downstream the entire river plunges through a deep crevice which is 

narrow enough for a man to leap across when the river is not at flood 

stage. High tides affect the river below this point, which is less than 

5 miles from Speel Arm. It would be difficult to navigate this river 

any great distance above the mouth because of sand and gravel bars 

and the swift current. Above the damsite the river level has a diurnal 

fluctuation of as much as 5 feet owing primarily to the diurnal temperature variation and its resultant effect on the glaciers and snowfields. The braided channels change locations rapidly with the shifting of sand and gravel bars in this part of the river.

The tidal flats at the head of Speel Arm are extensive, ranging in 

width from y2 to 1 mile, a result of the pronounced tidal range 

which averages 13 feet and at times is as much as 19 feet. The mate-
WATERPOWER SITES NEAR JUNEAU, ALASKA 75

rial in these flats has probably been carried by tidal action from the 

delta at the mouth of the Speel River. These extensive flats are of 

particular concern in planning wharf facilities for the delivery of 

construction equipment and materials.

Crater Lake (pi. 7) is about 2 miles long and over 400 feet deep in its 

deepest part. The maximum depth observed, 414 feet, was about twothirds the distance from the upper end of the lake. From this point to 

the outlet the lake becomes progressively shallower. The lake occupies 

a narrow trough about half a mile wide with steep to vertical sides. 

At the lower end, mainly north of the outlet, a well-rounded ridge 

whose crest is 400 feet above the lake extends between the high valley 

walls; probably postglacial erosion cut through this ridge. From 

the steep vertically walled canyon at the outlet of the lake Crater 

Creek descends to Speel Arm. Beginning at a point about 4,000 feet 

west of the outlet, glacial grooves on the walls adjacent to th'e lake 

are inclined upward as much as 3° to 5°, indicating that the upper ice, 

moving eastward, may have ridden over the slower moving debrisladen ice on what is now the lake bottom. It is noteworthy that the 

point at which this inclination first occurs coincides with the greatest 

depth sounded in the present lake. The high well-rounded hornblende-quartz diorite ridge and its smaller counterpart, which form 

the east boundary of the lake, presumably were left as a lip at the end 

of the valley when the valley glacier descended toward Speel Arm or 

joined the trunk glacier moving down the Speel River valley toward 

Speel Arm. The walls of the valley of the Speel River have been 

grooved by the main glacier.

The valley between Speel Arm and the Long River, in which First 

and Second Lakes are located, and the continuance of this depressed 

area northward through Indian Lake toward the Speel River, present 

several problems beyond the scope of this report which are too involved to be determined in a short field season. However, a brief discussion of the probable physiographic history of this area is justified 

in view of its bearing on the location of two prospective damsites: 

one on the Speel River and one in the valley or saddle previously 

mentioned. The origin of some of the present topographic features is 

postulated on admittedly limited investigation.

It will be noted (pi. 7) that Glacier Creek flows into Speel Arm 

rather than northward toward the Long River. The sharp rise, however, from Glacier Creek valley to the basin of First Lake causes the 

drainage in the valley of First and Second Lakes to flow northward 

to join the Long River near the head of Indian Lake, which is at an 

altitude of 177 feet. The altitude of First Lake is 234 feet and that 

of Second Lake is 193 feet. Glacial grooves were noted as high as an 

altitude of about 300 feet on the walls of the valley of First and Sec6Z0508 O -62 -Z
76 GEOLOGY OF WATERPOWER SITES IN ALASKA

ond Lakes. The large hill which flanks the east banks of First and 

Second Lakes, also of Indian Lake, is well rounded and grooved. The 

area between the outlet or northeast end of Indian Lake and the Speel 

River is a low-rounded divide, a large part of which is muskeg. The 

occurrence of bedrock at and close to the surface along the Long River 

seems to indicate that this divide is similar in origin to a threshold 

(Flint, 1947) that was caused probably by a decrease in erosive action 

of the tributary glacier from the valley of First and Second Lakes 

when it met the larger glacier flowing down the Speel River. The 

divide is about 30 feet higher than Indian Lake according to the Taku 

River A-5 quadrangle map.

The river at the damsite on the Speel River, at what has been designated the First gorge, is at a slightly lower altitude than Indian Lake. 

After passing through this gorge the river flows through two successively narrower gorges which are designated the Second and Third 

gorges. Examination of aerial photographs and the present reconnaissance of this area reveal no major fault which directly might cause 

this succession of gorges or the rather sharp eastward bend of the 

Speel River through the narrow valley. The Indian Lake glacier may 

have confined the flow of water from the Speel River glacier to a 

narrow channel along the common boundary of the glaciers in order 

to erode the three very narrow gorges on the Speel River. During its 

maximum advance the Speel River glacier apparently scoured out 

several rock basins upstream from each of the three gorges of the present Speel River, and the debris-laden stream cut through these basins 

along lines of weakness in the bedrock or through zones of softer 

rock, such as gneiss or schist in roof pendants noted in this vicinity.

The altitude of the river surface near the damsite in the First gorge 

ranges from 150 to 160 feet, that of Second Lake is 193 feet, and that 

of First Lake is 234 feet. A tilting of the valley northward, or deep 

erosion by a glacier or glaciers that occupied the valleys of First, 

Second, and Indian Lakes may be postulated. Whatever the regimens 

 of the glaciers in this area might have been, they exerted a definite influence on the course of the Speel River in its path to the sea, causing it 

to avoid the more direct route through Indian Lake.

Long Lake (pi. 7) is about 3% miles long and its deepest part is 

about 470 feet deep. The north side of the lake has a fairly smooth 

shoreline, but the south shoreline is marked in its western half by two 

prominent peninsulas whose origin may have been due partly to faulting and partly to a tributary glacier, a remnant of which exists on that 

side of the lake. The steep to vertical walls of the lake have been 

grooved by a glacier flowing eastward, as indicated by friction cracks 

in the walls of the lake. As at Crater Lake, a transverse ridge exists 

at the lower or outlet end of Long Lake where the damsite is located.
WATERPOWER SITES NEAR JUNEAU, ALASKA 77

The Long Lake ridge is much lower, however, than the ridge at the 

outlet of Crater Lake. This is probably due to the more extensive 

glacier in the former drainage basin rather than isostatic rebound in 

the Crater Lake area.

GENERAL GEOLOGY

The geology of the Port Snettisham area, including Speel Arm and 

vicinity, is described briefly by Spencer and Wright (1906) and Buddington and Chapin (1929). The fieldwork on which their reports 

were based included geologic mapping inland about as far as the 

head of Speel Arm. East of this point the geology of the region is 

not mapped. The earlier geologic mapping by Spencer and Wright 

within the area of this report included Crater and Long Lakes, but 

the mapping by Buddingtoii and Chapin included only Crater Lake.

Plate 7 is a geologic map resulting from reconnaissance work by 

the writer during the summer of 1952. Samples of bedrock were 

taken at fairly regular intervals at the damsites, near the tunnel 

courses, and in the reservoir area for further examination. The 

geology of the proposed dam and tunnel sites will be discussed following the description of the general geology of the area as a whole.

IGNEOUS BOCKS

The predominant rocks of the Port Snettisham area are quartz diorite of the composite Coast Range batholith and gneisses and schists 

of the metamorphic-complex belt lying adjacent to the batholith and 

the Juneau syncline. The Juneau syncline is referred to as the 

Seymour geosyncline by Payne (1955). The quartz diorite intrusive 

mass is contaminated with varying amounts of inclusions which are 

remnants or assimilation products that resulted from the breaking up 

or disintegration of schist. In the area investigated, and in the 

vicinity of the Speel River damsite in particular, relatively pure thin 

beds or stringers of marble occur locally, and these are interpreted as 

roof pendants that remain from the stoping action of the intrusive 

diorite masses. The dominant varietal minerals in the quartz diorite 

are hornblende and biotite and the quantity of these mafic minerals 

varies considerably in the areas examined. Samples were taken 

within each drainage basin to provide data for later thin-section 

examination.

The texture of this quartz diorite ranges from fine to medium 

grained, and the fabric commonly is equigranular. It is considered to 

be of Late Jurassic or Early Cretaceous age.

METAMORPHIC ROCKS

Metamorphic rocks occur in a few isolated patches. The principal 

locality of metamorphic rocks observed was that near the dariisite on
78 GEOLOGY OF WATERPOWER SITES IN ALASKA

the Speel River where small blocks of gneiss have been assimilated in 

the hornblende-quartz diorite mass. The intercalated beds or stringers of marble in the blocks of gneiss range from half an inch to as 

much as 6 inches in thickness. The main belt of metamorphic rocks 

lies to the west and south of the area here considered and consist principally of gneisses, schists, slates, and quartzites.

There has been some alteration of the quartz diorite, and flow structures are common. In some areas lineation and foliation are also very 

common and the rock is somewhat gneissic in character. The age of 

the rocks in the metamorphic belt is Carboniferous.

UNCONSOLIDATED DEPOSITS

In the valleys at the heads of the lakes and in the broad valley of 

the Speel River unconsolidated sand, gravel, and boulder deposits of 

hornblende-quartz diorite form the valley floor. Large blocks or 

boulders of quartz diorite quarried by glacial action occur in some of 

the valleys as lateral and end moraines and eskers. Large debris 

cones have accumulated in many places where the major streams or 

drainage courses from the steep mountains debouch on the relatively 

flat valley floors. In poorly drained areas the mud, silt, and decaying 

vegetation combine to form swamps. Large amounts of rock flour 

and glacial sediments are transported by the Speel River. During 

rainy periods streams entering the lakes carry a noticeable amount 

of fine sediments.

STRUCTURE

The four damsites considered are within or near the west edge of a 

batholithic intrusion.

Evidence of faulting was observed at one locality but it was not 

possible to determine the amount of movement. Plate 7 shows the 

principal line of weakness; it was mapped partly on the basis of 

field examination and partly from inferred topographic expression as 

revealed by a study of aerial photographs. In several localities these 

lineaments disappeared within relatively short, distances in the quartz 

diorite mass, and in most places it was difficult to follow their traces 

because of forest cover. Joints were mostly vertical or steep and 

widely spaced. Some slickensides were observed in areas near the 

western part of the Coast Range batholith.

SEISMIC ACTIVITY

The Preliminary Seismic Probability Map of the United States, 

compiled in September 1950 (unpublished) by the Coast and Geodetic Survey, indicates that these damsites are on the west edge of 

zone 1 which is considered an area of only moderate seismic activity.
WATERPOWER SITES NEAR JUNEAU, ALASKA 79

Nevertheless, the design of structures for power development in this 

area should take into consideration the probable effects of earthquakes.

GEOLOGY OF PROBABLE POWERSITES

Plans for power development are described by Johnson (1962). 

Two general plans of development have been proposed: water from 

Crater Lake, Long Lake, and the Speel River would be diverted by 

tunnels and penstocks to a single powerhouse located on or near the 

shore of Speel Arm about half a mile west of the mouth of Glacier 

Creek; or water from the three sources would be diverted to individual 

powerhouses. In the second plan the powerhouse for Crater Lake 

would be located near the head of the embayment about half a mile 

southwest of the mouth of Crater Creek; for Long Lake it would be 

on or near the Long River i/£ to % mile upstream from the head of 

Indian Lake; and for the Speel River either on the north shore of 

Speel Arm about half a mile northeast of the mouth of Glacier Creek 

or on the right bank of the Speel River about half a mile downstream 

from the damsite thereon. The alternate tunnel courses are shown 

by dashed lines on plate 7.

The first plan provides certain advantages from the standpoint of 

construction, operation, and transmission. The second plan, although 

requiring three individual powerhouses, requires a much shorter aggregate length of tunnels which may outweigh the advantages of the 

first plan. Either plan requires the development of storage to equalize the streamflow. At Crater Lake and Long Lake storage could be 

developed by: (a) the construction of dams at the lake outlets to raise 

the water above the natural lake level; (b) drawing the lakes below 

their natural levels; and (c) a combination of the two methods. The 

second method eliminates the need for a dam, and under the third 

method a dam of lesser height would be required than under the first 

method. The second and third methods do not provide as great a head 

for power development as the first.

The fieldwork for this report was directed primarily toward an evaluation of the geologic conditions relating to the first plan of development (a) described above. The information obtained, however, also 

permits an appraisal of the alternative plan (b) as suggested. Four 

damsites, designated as Crater Lake, Long Lake, Speel River, and 

Saddle, were examined during the 1952 field season along with the 

three reservoir sites and the probable tunnel routes from these three 

reservoir sites to a common powerhouse site located on tidewater. The 

location of the four damsites and the suggested tunnel routes are 

shown on plate 7.

The proposed tunnel routes, as indicated on plate 7, are all on the 

west slope of a steep-sided valley and would traverse fractured rock
80 GEOLOGY OF WATERPOWER SITES IN ALASKA

adjacent to the valley of First and Second Lakes and the Long 

River. Alternate tunnel routes are possible if some other powerhouse 

location should prove more feasible or if two or three powerhouses are 

used instead of one.

The examination of the damsites and reservoir sites was conducted 

to an altitude that would be well above the probable maximum flowline of any of the reservoir sites under consideration.

The tunnel routes are plotted as straight lines in a direction that is 

intended to avoid the deep ravines or canyons (pi. 7). These tunnel 

routes, however, intersect three fractured areas: one roughly following 

the Long River, another about 1 mile south of the Long River, and a 

third west of the valley containing First and Second Lakes. The 

tunnel headings for the Crater Lake and Long Lake reservoir sites 

would be near the lake outlets and those for the Speel River reservoir 

site would be near the dam in the saddle area near First Lake. The 

reservoir levels might vary as much as 150 to 200 feet, and the tunnels 

must be capable of withstanding the hydraulic pressure under these 

maximum heads of water. Sections along the proposed tunnel routes 

are shown on figures 16, 18, and 22. These illustrative sections contemplate tunnels with only sufficient gradients to meet the hydraulic 

demands, and they terminate in an underground penstock.

The character of the rock beneath the surface is inferred on the 

basis of field investigations and laboratory study of samples obtained 

near the proposed tunnel courses. The rock is part of the Coast Range 

batholith and no metamorphic rock was observed along the tunnel 

routes. Consequently the tunnels would probably be driven through 

rock of uniform composition. The rock is considered competent to 

withstand tunnel openings and the hydraulic pressure caused by the 

maximum reservoir level. The tunnel courses were selected and the 

profiles prepared from the topographic map. The locations of the 

penstocks were selected to assure a minimum cover of 100 to 200 feet 

of rock in place.

The engineering geology of each damsite and tunnel site discussed in 

the following pages is the result of reconnaissance in these areas. A 

more detailed examination aided by drill-hole data must be made prior 

to planning for construction. The three reservoirs would be in impervious rocks and, although they are affected by joints or lines of 

weakness, it is believed that there are no structural or geologic conditions which would cause abnormal leakage from them. The zones 

of weakness are believed to be virtually impervious to percolation 

of water.

The geology of the damsites, cross sections along the probable axis 

of each site, and sections along the tentative tunnel routes are shown
WATERPOWER SITES NEAR JUNEAU, ALASKA 81

on figures 16, 18, 20, and 22. The topography of each of the four 

damsite areas is shown on sheet 2 of the river-survey map entitled, 

"Plan and profile, Crater Lake and vicinity near Juneau, Alaska, with 

damsites," published by the U.S. Geological Survey in 1952. In the 

illustrations in this report only 50-foot contours have been shown on 

the damsite maps. Masonry or rockfill dams could be constructed 

at each of the four sites.

CRATER LAKE POWERSITE 

TOPOGRAPHY

Crater Lake is at an altitude of 1,022 feet and has a drainage area 

of 11.4 miles.

The catchment basin above Crater Lake has the appearance of a 

glaciated valley that has lateral moraines partly preserved along the 

valley walls and a small end moraine on the valley floor about half a 

mile west of the lake. Crater Creek, above the lake, is a braided 

stream threading its way between glacial gravel and boulders. There 

is a large rockslide on the left bank about half a mile from the head 

of the lake. Talus slopes are composed of large angular boulders, but 

the location of the major accumulations seem to coincide with the 

faults or crevices common to the peripheral zone of the Coast Range 

batholith. Except for the alder and brush on the relatively flat valley 

floor, there is very little forest growth in the upper valley of Crater 

Creek.

At least three partly developed cirques form part of the upper 

valley of Crater Creek. Snow was present in these cirques as late 

as August 27, 1952. Melt water from snow and from the ice which 

caps the surrounding ridges and the runoff from frequent rains flow 

into either Crater Creek or Crater Lake. The walls of the lake are 

steep and are timbered where the slopes are not too precipitous. 

Several rockslides have occurred around the borders of the lake.

Crater Creek, which drains Crater Lake, is 1.4 miles long and 

empties into an embayment of Speel Arm. Owing to a fall of more 

than 1,000 feet, the creek has many cascades along its course.

GEOLOGY

Rock types. The dominant rock of the reservoir area is hornblendequartz diorite of the Coast Range batholith (pi. 7). The content* of 

ferromagnesian minerals, mainly hornblende and biotite, varies 

slightly from place to place. On the south side of the lake these dark 

minerals increase where the rock exhibits rather intricate flow 

structure and attains a gneissic appearance.

At the outlet of the lake a large block of quartz diorite has slumped
82 GEOLOGY OF WATERPOWER SITES IN ALASKA

from the right bank, probably as a result of frost action and jointing 

in the locality. Flow structure is prevalent at this point. Several 

hundred feet north of the outlet the rock becomes more coarsely 

crystalline and lighter in color owing to reduction in the amount of 

hornblende. On the side of the lake west of the outlet, the quartz 

diorite is also light colored and fine grained, and is penetrated by small 

quartz veins. About a quarter of a mile farther west on the north 

side of the lake, biotite is abundant in the quartz diorite. From this 

point to the head of the lake the composition of the quartz diorite is 

about the average for the region. The upper valley of Crater Creek 

was investigated at points about 200 feet above the lake level, and 

the entire upper valley seems to be hornblende-quartz diorite also.

Structure. Evidence of folding was uncommon in the area. Infolded crystalline gneiss or schist was observed only at the Speel River 

damsite, and. this occurrence is probably a roof pendant. Sporadic 

occurrences of gneissic structure were also noted elsewhere.

The principal zones of weakness are shown on plate 7. The magnitude or direction of movement was indeterminable in most places 

because of forest cover, difficulty of access, and lack of definite correlation. These lineaments are shown by dotted lines where they are 

concealed and by dashes where they are plainly visible on aerial photographs. The lineament along Crater Creek from tideland westward 

was not identified at the lake. Schistosity, lineation, and flow structures of the quartz diorite on the south side of the lake near the outlet 

are evident.

In the upper valley of Crater Creek jointing and fracturing are more 

intense. The most pronounced fracturing trends N. 60° E. and dips 

slightly away from vertical toward the south. These zones are 

generally slickensided.

Large vertical crevices in the bedrock were noted in the north wall 

about 200 or 400 feet east of the outlet. On the nearly vertical south 

wall east of the outlet no definite arrangement of fractures was evident. The general direction of the south face is N. 75° E. It is not 

a smooth face and large blocks have been weathered or plucked out 

of it. About 200 feet above the outlet there is a bench above which 

glacial grooving is pronounced.

Evidence revealed by aerial photographs and field examination indicates that a topographically well-defined lineament roughly parallel 

to Glacier Creek may be more or less continuous southwestward 

across the Crater Lake basin. At the end of the lake two lineaments 

roughly parallel to Glacier Creek are intersected by a transverse one 

trending northwestward. A distinct lineament on the west side of 

the valley of First and Second Lakes passes near the point where 

Crater Creek debouches on the tidal flat.
WATERPOWER SITES NEAR JUNEAU, ALASKA 83

DAM SITE FOUNDATION

The damsite at the outlet of the lake is in hornblende-quartz diorite 

broken by northwestward-trending vertical joints or fractures. 

Only minor fractures were observed which intersected the principal 

joints or fractures. The principal fractures are transverse to the 

streamflow and could be grouted to prevent possible leakage. These 

joints and the geology of the site are shown on figure 15. The 

foundation and abutments would be in hornblende-quartz diorite 

which should be capable of supporting a dam 150 to 200 feet high. 

Leakage around the damsite seems unlikely, and, based on conditions 

observed in the field, no damage from rockslides or snowslides is 

anticipated.

The Crater Lake damsite (figs. 15, 16) is considered suitable for 

either a masonry or a fill type of dam. The use of the latter, however, would depend on the availability of suitable materials.

SPILLWAY

For a nonoverflow type of dam the location of the spillway may be 

on either bank. A location on the left bank or north wall will offer 

the least difficulty in construction as it is not as steep as the right 

bank.

An overflow type of spillway may be located near the middle of 

the dam to utilize the original stream channel. The flow from such 

a spillway would not cause excessive erosion as the quartz diorite 

crops out continuously along Crater Creek to Speel Arm. A spillway 

tunnel might be utilized to avoid ice jamming at the surface if spillway discharge were to occur while the lake was still ice covered.

RESERVOIR

The reservoir area of Crater Lake is entirely in quartz diorite. 

The jointing or fracturing within the reservoir area is not considered 

of sufficient magnitude or of such character as to cause leakage. The 

upper valley of Crater Creek contains alluvium (pi. 7) in part reworked by the stream but largely deposited by the valley glacier to 

an unknown depth.

The valley walls around most of the lake are steep and in places 

almost vertical. Near the head of the lake on the south bank there 

are large snowslides and rockslides. A few streams, which carry 

boulders and debris enter the lake on the north side. There is no evidence of snowslides of any consequence from the ridge across the outlet end of the lake. Landslides are not believed to pose a damage 

threat to the proposed reservoir or power facilities. Erosion by .frost 

and ice action may result in some movement of large blocks of rock 

into the lake.
84 GEOLOGY OF WATERPOWER SITES IN ALASKA

DATUM IS MEAN SEA LEVEL

FIGURE 15. Geologic map of Crater Lake powerslte.

Scattered brush and alder grow in the upper valley of Crater Creek 

on the alluvium and Pleistocene or post-Pleistocene glacial deposits. 

A small well-preserved terminal moraine occupies the floor of the 

valley south of the creek and about half a mile west of the lake. Eastward from this point there are remnants of lateral moraines, small 

eskers, and deposits built by preglacial streams. The upper end of 

the valley was largely snow covered from the previous winter season 

at the time of the field investigations in 1952.
WATERPOWER SITES NEAR JUNEAU, ALASKA 

EXPLANATION

Hornblende-quartz diorite

Principal jointing

Projection of surface trace of lineament 

Depth of fracture not known

100' 200' 300'

85

-1100'

Section along line A-A'

2000' -|

1000' - 1000'

Section along line E-F (Plate 7)

FIGURE 16. Geologic sections of Crater Lake powersite.

The sparse timber on the slopes of the lake basin up to the maximum level of the proposed reservoir would present no operational 

problems.
86 GEOLOGY OF WATERPOWER SITES IN ALASKA

TUNNEL

A profile of the probable tunnel route, E-F, from Crater Lake to 

tidewater at Speel Arm is shown on figure 16. The profile was derived 

from sheet 1 of the river-survey map "Plan and profile, Crater Lake 

and vicinity near Juneau, Alaska, with dam sites," published by the 

U.S. Geological Survey, 1952.

The suggested tunnel route would intersect the lineation, plotted 

about 1,500 feet east of the lake, which is parallel to a well-defined 

joint set, but the wallrock is considered capable of maintaining the 

tunnel openings. Another line of weakness is shown on the profile 

about 800 feet from Speel Arm. Between Glacier Creek and Crater 

Creek sets of joints have resulted in successively lower rows of blocks 

of quartz diorite on the west side of the valley of First and Second 

Lakes. This zone of weakness may cause some difficulty in driving 

tunnels in this area, because of fractures and the possibility that the 

blocks might settle. The length of the tunnel from the lake to the 

powerhouse would be about iy2 miles.

Other than the zones of weakness or joint sets described, no areas of 

unstable rock were observed and no other unfavorable geologic or 

engineering conditions are believed to exist along the tunnel routes.

If Crater Lake were to be developed independently, a powerhouse 

site could be selected at some point near the head of the embayment 

about 2,000 feet southwest from the mouth of Crater Creek. The 

conduit route from this point to a point on Crater Lake 500 to 1,000 

feet southwest of the outlet would have an overall length of about 

4,200 feet, 1,300 feet of which would be tunnel and 2,900 feet penstock. 

The tunnel might intersect one joint system near its heading and the 

penstock would cross another set of joints near the powerhouse, but it 

is assumed that the tunnel would be under a minimum cover of 100 

feet of rock in place.

CONSTRUCTION MATERIALS

The coarse aggregate for concrete construction could be obtained 

by quarrying and crushing the quartz diorite rock in the vicinity of 

the damsite. Sand and gravel, derived mainly from biotite- or hornblende-quartz diorite, may be obtainable in the valley upstream from 

Crater Lake. If this source is unsatisfactory another possibility 

would be from points along Speel Arm. The coarse material for a fill 

type of dam could also be obtained by quarrying and crushing rock 

in the vicinity of the site. The finer materials required for a fill type 

of dam are not available in the immediate vicinity of the site. Possible sources would be the valley upstream from the lake or from points 

along Speel Arm.
WATERPOWER SITES NEAR JUNEAU, ALASKA 87 

LONG LAKE POWERSITE 

TOPOGRAPHY

The catchment basin above Long Lake is a typical glacial valley. 

It was formed by Pleistocene glaciation and is at present being modified by three valley glaciers, the largest of which is at the headwaters 

of the Long River. The melt water from these glaciers and from 

the permanent icecap feeds into this basin and into Long Lake.

The valley floor at the head of Long Lake is relatively flat and the 

Long River is a braided stream with several channels which thread 

through the gravel and sandbars formed of glacial outwash. Where 

stream cutting is not active the valley is covered by a dense tangled 

growth of alder and brush.

The sides of the valley are steep and there are many large snowslides and rockslides which are difficult to cross. The slopes below 

the permanent icecap support a fairly dense growth of timber except 

where the valley walls are nearly vertical or where the slopes have 

been denuded by snowslides.

GEOLOGY

Rock types. The rock at the proposed damsite is quartz diorite 

containing hornblende and biotite, the same as the rock that surrounds 

Long Lake. In places the rock has a gneissic appearance. Rock samples were taken at different points along the shore of Long Lake and 

also in the valley above the lake.

The rock on the north side of Long Lake immediately west of the 

outlet is hornblende-quartz diorite with only a minor proportion of 

biotite. Between this point and the first lineament west of the outlet 

rock alteration becomes more pronounced. Flow structures are evident. A sample west of the second lineament west of the outlet showed 

quartz diorite with hornblende as an accessory mineral. Between 

these two lineaments on the north side of the lake dark-colored rocks 

are predominant. West of the fourth lineament on the north side 

Long Lake (pi. 7) the rock is fine grained. The jointing at this point 

strikes N. 50° E. and dips about 30° toward the lake. The rock of the 

valley walls above the head of the lake in the upper valley of the Long 

River is mainly hornblende-quartz diorite. On the south side of the 

lake the changes in color and composition of the rocks westward are 

similar to those on the north side.

Structure. The lineaments shown on plate 7 were examined where 

they intersect the lake, and their continuance beyond those points was 

verified by aerial photographs.

Only one zone of weakness might affect construction, and this occurs southwest of the outlet of the lake. Many fractures occur in the
88 GEOLOGY OF WATERPOWER SITES IN ALASKA

rock adjacent to the south abutment of the damsite and in the damsite 

foundation. This lineament probably continues on the southern side 

of Indian Lake.

DAMSITE FOUNDATION

The proximity of the Long Lake damsite to the fractured quartz 

diorite at the outlet of the lake requires that it be thoroughly explored 

by drilling as the first step in any plans for development.

The bedrock is chiefly quartz diorite containing biotite with minor 

amounts of hornblende and quartz in small veins and stringers. There 

is a slight lamination of the dark minerals present, mainly in the 

rock on the south side of the damsite. The geology of the Long Lake 

damsite and a geologic section along the axis of the probable dam are 

shown on figures 17, 18, on which the major intersecting joints or 

fractures are also indicated. There is doubt as to the continuity of 

the fractures at depth, but, as indicated by observations downstream

DATUM IS MEAN SEA LEVEL

FIGURE 17. 'Geologic map of Long Lake powersite.
WATERPOWER SITES NEAR JUNEAU, ALASKA 

EXPLANATION

89

1000' 

900' 

800'- 

700'

Hornblende-quartz diorite

Principal fracture* 

and jointing

Projection of surface trace of lineament 

Depth of fracture not known

+ 1+ + +, + + + + + * T * + k + * + * 

400'

+'+'+ + +

600' 800' 1000' 1200'

1000 

900' 

800' 

700'

Section along line B-B1

2000' -

1000'-

0-

-1000'

2000' 4000' 6000' 8000' 10,000'

Section along line F-G (see Plate 7) 

FIGURE 18. Geologic sections of Long Lake powersite.

and at the damsite, excessive leakage may be induced by the northwestward-trending fractures at the outlet unless extensive scaling or 

removal of perhaps 20 to 50 feet of bedrock is done in addition to 

grouting. An extensive drilling program will be necessary to determine how deep these fractures extend and whether excessive leakage 

may be expected. If exploratory drilling verifies the lack of an impervious foundation it may be possible to select another site about 150
90 GEOLOGY OF WATERPOWER SITES IN ALASKA

feet downstream to avoid this fractured area. Such a location would 

add about 50 feet to the height of the required dam.

As pointed out above, there is some question as to the conditions in 

the channel part of the cross section. The abutments, however, appear 

to be entirely satisfactory and should be adequate to support a dam 

as much as 200 feet high. This lake is ice covered during 7 to 8 

months of the year and the effects of the ice must be considered in any 

design.

SPILLWAY

The selection of a location for the spillway will depend on the type 

and height of dam planned. For a nonoverflow type of dam it seems, 

from a geologic viewpoint, that the spillway could be located on either 

bank. Erosion of the stream channel by discharge from the spillway 

would not be extensive because the quartz diorite crops out in the 

stream channel at the damsite and at least 500 feet downstream from it.

RESERVOIR

It was mentioned earlier that the bedrock of the Long Lake reservoir 

area is quartz diorite with biotite or hornblende as the principal 

accessory minerals. The major part of this rock is lacking in permeability. The joints or fractures within the reservoir area are not 

considered extensive enough to cause leakage; however, the movement 

along the northeastward-trending lineament immediately south of the 

outlet of the lake may have been accompanied by fracturing which 

would render the rock unable to impound water. Exploratory drilling 

of this area will be necessary to determine whether or not treatment 

will be required to prevent leakage. Plate 7 shows the relation of the 

principal lineaments to the reservoir area, the extent of the alluvium 

in the upper valley of the Long River which is the reworked outwash 

of a large glacier at the head of the valley, and glacial deposits of 

unknown thickness from other sources.

Remnants of lateral moraines are present along the south side of 

the upper valley and at the head of the lake along the- west side of the 

peninsula (pi. 7). A small end moraine occurs at the outlet of the 

small lake about 1 mile southwest of the head of Long Lake. The 

upper end of this lake is at the terminus of a glacier. Several long 

talus slopes and rockslides were noted in the upper valley. The steep 

lower slopes of the valley are barren of vegetation, but the valley floor 

is covered with a. thick growth of willows, alder, and brush 8 or 10 

feet high.

Evidence of snowslides was observed at several points along the 

lake, but the probability of future slides is not considered a hazard 

to the reservoir. From the lake to a height of 50 feet or more the
WATERPOWER SITES NEAR JUNEAU, ALASKA 91

slopes are generally barren and little clearing of timber would be 

required.

Extensive exploratory drilling seems necessary in the vicinity of 

the outlet of the lake to determine the extent of the separation of 

bedrock along the lines of weakness indicated.

TUNNEL

A profile, F-G, of the probable tunnel route would intersect the 

main fracture zone near the Long River within 1,000 feet of the lake 

(pi. 7). Between the lake and this zone unstable rock may be present. 

Between these fracture zones and the head of the penstock the tunnel 

would run through relatively undisturbed rock until it intersects the 

Glacier .Creek fracture zone. The penstock section would penetrate 

the system of joints on the west side of the valley of First and Second 

Lakes.

The surface rocks along the shore of Speel Arm crop out west from 

Glacier Creek in a series of steps and blocks as a result of frost and 

glacial plucking acting on the joint system; therefore, some unstable 

rock may be found in that area and would likewise affect the proposed tunnel routes from Crater Lake and from the Speel River 

reservoir.

The length of the tunnel from the lake to the head of the penstock 

would be about 7,000 feet and the length of the penstock section about 

2,800 feet, depending to some extent on the length of the outlet tunnel 

and the altitude of the powerhouse with reference to sea level.

In an alternative scheme of development, water would be diverted 

from Long Lake to a powerhouse site on or near the Long River within y2 to % mile of the lake outlet. The exact location will depend on 

whether the Speel River development materializes. In this scheme 

the powerhouse location would be determined by the maximum flowline of the Speel River reservoir site. This would probably be at an 

altitude of 300 feet or more. If this development is not found feasible 

then the powerhouse would be located at an altitude of about 200 feet. 

A location at this altitude south of the Long River could be reached 

with about 3,000 feet of conduit, 2,000 feet of which would be tunnel 

and 1,000 feet penstock. Such a tunnel location would intersect the 

fracture zone along the Long River. A powerhouse location could 

also be selected on the north bank of the Long River (pi. 7) at an altitude of 200 or 300 feet. This could be reached from the lake with 

about 3,500 feet of conduit. About 2,000 feet of this distance would 

be tunnel and 1,500 feet would be penstock. Such a tunnel location 

would avoid the fractured area along the south side of the Long River.
92 GEOLOGY OP WATERPOWER SITES IN ALASKA 

CONSTRUCTION MATERIALS

Coarse aggregate for concrete construction, as well as the coarse 

materials for a fill type of dam, could be obtained by quarrying and 

crushing the hornblende-quartz diorite rock in the vicinity of the 

damsite. Sand and gravel for concrete may be obtainable in the valley 

upstream from Long Lake, from the valley at the head of Indian 

Lake, or from points along Speel Arm. The finer materials needed 

for a fill type of dam might be available from these same sources.

SPEEL RIVER POWERSITES

The development of the Speel River powersite, as previously mentioned, will require a dam on the main river as well as an auxiliary 

dam in the low divide or saddle at First Lake between the Long River 

and Speel Arm. Water would be conveyed from some point near the 

saddle dam to a tidewater powerhouse site along the shore of Speel 

Arm, or from the dam on the Speel River to a powerhouse site along 

the river about three-fourths of a mile downstream.

T O POGB AJPH Y

The Speel River has a drainage area of 226 square miles above the 

damsite which is located about half a mile downstream from the Long 

River. This area includes the Long Lake basin which is discussed 

on page 87. The Speel River basin upstream from the Long River 

is composed of three principal tributaries which join to form the main 

river at a point about 3 miles upstream from the Long River. These 

tributaries flow from the northwest, north, and northeast. The latter 

originates in the Speel Glacier, and the other two tributaries also drain 

extensive glacier areas. The main stream as well as the three tributaries all have braided channels and carry a heavy silt load.

The saddle area between the Long River and Speel Arm is of special 

interest in this study since a dam will also be required in it. The valley in which this saddle is located trends northeastward and is about 

2 miles long between Speel Arm and the Long River. First and Second Lakes are both located in this valley. First Lake is nearer Speel 

Arm at an altitude of 234 feet; Second Lake is at 193 feet. The saddle 

dam would be located at the south end of First Lake.

Both the east and west slopes of this drainage area are steep and 

thickly wooded. The east slope becomes nearly vertical along Second 

Lake near its outlet. The ridge along the east side of the valley rises 

to an altitude of 2,300 feet and has been well rounded by glacial action. 

The crest is largely bare of vegetation. The west slope is more rugged 

and rises to an altitude of more than 4,000 feet where higher parts are 

covered with permanent snow and ice. Most of the runoff is from
WATERPOWER SITES NEAR JUNEAU, ALASKA 93

the west slope. First Lake drains northward into Second Lake and 

Second Lake drains into the Long River and Indian Lake. There is a 

low divide in the ravine west of the saddle and only the runoff south 

of the saddle drains into Glacier Creek and thence to Speel Arm. 

Glacier Creek carries the melt water from the icecap in addition to the 

runoff within its restricted basin. Muskeg occurs on flat areas above 

the level of the creek between the saddle and Speel Arm. A few very 

large erratic boulders were observed on the muskeg about half a mile 

from. Speel Arm.

GEOLOGY

Rock types. The bedrock was investigated on the Speel River in 

the vicinity of the damsite, between the Speel River and Indian Lake, 

around Indian Lake, and in the drainage areas of First and Second 

Lakes. The geology of the Speel River area above the Long River 

was only investigated for a short distance on the right bank, but observations from the air and examination of aerial photographs indicate that, as within the other reservoir areas, the bedrock is quartz 

diorite. Near the damsite on the Speel River significant changes in 

texture and composition of the quartz diorite were noted. The bedrock shows an increase in mafic minerals, and at the damsite an intrusion of calcareous gneiss was observed.

The broad relatively flat divide between the north end of Indian 

Lake and the Speel River is partly covered by muskeg, forest growth, 

and alder which obscure the underlying rock between the valley walls. 

The surrounding valley walls are hornblende-quartz diorite. Below 

the outlet of Indian Lake on the left bank of the Long River a limited 

exposure shows some schistosity within the quartz diorite; the foliation at that point strikes N. 40° W. and dips about 35° NE. About 

10 feet of glacial gravel covers this rock in the bank of the stream. 

It is assumed that beneath the muskeg and forest growth this quartz 

diorite forms a buried ridge between Indian Lake and the Speel 

River to the north. About 1 mile northwest of the confluence of the 

Long and the Speel Rivers, the bedrock is hornblende-quartz diorite. 

On the south side of the Long River near the damsite an increase in 

gneissic texture and mafic minerals in the bedrock is apparent, in 

contrast with rock of porphyritic appearance about three-fourths of 

a mile west along the right bank of the Long River.

Around Indian Lake the bedrock is also hornblende-biotite diorite. 

The dark minerals in the quartz diorite are more abundant on the 

north side of the lake than on the south side.

Southwestward in the valley of First and Second Lakes the bedrock is hornblende-quartz diorite. Near the west end of the saddle 

area between Indian Lake and the Speel Arm drainage there is a low
94 GEOLOGY OF WATERPOWER SITES IN ALASKA

ridge of quartz diorite. The west side of this ridge is almost vertical 

in its descent to the relatively narrow drainage channel. A low 

divide in this flat-bottomed ravine separates the Glacier Creek drainage from that draining northward through Second Lake. The valley 

walls are quartz diorite, and on the west side of the ravine some 

glacial grooving was observed directly west of the head of Second 

Lake. The east slope of the valley is steep, and adjacent to Second 

Lake the quartz diorite forms a nearly vertical cliff.

From the outcrops of quartz diorite on the saddle, northward to 

the west side of First Lake, the valley floor is covered principally 

with muskeg. The amount of glacial gravel and sand, if any, that 

may underlie the muskeg is unknown.

At two localities in the valley of First and Second Lakes very small 

exposures of glacial lake sediments were observed. One such deposit 

about 10 feet thick and 20 feet long was exposed in a small ravine in 

the quartz diorite east of the head of First Lake. The sediments 

were mainly beds of gray silty sand % to 1/4 inch thick. These beds 

were horizontal where exposed in the hillside and were 50 to 75 feet 

above the present valley floor. These beds are assumed to have been 

deposited in an ice-marginal lake created by a glacier in Speel Arm 

which dammed the water in this valley. Another outcrop of similar 

beds was observed to the northwest in a small cove on the west side of 

Second Lake. At this point the exposure of gray lakebeds was about 

75 feet long and at lake level. They strike about N. 15° W. and consist of sandy beds of clay 14 to i/£ inch thick and fine sand layers at 

about 6-inch intervals. The dip of these layers is about 20° NE. No 

other outcrop of lakebeds was observed in the area.

Structure. The zones of weakness that occur in the damsite and 

reservoir areas are shown on plate 7. These fractures were traced on 

the aerial photographs and where possible they were verified by field 

examination. Near the fork of Glacier Creek an exposure of a fault 

plane with breccia and gouge was observed. The strike of this fault 

here is due east and the dip is 60° S. The throw and lateral extent of 

this fault were not determined. It is not certain that this fracture 

zone extends to Crater Lake but the aerial photographs indicate that 

possibility. On the aerial photographs, however, the lineaments appear very definite, but whether these fractures are continuous beneath 

the alluvium and forest growth is conjectural.

SPEEL RIVER DAMSITE FOUNDATION

The Speel Eiver damsite is located in the first gorge on the river 

half a mile downstream from the mouth of the Long River. The rock 

at this site (fig. 5) is of varied composition on both banks of the river.
WATERPOWER SITES NEAR JUNEAU, ALASKA 95

The principal rock type is quartz diorite in which the proportion of 

dark mineral varies greatly. The fme-textured rock on the right 

bank exhibits considerable lineation. The blocks of gneiss on the left 

bank which contain beds and stringers of marble or calcite appear to 

have been stoped from overlying beds by the quartz diorite instmsion 

and may be pendants. The gneiss inclusions in the quartz diorite 

show vertical foliation in a north-south direction. The rock on the 

right bank is a biotite gneiss and that on the left bank, west of the 

large block of gneiss containing marble, is biotite-quartz diorite.

The foundation rock and the abutments appear capable of supporting a masonry dam 160 feet high, which is the maximum height 

that would likely be considered for this site.

The right abutment wall is fractured to some extent and may require 

grouting, while the left abutment area appears less fractured. The 

bedrock above an altitude of 200 feet is not well exposed, but aerial 

photographs and the observations in the escarpments above river level 

indicate that some fracturing may be present. Section C-C', figures 

19,20, show the general character of bedrock.

SADDLE DAM SITE FOUNDATION

The Saddle damsite at the head of First Lake (pi. 7) is suitable for 

either a flexible fill type or a masonry dam of sufficient height to raise 

the water to an altitude of 400 feet, the maximum that would likely 

be considered for this site. A small auxiliary dam would be necessary 

in the low divide between Glacier Creek and the Second Lake drainage. The depth of alluvium or glacial fill in this divide may be as 

much as 50 feet or even more. The steep descent of about 100 feet on 

the south side of the damsite may, however, represent the depth of 

valley fill. Core drilling would be necessary to establish the depths to 

bedrock. The geology of the Saddle damsite, map and section, is 

shown on figures 21 and 22.

SPILLWAY

Either an overflow type of dam with a spillway apron or a nonoverflow type having an open spillway on the left (north) bank could be 

constructed at the Speel River site. The open spillway would not become clogged by slides from the hillsides and the discharge would 

not cause rapid erosion of the bedrock. However, both the dam and 

spillway design should take into account the effects of ice during the 

winter months.

The Speel River dam, if built, would be constructed in conjunction 

with the Saddle dam. A spillway will be required at only one of these 

two dams as they both control the same reservoir. A spillway at the 

Speel River dam may be preferable since the water can then be re-
96 GEOLOGY OF WATERPOWER SITES IN ALASKA

400 80O Fttt

DATUM IS MEAN SEA LEVEL

FIGURE 19. Geologic map of Speel River powersite.

turned to the main river channel. A spillway at the Saddle dam 

would require construction of a channel from the dam to Speel Arm 

for the discharge.

The main or longer part of the Saddle dam would probably be a 

nonoverflow type owing to the character of the alluvial cover in the 

valley. If a dam of this design should be constructed a spillway at 

the west side of the valley (figs. 21, 22) would require a minimum of 

preparation. A spillway at this point would utilize the small valley 

west of the main dam and would require little excavation. Both abutments of the smaller dam would be in the quartz diorite and the depth 

to bedrock in the ravine probably does not exceed 10 feet. A spillway 

of an overflow type could be constructed at the location indicated.

RESERVOIR

The reservoir which would be formed by the Speel Eiver and Saddle 

dams would back water up the Speel Eiver beyond the junction of 

the three main tributaries, the exact distance depending on the height
WATERPOWER SITES NEAR JUNEAU, ALASKA 

EXPLANATION

97

Hornblende-quartz diorite

Sand 

Indefinite contact

400'-I 

300'- 

200'- 

100'- 

0-

400' 

300'

 200' 

100'

-0

200' 400' 6OO' 800' 1000'

Section along line C-C1 

FIGURE 20. Geologic section of Speel River powersite.

of the dam selected. It would also inundate First, Second, and Indian 

Lakes and extend some distance up the Long River beyond Indian 

Lake.

The Taku River A-5 quadrangle map indicates a rise of 30 feet 

or more to the 200-foot contour between Indian Lake and the Speel 

River. This broad area and the bed of the Long River would be 

under about 100 feet of water if the proposed Speel River dam raised 

the water level to an altitude of 300 feet.

The geology of the Speel River valley was not investigated above 

the low ridge or divide mentioned in the preceding paragraph, or on 

the left bank above the Speel River damsite. A sample of the bedrock, 

however, was taken about 1 mile northwest of the confluence of the
98 GEOLOGY OF WATERPOWER SITES IN ALASKA

Lake beds (glacial), 

limits not determined

DATUM IS MEAN SEA LEVEL

FIGURE 21. Geologic map of Saddle damsite and Speel River powerslte.

Long and the Speel Eivers. This exposure was hornblende-quartz 

diorite. Observations hi the vicinity of the damsite and inspection 

of aerial photographs indicate that the rock comprising the remainder 

of the reservoir area is probably similar to that in the other areas 

examined.

The divide may be inferred to represent a down-drop block, as 

shown on plate 7. None of the inferred fault zones or zones of weakness shown within the reservoir are considered sufficiently open to 

permit escape of water impounded by the two dams.

A notably unfavorable factor in the consideration of a reservoir 

site on the Speel River is the large quantity of sediment carried by this 

stream. A serious question arises as to the probable useful life of a 

reservoir under these circumstances and consequently the feasibility 

of a hydroelectric development on this stream. The effect of silt in 

a reservoir during earthquakes is not known but is probably negligible.

TUNNEL

Water from the Speel River reservoir could be conveyed to one of 

three powerhouse sites. Field investigation was directed toward a
WATERPOWER SITES NEAR JUNEAU, ALASKA 

EXPLANATION

99

Alluvium and glacial deposits 

Locally covered by muskeg

Hornbiende-quartz dlorite 

Indefinite contact

Lineament determined from aerial photographs 

Dashed where inferred

Projection of surface trace of lineament 

Depth of fracture not known

45tf-, 

400'-

3001 - 

ZOO'- 

100'-

-450'

-400'

-300'

-200'

-100'

200' 400' 600' 800' I OOP1 1200' 1400' 1600' MOO' ZOOO1

Section along line D-D'

1000'

1000' 2000' 3000' 400O'

f* * Intuit

5000'

1000'

Section along line F-H (see Plate 7} 

FIGDRE 22. Geologic sections of Saddle damsite and Speel River powersite.

route from the Saddle dam to a powerhouse site on the shore of Speel 

Arm about 3,000 feet west of the mouth of Glacier Creek. This route 

is indicated at F-H on plate 7 and is shown in section on figure 20.

The entire tunnel course would be in quartz diorite. The rock, however, is disturbed in varying degrees by the fracture zones noted. No 

serious hindrance to tunneling is anticipated in these zones, and the 

rock is considered stable enough to maintain tunnel openings. The required tunnel would be about 3,800 feet long, and the penstock about
100 GEOLOGY OF WATERPOWER SITES IN ALASKA

2,300 feet, depending on the length of the outlet tunnel. A cover of 

100 feet of rock in place is assumed.

If the Speel Eiver site were to be developed as an individual site 

without regard to combining its flow in a powerhouse with the water 

from Crater Lake and Long Lake, two methods are possible. One 

would utilize its flow in a powerhouse on the north shore of Speel Arm 

about half a mile northeast of the mouth of Glacier Creek. This location would require a conduit about 3.000 feet long, 2,100 feet of which 

would be tunnel and 900 feet penstock. Such a tunnel may traverse 

fracture zones near the Saddle dam, but otherwise would be in undisturbed rock (pi. 7). Another location would be on the right bank of 

the Speel Eiver, about three-fourths of a mile downstream from the 

dam on the river in the first gorge. A powerhouse there would require an overall conduit length of about 4,100 feet, 1,000 feet of which 

would be penstock section. This conduit route would not traverse any 

fault zones and would be in undisturbed rock throughout its entire 

length (pi. 7).

CONSTRUCTION MATERIALS

The same situation exists at the Speel Eiver and Saddle damsites 

as at the Crater Lake and Long Lake sites, in that the coarse aggregate for concrete construction or the coarse materials for a fill type of 

dam could be obtained from quarrying and crushing quartz diorite 

rock in the vicinity of the sites. Sand and gravel may be obtainable 

from the bed of the Speel River, or from points along Indian Lake, 

Glacier Creek, and Speel Arm. These same areas might be sources 

for the fine materials for a fill type of dam. It is recognized, however, that the possible absence of appropriate materials may preclude 

consideration of a flexible fill type of dam at any of the sites discussed 

in this report.

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. Forest Service, 1947, Water powers, southeast Alaska. 

Flint, F. F., 1947, Glacial geology and the Pleistocene epoch: New York, John

Wiley & Sons. 

Johnson, F. A., 1961, Waterpower resources, southeastern Alaska mainland,

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

1529 (inpress). 

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

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

Spencer, A. C., 1906, The Juneau gold belt, Alaska, and A reconnaissance of

Admiralty Island, Alaska, by C. W. Wright: U.S. Geol. Survey Bull. 287.
INDEX

Abstract.

Page

71

Clay, varved__________________ 74

Coast Range batholith__..__.__... 77,78,80

Crater Creek..________________ 81

Crater Lake, altitude..____________ 73,81

drainage area ____________ 81

depth___._______._.......... 75

physical features_____________ 75 

storage possibilities.____________ 79 

Crater Lake powersite____________ 81 

available construction materials..____ 86 

dam spillway-_______________ 83 

damsite foundation.____________ 83 

petrography________________ 81,82 

proposed tunnel route________.__ 86 

reservoir._________________ 83 

structure..________._____ 82

Development of powersites, alternate plans... 79

Faulting__.___________...__... 78

First gorge____________....__.. 76

altitude at damsite 76

proposed damsite_____________ 76

First Lake, altitude....._____._ 73,75,76,92

varved clay deposits in____ 74

Glacial lake, evidence of.________ 74,94 

Qlaciation in area_____._ ._.. 74,75,76 

Gneiss..._._______________ 77,78,93,95

Hanging valleys_________ 74 

Indian Lake, altitude____________.. 75

Joints....................... ..-. -... 78,83

Juneau, population________ 73 

Juneau syncline__ 77

Long Lake, altitude 73 

depth.. . . .. . 76 

physical characteristics____ 76 

storage possibilities 79

Long Lake powersite, available construction

materials______ 92 

dam spillway 90 

damsite foundation______ 88,89,90 

petrography........ . 87

proposed tunnel route 91 

reservoir._____........ 90,91

Long Lake powersite Continued 

structure_______.........

Marble 

Metamorphic rocks, age____...

Page

87

77

78

Quartz diorite, age ..___.__ . 77 

Crater Lake powersite._____.__ 81,82,83 

inclusions in .. _______ 77 

Long Lake powersite 87,88,90,92 

relation to Coast Bange batholith---__ 77 

Saddle damsite___.____._... 93,94 

Speel River damsite________ 93,94,95,99

Quartzite_______________ 78

Saddle damsite, foundation..__... ...... 95

location_ __________________ 92,95 

petrography- 94 

proposed tunnel rente____.__ 99,100 

relation to Speel River damsite. 95,96 

See also Speel River damsite. - 

Schist.. ...... .-....- 77,78

Second gorge 76

Second Lake, altitude.. __ . 73,75,76,92

varved clay deposits in 74

Seismic activity.._ 78,79

Seymour geosyncline 77

Slate__._.__... ... _ 78

Speel Arm, powerplant on 73

Tidal flats.. . 74,75

Speel River, annual rainfall at.. 73 

drainage area at damsite ___.. __ 92 

glacial sediment in_____...... 74,78,98

physical features 74 

proposed damsite________.___ 74 

Speel River damsite, available construction

material___....__ ... 100

dam spillway..____________.... 95,96

folded metamorphic rocks at. . 82 

foundation________________.. 94,95 

marble at . .. 77,95 

metamorphic locality.________.... 77,78

petrography__ . 93,94 

proposed tunnel routes. 98,99 

reservoir ______________- 96,97,98 

structure... 94 

See also Saddle damsite. 

Speel River powersites, proposed locations 99,100

Third gorge.. ......

Unconsolidated deposits 

101

U.S. GOVERNMENT PRINTING OFFICE: 196Z O - 6Z0508