DEPARTMENT OF THE INTERIOR

FRANKLIN K. LANE, Secretary

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, Director

Bulletin 685

RELATION OF LANDSLIDES AND GLACIAL 

DEPOSITS TO RESERVOIR SITES

IN THE

SAN JUAN MOUNTAINS, COLORADO

BY

WALLACE W. ATWOOD

WASHINGTON

GOVERNMENT PRINTING OFFICE. 

1918

CONTENTS.

Page.

Introduction............................................................... 5

Location.................................................................. 7

Physical geography........................................................ 7

Physiographic evolution of the region .................................. 7

Great canyons........................................................ !)

Glacial deposits in the canyons........................................ JO

Terminal moraines................................................ 10

, Recessional moraines.............................................. .11.

Lateral moraines................................................. 12

Medial moraines.................................'.................. 13

Outwash deposits......................:.......................... 13

Deposits of distinct glacial epochs..................................... 14

Landslide deposits...................................................... 14

Lakes among the mountains...;........................................ 1C

Torrential deposits in the large canyons............. 1................... 16

Stream courses in the larger valleys..................................... ' 17

Reservoirs ........................................................... '.... 18

Farmers' Union or Rio Grande reservoir................................ . 18

Ignacio reservoir....................................................... 20

Road Canyon reservoir................................................ 22

Santa Maria reservoir.......... '. ...................................... 23

Mosca reservoir, by J. F. Hunter....................................... 26

Terrace reservoir...................................................... 33

La Jara reservoir..................................................... 36

Proposed reservoir in Clear Creek....................................... 37

Proposed reservoir on South Fork of Clear Creek ........................ 37

ILLUSTKATIONS.

PLATE I. A, Farmers' Union reservoir from a point on the Weminuche 

trail; B, Dam of the Farmers' Union reservoir, from the 

south; C, South margin of the dam on the downstream 

side....................................................... 18

II. A, North margin of the Farmers' Union dam from downstream side; 

B, Seepage through the landslide mass just below the Farmers' 

Union dam; C, Ignacio reservoir and dam................... 19

III. A, Santa Maria reservoir from the north; B, Santa Maria reservoir

dam from the north; C, Outlet of the Santa Maria reservoir... 24

3
ILLUSTRATIONS.

PLATE IV. A, Lower end of Mosca reservoir; B, Mosca reseryoir dam .and 

spillway; C, Tunnel outlet at Mosca reservoir.................

V. A, Terrace reservoir; B, Mosca reservoir site....................

VI. A, Terrace reservoir dam from the north; B, Crest of Terrace 

reservoir dam; C, Terrace reservoir dam from below.......

VII. A, General view of Terrace reservoir dam from the south; E, 

Outlet of Terrace reservoir tunnel; C, Postglacial gorge below 

Terrace reservoir..........................................

VIII. A, La Jara reservoir dam; B, La Jara reservoir; C, Tunnel opening 

in the La Jara dam .......................................

FIGURE 1. Index map showing location and drainage features of reservoir 

projects in the San Juan Mountains...........................

2. Diagrammatic cross section showing the physiographic evolution 

of the San Juan region since the beginning of Tertiary time.......

3. Map of a part of the San Cristobal quadrangle, showing the 

location and topographic relations of the Farmers' Union or Rio 

Grande reservoir.............................................

4. Sketch map showing the geologic conditions immediately adjoining 

the dam of the Farmers' Union reservoir........................

5. Cross section along the line A-B on figure 4........................

6. Map of a part of the Engineer Mountain quadrangle, showing 

the location and topographic relations of the Ignacio reservoir...

7. Map of a part of the San Cristobal quadrangle, showing the location and topographic relations of the Road Canyon reservoir....

8. Map of a part of the San Cristobal quadrangle, showing the location and topographic relations of the Santa Maria reservoir.......

9. Diagrammatic north-south section through the Santa Maria trough.

10. Map of a part of the Creede quadrangle, showing the location 

and topographic relations of the Mosca reservoir..................

11. Sketch map of Mosca reservoir project and environs, showing approximate boundaries of geologic formations..................

12. Profile section across Beaver Creek at Mosca dam...............

13. Sketch map of the lower end of the Terrace reservoir, showing the 

general distribution of the frontal moraine that determined the 

location of the dam and reservoir site........................

14. Cross section of the earth dam of the Terrace reservoir.....'........

15. Cross section of the gorge in which the earth dam of the Terrace 

reservoir has been placed ....................................

16. Sketch map showing geologic conditions of the area immediately 

surrounding the lower end of the La Jara reservoir.............

17. Map of a part of the San Cristobal quadrangle, showing the location and topographic relations of the proposed reservoir sites in 

Clear Creek and South Fork of Clear Creek....................

Page.

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RELATION OF LANDSLIDES AND GLACIAL DEPOSITS

TO' RESERVOIR SITES IN THE SAN JUAN

MOUNTAINS, COLORADO.

By WALLACE W. ATWOOD.

INTRODUCTION.

With the increase in farming on the. lowlands bordering the San 

Juan Mountains, Colo., and on the broader valley floors within 

the range, there has come a demand for a large supply of water to 

be used for irrigation. This demand has been particularly urgent on 

the east side of the range, among the farmers arid ranchmen of the 

San Luis Valley. Numerous reservoirs have been planned in which 

the waters from the melting snows and the surplus floods from heavy 

rains may be stored, to be later released as needed during the growing season.

Most of the reservoir projects are associated with the great.glaciated canyons of the range or with lake basins in which the waters 

have been artificially raised. In some of them the- selection of the 

reservoir site has apparently been determined by the occurrence of 

a narrow gorge or "constricted, portion in the canyon, downstream 

from a broad, open, parklike portion. In many of these mountain 

valleys, however, the narrow portions have been formed by the deposition of large masses of loose material, either of landslide or of 

glacial origin, and such materials have not proved to be watertight. 

Most of the lakes in the mountains are also held back by glacial 

or landslide deposits. In many of the projects a perfectly good 

watertight dam has been constructed, but serious leakages have 

occurred, the waters finding a way underneath or around one end, 

or even both ends, of the dam. For these reasons, some projects 

appear to have been abandoned, a-nd others are continued with heavy 

expenses for repairs. On one reservoir large additional construction has been undertaken to prevent disastrous leakage through a 

glacial moraine and the threatened loss of the entire amount of 

capital invested. Nevertheless, other reservoirs are being planned in 

places where just such loose materials border the sites of the proposed dams.

5
FIOUKE 1. Index map showing location and drainage features of reservoir projects in the San Juan Mounta

3, Road Canyon reservoir; 4, Santa Maria reservoir; 5, Mosca reservoir or Beaver Creek project; 6, Ter

Creek; 9, proposed reservoir site on South Fork of Clear Creek. Scale about 17J miles to an inch.
PHYSICAL GEOGRAPHY. 7

Inasmuch as experience has shown that many landslide masses and 

certain of the glacial deposits are not able to withstand the pressure 

of a high head of water without serious; leakage, it seems desirable to 

publish a description of the mountain canyons and the deposits commonly found in them and of the geologic conditions1 associated with 

the lakes in the mountains'; so that, in the future, no expensive errors 

need be due to a failure to recognize the geologic formations bordering a proposed reservoir site.

LOCATION.

The San Juan Mountains are in the southwestern part of Colorado. 

(See fig. 1.) The loftier summits in the range rise to elevations of 

over 1,400 feet, and the highest peak, known as Uncompahgre, has 

an elevation of 14,306 feet. Much of the range is above 10,000 feet 

in elevation. To the west the bordering lower lands form the Colorado Plateau, whose elevation is between 7,000 and 8,000 feet above 

the sea. The Uncompahgre Plateau lies at the northwest and ranges 

in elevation from 9,000 feet near the mountains to 8,000 feet, and at 

a still greater distance from the mountains, to 7,000 feet above sea 

level. North of the range, in the vicinity of Montrose, in the great 

valley of the Uncompahgre Kiver, there is an area at about 5,000 

feet, but just east of this area is another plateau district connecting 

the San Juan Mountains with the West Elk Mountains, and into 

that plateau the Black Canyon of the Gunnison has been cut. To the 

east is the broad San Luis Valley, with a general elevation of about 

7,500 feet, and on the south the plateau and mesa country of northern 

New Mexico.

PHYSICAL GEOGRAPHY.

PHYSIOGRAPHIC EVOLUTION OF THE REGION.

The San Juan district has passed through a remarkable geologic 

history. The earlier chapters of that history need not be reviewed 

here,1 but the later chapters, which cover an interpretation of the 

present topography, should be outlined.

At the end of Cretaceous time the range appeared as a great mountain mass a huge dome. (See fig. 2.) Rivers went to work upon 

that uplifted ]and and carved out the early Tertiary San Juan Mountains. Alpine glaciers were formed in the basins among those mountains and descended to the neighboring lowlands. In time, through 

the combined efforts of rivers, glaciers, winds, and all agents of 

weathering, the mountains were removed and the region was re1 The early geologic history of the range is clearly stated in the Telluride, Sllverton, 

Needle Mountains, and Ouray folios of the Geologic Atlas, published by the United States 

Geological Survey.
RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO.

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duced to one of low relief, not far above sea 

level. Before this work was completed volcanoes had broken out in the range, and by 

the end of the Oligocene epoch vast amounts 

of sand and gravel were spread out over 

the western part of the area.

Next came a second series of violent and 

extensive volcanic outbursts. From numerous vents fragmental materials were thrown 

into the air, and lavas were poured out over 

the surface. Little by little these formations, 

coming from many centers, built up an extensive volcanic plateau, which, if restored, 

would rise at least a few thousand feet 

above the highest summits of the present 

range. By this time the Pliocene or late 

Tertiary epoch had been reached.

After the development of the great volcanic plateau the streams again undertook 

their work of dissection. Another range of 

mountains was carved out, and once more, 

after the rivers had worked for a long time, 

the region was reduced to a lowland not 

much above sea level. The old surface produced at that time is represented by many 

of the broad summits and intercanyon ridges 

of the present range.

Still later the San Juan area was again 

uplifted. This time the erosion surface that 

had just been produced was domed. The 

streams were invigorated by the greater 

elevation of the mountains and began at 

once to lower their courses. A long period 

of stream erosion followed, and associated 

with it were at least three stages of glaciation, during each of which the mountain 

canyons were occupied by ice. Through the 

combined efforts of ice and water the major 

features of the present topography were 

developed.

Since the melting away of the last glaciers 

there has been a renewed and vigorous attack 

on the mountains by streams and by all those 

agents which assist in 'the weathering or 

disintegration of rocks. Vast quantities of
PHYSICAL GEOGKAPHY. 9

loose material have been taken down the canyon walls. The mountain slopes left too steep to stand when the, last ice melted have 

loosened, and great areas of landslides have been formed. The 

morainic deposits left by the glaciers contain lake basins, and at 

many places such deposits have partly filled the valleys and even 

ponded the streams. In the higher mountains there are places where 

the ice actually gouged basins out of the solid rock, and in such 

basins waters have accumulated. Mud flows and torrential fans have 

also ponded certain of the streams and caused lakes to come into 

existence. Some lakes have been filled and others have been drained. 

Broad flood plains have been developed, and by the intrenchment of 

stream courses below former flood plains many terraces have been 

left. The range is to-day being actively eroded, and great masses are 

slipping or sliding from the oversteepened mountain slopes. There 

are reasons for believing that the range is also being slowly uplifted.

GREAT CANYONS.

The great canyons of the San Juan region head in the central 

part of the mountain area and lead radially to the bordering plateaus. 

At their heads and at the heads of the several tributaries to these 

major canyons there are broad, open semicircular areas where the 

snows collected during the several glacial stages and formed the 

alpine glaciers. The vigor of ice action, even at the very beginning 

of movement, is recorded in these ancient catchment basins or cirques 

by deep gouging into the solid rock and by grooved, polished, and 

striated surfaces. The bordering wall of a cirque is usually very 

steep, in places precipitous, for the work underneath the glacier led 

to a continual enlargement of the catchment area, so that the intercanyon or intercirque ridges became narrower and narrower, locally 

even coming to have sharp, jagged, sawtooth forms. During the 

period of maximum formation of ice the sharp peaks were all that 

rose above the glaciers and the associated snow fields. The catchment areas as examined to-day range in area from a fraction of a 

square mile to as much as 15 square miles.

Downstream from the cirques the canyons usually retain an open 

or U-shaped form, being in sharp contrast to unglaciated canyons, 

such as the Black Canyon of the Gunnison, at the north margin of 

the range, and the Toltec gorge, at the southeast, for such streameroded canyons are V-shaped. The walls of the great canyons in the 

range are in places smooth and even polished from ice action. The 

bold or fantastic rock features which certainly must have existed 

there before the ice passed through the canyons have been removed, 

and there is a general simplicity to the canyon walls. On the floors 

of the canyons it is evident, at many places, that there has been
10 RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO.

vigorous ice action. Grooved, polished, and striated surfaces are 

still preserved, and vast deposits of glacial debris are lodged in the 

lower portions of the valleys.

Many of the tributary canyons terminate several hundred feet 

above the bottom of the main canyon, so that their waters now 

tumble or fall into the main gorge. Such hanging valleys were 

usually occupied by ice, but the deepening work of the tributary 

glaciers fell far short of that accomplished by the glaciers in the 

main canyons.

The floors of the glaciated canyons are very irregular and do not 

have the normal gradients of stream courses, for the work of glacier 

ice is not controlled by the same laws that control the work of running water. In some places the ice has gouged out holes much below 

the general gradient; in others rock hills or knobs have been left on 

the floors of the valleys. The debris left by the ice during its advance and upon its final melting has added numerous details in the 

topography of the canyon floors and the lower slopes, so that many 

of the postglacial streams in such canyons flow here on gentle gradients through meadow lands, there over rocky places where cataracts 

and rapids exist, or through some narrow notch cut in a rock ledgu 

or morainic ridge. Some of the lakes in the canyons are due to 

these irregularities in the, stream bed, either in the solid rock or in 

the glacial debris. At many places vast quantities of material have 

slumped into the canyon from one side or the other, or from both 

sides, so as to block the course of the stream. At a few places muds 

have formed and come down the canyon slopes, and opposite the 

mouth of each small tributary stream there is usually a torrential 

fan. The recognition of these looser deposits is of special significance in the selection of reservoir sites, arid each class is described in 

detail below.

GLACIAL DEPOSITS IN THE CANYONS. 

TERMINAL MORAINES.

While a glacier occupies/ a mountain canyon there is constant 

movement of the ice down that canyon. The walls and floor of the 

canyon are scraped bare of loose material. At the margin of the ice 

in the position of maximum advance excessive melting is taking 

place and heavy deposits of glacial drift accumulate. Upon the 

final melting away of the ice this debris is uncovered. Such a 

deposit, which should normally cross the valley as a curved or crescentric belt or ridge, is known as a terminal moraine. As the ice 

carries different kinds and amounts of debris in its different portions, the deposit will not be of uniform composition or uniform 

thickness. As the ice during seasonal variations moves forward or
PHYSICAL GEOGRAPHY. 11

retreats irregularly at its margin, such a deposit will have a very 

irregular or hummocky surface form. The terminal moraine in 

many valleys in the San Juan Mountains is a belt that crosses the 

valley and ranges from a few rods to a mile in width. At either 

side of the canyon this moraine may blend or connect with a lateral 

moraine deposited at the side of the valley or on the lower slopes, as 

the ice melted away.

The recognition of the terminal moraine rests upon (1) its topography, which is rough or hummocky, with knobs and kettles; (2) 

its- composition, for it is a mass of stones and boulders of various 

dimensions, as much as 10 or 15 feet in diameter, intermingled with 

gravels, sands, and clays; (3) its structure, for in most cross sections it is apparent that much of the material was not carried and 

deposited by running water, or deposited in standing water, the 

greater part of the material being unstratified, though within many 

such deposits there are pockets of stratified sands and gravels; (4) 

the subangular shapes of the stones; and (5) the scratches or striae 

found on many of the stones.

When such a terminal moraine is approached from the downstream side it may appear as a rough or hilly belt, usually well 

wooded, extending across the valley. The stream may have cut 

but a narrow notch through the moraine, and rapids or cataracts 

occur at such places. As seen from the upstream side the terminal 

moraine is not commonly so conspicuous, for on that side the morainic deposits usually become gradually less and less.

EECESSIONAL MORAINES.

When the ice front remains stationary for a considerable time 

during the general melting and recession of the glacier morainic 

deposits accumulate at such marginal positions. Those deposits resemble the terminal moraines very closely in topography, material, 

and topographic relations, and in being frontal, but they are unlike 

the terminal moraines in that they do not represent the position of 

the ice front at its maximum advance. Several such recessional moraines may be lodged in a single valley, and at each moraine the 

.stream course may be somewhat constricted. Upstream from such 

moraines, as from the terminal moraine, the valley floor may be less 

heavily mantled with glacial drift and so appear broader or more 

open. Many recessional moraines serve as natural dams, blocking 

the drainage and causing lakes. In certain of the valleys among 

the San Juan Mountains there are chains of lakes caused in this 

way and connected by small streams. The lowering of outlets has 

drained many lakes that existed for a time after the melting of the 

ice, and those lake basins now appear as meadows or swampy lands in
12 RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO.

the course of the stream. Thus the mountain stream may flow for 

some distance through a meadow where it has a low gradient and 

meander there much as rivers of extreme old age wind about in their 

flood plains. Where such a stream leaves the old lake bottom and 

crosses the moraine it may have to flow through a narrow gorge, and 

with falls and rapids in its course it ma}' literally tumble down to 

the level of the next meadow, again to meander through an old lake 

bottom and tumble tor a still lower level, and so on, until the glaciated 

portion of the canyon is passed. The constricted courses of the 

streams just below such lake-bottom meadows have often been selected as dam sites, and the former lake basins as the sites for 

reservoirs.

LATERAL MORAINES.

When a valley glacier is in place vast quantities of material loosened by weathering, rain work, or streams are carried down the 

mountain slope and deposited as ridges on the ice but near its margins. Such ridges of debris are lateral moraines, and on the final 

melting of the ice they come to rest at the sides of the valley. They 

commonly connect at the downstream end with the terminal moraine 

or with one of the recessional moraines. Upstream they become less 

and less prominent and die out before the catchment basin is 

reached.

The lateral moraine commonly resembles a ridge. Its upper surface may have so low a gradient as to suggest an old railroad grade 

on the canyon wall. There may be a depression between the lateral 

moraine and the mountain slope, and locally small lakes are held in 

such depressions. Where a lateral moraine crosses the mouth of a 

tributary stream ponding may follow and a small lake thus come 

into existence. Such lakes have commonly overflowed, their outlets 

have been lowered, and to-day the formerly ponded tributary streams 

flow through meadows occupying the sites of the former lakes, then 

cross the lateral moraines, and come into the main valleys.

The position of the lateral moraine on the valley wall represents 

the minimum elevation of the ice that occupied the main canyon. 

The ice was certainly somewhat greater in thickness than the elevation of the lateral moraine above the stream channel, for the material 

of such a moraine is let down perhaps 100 or 200 feet during the melting of the ice before it becomes lodged on the slope.

At some places in these mountain canyons the walls are too steep 

to permit the lodgment of such lateral deposits, and the material that 

would otherwise have formed a lateral moraine comes to rest on the 

floor of the valley. There it does not have a distinct ridgelike form
PHYSICAL GEOGRAPHY. 13

but is mingled with the general mantle of ground moraine left on 

the final melting of the ice.

In composition and structure the lateral moraines resemble the 

terminal and recessional moraines, but they do not contain pockets 

of stratified materials such as are common in the frontal moraines. 

The stones in the lateral moraines are more commonly angular than 

those that were carried near the base of the glacier.

MEDIAL MORAINES.

When a tributary glacier unites with ice in the main canyon the 

lateral moraine on the upstream side of the tributary glacier joins 

the lateral moraine of the main glacier and forms a medial moraine. 

Material so accumulated rests upon the surface of the glacier, but 

on the final melting of the ice it is deposited on the floor of the 

canyon. Here and there a distinct medial-moraine ridge noAv exists 

on the floor of a glaciated valley. It would seem that as the material 

of the medial moraine was gradually let down during the melting of 

the ice, it became distributed or dispersed, and so became a part of a 

general mantle of drift on the floor of the valley. Short medialmoraine ridges may be present at the junction of a tributary with 

the main valley.

OUTWASH DEPOSITS.

Downstream1 from the terminal and recessional moraines vast quantities of sand and gravel are carried out by the waters that come from 

the melting ice. These sands and gravels load the streams to their 

utmost capacity and thus commonly lead to the aggradation or building up of the floors of the valleys beyond the position of the ice 

front. Such outwash deposits are called valley trains. They extend 

from a few miles to as much as 20 or 30 miles below the terminal 

moraine. When the glaciers finally melted away and the streams became clearer, having less debris to carry, their velocities were increased and they were able to lower their courses through these 

loose deposits of sand and gravel. Possibly an uplift of the range 

helped to quicken their velocities. Thus the stream channels are 

to-day intrenched below the old flood-plain levels of glacial time, 

and the remnants of the old flood'plains appear as terraces or benches 

as much as 100 feet above the present streams. At each stage of the 

retreat of the ice vast quantities of silts were washed out beyond the 

ice front, so that bodies of stratified sand and gravel, may be found 

at several localities in the valley, but usually they lie just below the 

frontal moraines.
14 RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO. ' 

DEPOSITS OF DISTINCT GLACIAL EPOCHS.

In the San Juan region proof has been obtained of at least three 

distinct glacial stages during the last long period of erosion.1 In the 

earliest of these, which is called the Cerro stage,2 the glaciation was 

the most extensive, and the moraines of this stage have been largely 

removed. A few scattered remnants, all of which are above or outside of the modern canyons, have been found at several places- about 

the margin of the range.

The glaciers of the intermediate stage, known as the Durango, 

were intermediate in extent, but they occupied the modern canyons, 

and their moraines are farther down those canyons! and higher on 

the slopes than those of the last stage. The Durango glaciers were 

as a rule from 2 to 5 miles longer than the glaciers of the last or 

Wisconsin stage. Most of the terminal moraines of the Durango 

glaciers have been removed by subsequent erosion. The lateral moraines remain, and at many places the outwash deposits of that 

stage may be identified.

The glacial deposits which are of immediate significance in the 

present discussion are those of the last or Wisconsin glacial stage, 

which have been described above in detail.

LANDSLIDE DEPOSITS.

When the last glaciers disappeared from the San Juan Mountains 

the canyon walls were at many places so steep that as weathering 

and disintegration of the rocks progressed vast quantities of the 

material slumped off and slid to the bases of the cliffs. The geologic 

conditions at many places are especially favorable for the development of landslides. Loose or soft formations underlie heavy flows 

of lava, and as the less resistant formations are weathered out', the 

heavy load above is undermined, breaks off, and pushes a great mass 

of the 'fine material with it down the slope. Again the volcanic 

rocks become very thoroughly weathered or disintegrated, and such 

a mass when saturated from heavy rains or from the melting of 

snow may move or slide down the mountain. Where there is a somewhat impervious layer overlain by loose materials the ground waters 

collect just above the impervious layer. Such waters may issue as 

springs at the base of the loose material, but they also sBrve as a 

lubricant and hasten the movement of great masses of the overlying 

rock down the hill. Thus the cliff is forced back and may retreat for

1 Atwood, W. W., and Mather, K. F., The evidence of three distinct glacial epochs in 

the Pleistocene history of the San Juan Mountains, Colo.: Jour. Geology, vol. 20, pp. 385- 

409, 1912.

2 Atwood, W. W., Eocene glacial deposits of southwestern Colorado: U. S. Geol. Survey 

Trof. Paper 95, pp. 14-15, 1915.
PHYSICAL GEOGRAPHY. 15

many hundreds of feet. At some localities this process has continued 

until the cliff is a large fraction of a mile back of its former position.

The landslide masses as- they accumulate have an exceedingly 

rough topography. This topography may resemble somewhat that of 

a glacial moraine, but in contrast to- the moraine the individual landslide mass commonly has one longer axis, which makes it ridgelike, 

and the longer axis or crest of the ridge will be approximately parallel to the cliff from which the mass slid. The landslide materials 

differ from the glacial deposits in that they are angular, and the individual stones do not haA^e polished and striated surfaces such as 

are present on the stones of the glacial drift. The landslide masses 

do not contain so large a variety of stones as the glacial deposits, for 

their materials must come from the few formations represented in 

the cliffs above them. The landslide masses- are also of smaller extent 

than the glacial deposits, and the cliffs from which they have come 

are usually to be seen. There is no assortment of the material in 

the landslide masses, and in that respect they resemble the deposits 

left by the melting ice.

Among the landslide masses there are small undrained depressions, 

and in such depressions lakes may form. Thus the presence of 

lakes or ponds in the loose deposits of the mountains is not a proof of 

glacial origin for those deposits. At some places landslide masses 

check drainage lines and pond the streams, so that lakes have come 

into existence. Landslides may come from the opposite sides of a 

mountain canyon and meet on the floor, or they may come with so 

great velocity from one wall of a canyon that they cross the stream 

course and ride several hundred feet up the farther slope.

The accumulation of loose angular material at the base of a cliff in 

the upper portions of a canyon, especially in the. great catchment 

areas, is spoken of as talus. The individual blocks have fallen a few 

at a time. Locally in these basins, and in a few places farther down 

the canyons, huge masses of talus material have ridden out or flowed 

out from the cliffs, and as this process is continued, one mass after 

another folloAving the same track down the mountain' slope, land 

forms 'are produced that resemble great mud flows, but they are- composed of angular talus blocks. Such deposits have been described as 

rock streams 1 and are sometimes called rock glaciers, the term implying that the mass was frozen and moved as a glacier, although 

it was heavily loaded with the angular fragments of rock waste. The 

significance of the landslide and talus accumulations is referred to 

in the descriptions,^ special reservoir sites.

1 Howe, Ei-nest, Landslides In the San Juan Mountains, Colo.: U. S. Geol. Survey Prof. 

Paper 67, 1909.
16 RESERVOIR SITES IN" SAN JUAN MOUNTAINS, COLO. 

LAKES AMONG THE MOUNTAINS.

By far the greater number of the lakes in the San Juan Mountains are due in one way or another to glaciation. High among the 

mountains, where ice erosion was intense, small basins were gouged 

out of the solid rock, and in those basins waters have accumulated. 

In the valleys below the basins, where deposition predominated over 

glacial erosion, the deposits of drift have been so irregularly lodged 

on the floors of the canyons that many undrained areas were left, and 

in those" undrained areas waters from subsequent rainfall have 

formed small lakes or ponds. In the terminal and recessional 

moraines the depressions are relatively small, being sometimes appropriately described as "kettle holes," and many of them contain 

water. Upstream from the great recessional moraines the waters 

are sometimes held on the floor of the valley awaiting overflow and 

discharge. In the tributary valleys there are examples of lakes 

held in by the lateral moraine of the main canyon glacier. Such a 

lake may in time rise until it overflows, and in cutting its outlet 

through the lateral-moraine dam it may soon lower its water even to 

the point of extinction. In a few places lateral moraines retain waters 

against the mountain slopes, and occasionally a lake may come into 

existence where two lateral moraines from adjoining canyons meet 

to form a medial moraine in a small triangular space just above the 

junction of the lateral moraines. The blocking of drainage by landslides, as has been suggested above, accounts for some of the lakes in 

the range, such as Emerald Lake, in the valley of Pine Creek, and 

Lake Santa Maria. Mud flows may be of such dimensions as to 

block drainage. One of the most remarkable lakes in the range is 

caused by a mud flow coming across the canyon of the Lake Fork of 

the Gunnison about 4 miles above Lake City. The ponding of the 

waters by this mud flow explains the presence of Lake San Cristobal. 

Glacial deposits and landslides, or other combinations of the loose 

deposits so commonly found in the mountain canyons, may form 

undrained depressions. Many of the lakes among the mountains invite investigation by those looking for reservoirs. The outlets may 

be narrow V-shaped canyons, and the retaining walls may appear high 

enough so that, if the outlet is blocked, a large supply of water may 

be readily stored, but the geologic conditions of the areas bordering 

the present lakes, and especially above their present high-water 

mark, are of great significance in the selection of reservoir sites.

TORRENTIAL DEPOSITS IN THE LARGE CANYONS.

In addition to the glacial, landslide, and'mud-flow deposits in the 

canyons there are great fanlike accumulations opposite the mouths 

of tributary streams. The side stream usually comes over a very
PHYSICAL GEOGRAPHY. 17

steep gradient and brings large quantities of fragmental material or 

stones, which have been somewhat rounded, and when the gradientis decreased as the tributary reaches the main valley it must deposit 

its load. The heavier material is dropped first, the stream separates 

into several distributaries, and more and more of the load is dropped, 

until in the small streams that result from the continual division of 

the waters the fine sands and silts are laid down. Such an accumulation has its apex at the mouth of the tributary gorge and slopes 

radially for nearly 180° from that point toward the bottom of the 

valley, so that it comes to have a fanlike form. Deposits of this 

kind from the two sides of the canyon may meet, or a single fan may 

advance to the farther wall of the canyon, and thus the stream 

course will be blocked and a lake will come into existence.

STREAM COURSES IN THE LARGER VALLEYS.

The actual channel which the stream follows through a canyon 

that has been glaciated and affected by landslides, mud flows, and 

torrential deposits is extremely varied. In places, more commonly 

in the upper portion of the course, the channel lies on bedrock. Falls 

and rapids occur where the ice has left a cliff or a very steep slope 

on the floor of the canyon. Again the stream is diverted to one side 

or the other because of some huge mass of loose material. The 

stream course may pass through an old lake bottom or wind in and 

out among the heavy deposits of a recessional or terminal moraine. 

Here and there the stream has a meandering course above torrential 

deposits, or perhaps it has been forced to cut into solid rock far to 

one side of the canyon by the alluvial fan deposit of a tributary 

stream- Throughout its course it is usually vigorous. Certain 

streams have rock walls on one side and at first glance may appear 

to be cutting into solid rock, but the opposite wall consists of loose 

material of glacial or landslide origin, and after waters are ponded 

by a dam at such a location the ponded waters may find easy passage through the loose materials. What appears to be a firm rock 

gorge has in fact but one wall of firm rock. Or a stream may be 

in a rock gorge far to one side of a valley, and the preglacial channel 

may be much lower and in the middle of th,e valley but filled with 

debris. The conditions are so variable and the number of possible 

combinations of the factors that have influenced the location of the 

stream is so great that the selection of a site for the dam for a reservoir requires a careful examination of the canyon for some distance 

up and down stream from the proposed site, and a full appreciation 

of the geologic conditions across the entire width of the canyon just 

below the reservoir.

65633° 18 Bull. 685 -2
18 RESERVOIR SITES IIST SAN- JUAN" MOUNTAINS, COLO.

The streams of the San Juan Mountains are all youthful. Even 

after the great deepening accomplished during the three stages of 

Pleistocene glaciation the streams are lowering their courses. It 

appears that the mountains are still growing, and the base-level to 

which the streams are deepening their courses is being lowered in 

the great mountain mass.

RESERVOIRS.

FARMERS' UNION OR RIO GRANDE RESERVOIR.

The headwaters of the Rio Grande are in the central portion of 

the range, Avhere the relief is very great and the combined rainfall

Modern alluvium Torrential wash Landslides Glacial moraines Lava flows or

of the other volcanic 

Wisconsin stage formations near 

or at the surface

FIGURE 3. Map of a part of the San Cristobal quadrangle, showing the location and 

topographic relations of the Farmers' Union or Rio Grande reservoir and the distribution of the loose or unconsolidated formations within the area shown.

and snowfall are so large that each of the streams contributing to 

this large trunk channel is a vigorous mountain torrent. The portion of the valley that was selected as a reservoir site (PL I, A) is in 

the central part of the San Cristobal quadrangle and extends from 

the mouth of Ute Creek 7 miles downstream to a point near the mouth 

of Big Squaw Creek (fig. 3). This portion of the valley of the Eio 

Grande is south of Lost Lakes and Long Park. The reservoir site 

may be reached by road most readily from Creede, about 33 miles 

away.
U. S. GEOLOGICAL SURVEY BULLETIN 685 PLATE I

A. FARMERS' UNION RESERVOIR, LOOKING DOWN THE VALLEY OF THE

RIO GRANDE FROM A POINT ON THK WEMINUCHE TRAIL. 

View showing the location of the dam and the great landslide area to the north.

B. DAM OF Till-; FAHMKHS' l.MON I!IJSKlt\ ()!li KHOM THE SOUTH.

The spillway appears in the foreground, and the reservoir is to the left. Control gates are 

near the roadway on the crest of the dam.

C. SOUTH MARGIN OF THE DAM ON THE DOWNSTREAM SIDE, SHOWING LEAKAGE AROUND THE SOUTH END OF THE DAM. 

This leakage is through fractured rock. The tunnel opening is near the left margin of the view.
U. S. GEOLOGICAL SURVEY BULLETIN 685 PLATE II

NORTH MARGIN OF THE FARMERS' UNION DAM Fll()\l DOWNSTREAM SIDE. 

The landslide mass appears at the right, and near the edge of the dam seepage is shown.

. SEEPAGE THROUGH THE LANDSLIDE MASS JUST BELOW THE FARMERS'

UNION DAM. 

... . . At the right is the opening of a prospect tunnel.

C. IGNACIO RESERVOIR AND DAM.
FABMEBS UNION EESEEVOIE. 19

The floor of the valley where the waters have been stored was formerly a broad stretch of meadowland. Much of that land had been 

taken up by ranchmen, who had established their 

homes. The site of 

the dam was a narrow place in the 

stream course just 

below the broad 

flat - bottomed portion of the valley. 

The constriction 

where the dam has 

been placed is due

to a large landslide

area associated with

the neighboring

cliff at the north.

(See fig. 4.) In

this cliff there is a

thick, heavy flow of

lava over a volcanic

tuff or breccia.

Such conditions are very favorable for landslides and have caused

similar accumulations at many places in the range. 

The dam is built of earth and rock and contains a concrete core

wall. (See PI. I, B.) It is about 100 feet high and 400 feet wide and

appears to be watertight. The tunnel and spillway (fig. 5) are near

the south side of the valley 

and are cut through rock, 

which is in place, though 

somewhat fractured. The 

level of the spillway permits waters to accumulate 

in the reservoir to a height

PiGUK.5.-42ross section along the line A-B on °f 85 feet } USt above the

figure 4. The diagonal lining simply indicates dam, and with that height

the extent of the consolidated rock formations, Qf wate th jd b

which lie nearly, horizontal. 

reservoir 7 miles in length

and from a quarter to half a mile in width. The capacity of the 

reservoir has been estimated at 4,500 acre-feet.

Before constructing the clam some prospecting was done by tunneling into the great landslide mass at the north side of the stream 

course. Bedrock was not found, and it could not have been expected

FIGURE 4. Sketch map showing the geologic conditions Immediately adjoining the dam of the Farmers' Union 

reservoir. 1, Control gates; 2, lower opening of tunnel; 

3, 4, 5, 6, places where seepage flows have heen noted; 

7, angular debris opposite lower end of spillway; 8, construction camp; 9, 10, canyon walls; A-B, position, of 

cross section shown in figure 5.
20 RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO.

unless the tunneling had gone perhaps a quarter of a mile, so as to 

reach the north wall of'the canyon. (See fig. 5.)

This reservoir has proved to be very serviceable, and the waters 

are furnished to ranchmen and farmers in the San Luis Valley. 

Some difficulty has been encountered, however, for the waters have 

seeped through the landslide mass around the north end of the dam, 

and have come out into the stream a short distance below the dam. 

(See fig. 4.) There is one locality where seepage is noticeable from 

the dampness of the ground, and one other very near the dam (3, 

fig. 4) where, during September, 1915, there was some water flowing. 

These indications of seepage were present at a time when the water 

in the reservoir was very low. In June, 1916, when the waters stood 

much higher in the reservoir than during the preceding fall and 

yet far below the maximum possible height, the seepage at both the 

north and south ends of the dam was notably free. At the south 

the waters came through the much fractured rock near the lower tunnel opening and issued as a cascade 6 to 8 feet wide. (See 2, fig. 4, 

and PI. I, C.} More of the fractured rock should have been removed 

before the dam was constructed. At the north side of the dam small 

streams were issuing at points marked 3, 4, 5, and 6 in figure 4, 

through the landslide material. (See PL II, A and B,} Between 

points 4 and 5 the entire bank was saturated, and waters were flowing freely into the stream. At point 5 an old testing tunnel had 

become the channel of an underground stream which issued here.

The possibility of checking this seepage by cribbing is under consideration, and every effort will be made by those in charge of the 

project to prevent this leakage from becoming serious or endangering 

the success of the project. The landslide mass through which the 

waters seep appears to consist of very coarse material mixed indiscriminately with finer detritus. If a body of landslide material 

contains sufficient clay to block up the interstitial spaces and thus 

form an impervious mass, it may be safely used to retain water. 

Here the waters have evidently passed through an eighth to a quarter 

of a mile of this material. It is conceivable that as the waters pass 

through they may carry and deposit clays in just such places as to 

block up the subterranean routes, but on the other hand there is evidently the danger of waters going through in such volume and with 

such velocity that the finer materials in the landslide mass may be 

washed out and the underground routes become larger and larger.

IGNACIO RESERVOIR.1

On a high bench west, of the canyon of Animas Kiver in the Engineer Mountain quadrangle a. large supply of water is held in what

1 The report on this reservoir is based largely on field work done by Klrtley F. Mather.
ICSTACIO RESERVOIR, 21

is called tlie Ignacio reservoir (fig. 6). The nearest railroad station 

is Tacoma, but the reservoir may be easily reached from Durango 

by road, a distance of about 22 miles. The waters are turned through 

a great flume into a lesser reservoir near the brink-of the canyon 

wall, and then through a smaller flume down the canyon wall and 

through a power plant.

The reservoir site is in a country of slight relief and rolling topography, where glacial ice has rubbed off the hilltops and removed

EXPLANATION

Lava flows or other

volcanic formations hear

or at the surface

FIOUKBG. Map of a part of the Engineer Mountain quadrangle, showing the location 

and topographic relations of the Ignacio reservoir and the distribution of the loose or 

unconsolidated formations within the area shown. The bench on which the reservoir 

lies has been severely glaciated, and there is a scattering of glacial drift over most 

of Its surface.

most of the loose material. There is a thin scattering of glacial 

drift on the bench in the vicinity of the reservoir, and a small area 

of ground moraine at the west side near the lower end. The area 

now covered by the waters of the. reservoir may have held several 

small glacial lakes, similar to the ponds and marches on the same 

bench north and south of the reservoir site.

A dam with a vertical face of about 52 feet in the center was constructed, and several small streams were turned into the basin. The
22 RESERVOIB SITES IN SAN" JUAN MOUNTAINS, COLO.

dam is built of log cribbing with rock fill. (See PI. II, C.) It is 

faced with three thicknesses of planking with tar paper between. 

Ordinarily this dam will hold from 48 to 50 feet of water. No overflow is allowed, for the waters come into the reservoir through flumes 

and, when necessary, may easily be diverted. The outlet from the 

reservoir is through a large pipe in the dam, and thence into a 6-foot 

concrete tunnel through a morainic hill. The intention may have 

been to rest this dam upon bedrock throughout its length, but as 

several serious leakages have occurred beneath the dam it appears 

that the base of the dam was not at all places below the glacial debris. 

The dam was not constructed at so great expense or with so much 

care as many of the more modern structures, and there is always 

some leakage. It has been reported that twice in the history of the 

reservoir large leaks have developed. The timbers are now very 

badly rotted, and the remaining life of the dam would appear to be 

relatively short.

BOAD CANYON RESERVOIR.

A short distance above the junction of Road Canyon with the 

canyon of Crooked Creek, in the central part of the San Cristobal

quadrangle, there is a 

small reservoir (fig. 7). 

The valley has a broad, 

open, flat-bottomed area 

immediately upstream 

from a few morainic hills, 

and it was conceived by 

persons who wished an 

extra supply of water 

that a small dam thrown 

across the valley at these 

morainic hills would 

make it possible to store 

water. The dam is a mere 

embankment of earth and 

stone, of very simple construction, and though it 

is leaking badly not very 

much has been invested 

in it nor very much expected from it. The reservoir is now used as a fish pond.> It illustrates the use of a recessional moraine and the meadowland, probably a former lake basin, 

just upstream from the moraine.

EXPLANATION

Lava, flows oro'flifirvbl'1

came formations 

near or atfhe surface

i Mile'

FIGDBE 7. Map of a part of the San Cristobal quadrangle, showing the location and topographic relations of the Road Canyon reservoir and the distribution of the loose or unconsolidated sediments 

within the area shown.
RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO. 

SANTA MARIA RESERVOIR.

23

Santa Maria Lake is a beautiful body of water resting in a long, 

narrow trough just west of Bristol Head, 4 miles northwest of Antelope Springs, near the eastern margin of the San Cristobal quadrangle. (See fig. 8 and PL III, A.) This reservoir may be reached 

from Creede by road, a distance of approximately 19 miles.

EXPLANATION

Lava flows or other

volcanic formations near

or at the surface

FIGURE 8. Map of a part of the San Cristobal quadrangle, showing the location and 

topographic relations of Santa Maria reservoir and the distribution of the loose or 

unconsolldated formations within the area shown.

The nearly vertical wall of Bristol Head, to the east of the lake, 

rises over 3,000 feet above the lake level. To the west there is also a 

steep wall, but this one rises only 800 feet above the average level of 

the lake. Vast quantities of debris have fallen from the cliff immediately below Bristol Head and partly filled the depression just west 

of the mountain and south of the lake. (See fig. 8.) There is now 

a landslide mass at this locality fully 1 mile wide and 500 feet 

thick, extending for a distance of nearly 2 miles along the axis of the
24 RESERVOIR SITES IN SAN JUAN MOUNTAINS, OOLO.

trough. At the north end of the lake basin there are deposits of 

glacial drift resting upon bedrock.

To provide for the storage of flood waters in this lake basin an 

earth dam with a concrete-core wall was constructed at the north end, 

a tunnel was driven at the northwest corner near the dam, a spillway 

was built near the dam, and an aqueduct was constructed so that 

the surplus waters of Clear Creek could be conducted to this basin. 

(See PL III.) The waters of several small streams from the east 

were also led into the lake basin. The tunnel driven through bedrock near the northwest corner is the outlet of the reservoir, 'and the 

waters as they issue from the tunnel follow an open ditch and soon 

enter Clear Creek, thence flowing into the Rio Grande. These 

waters are furnished to the ranchmen and farmers of the San Luis 

Valley. The engineering work and the mechanical construction appear to have been excellent. There is no leaking through or about 

the dam, and a very considerable head of water has been successfully 

held. At the south end, where the recent landslide masses form the

SmaU laKe

FIGURE 9. Diagrammatic north-south section through the Santa Maria trough. The 

actual position of the bedrock surface beneath the lake and beneath the landslide mass 

i south of the lake can not be given accurately.

rim of the lake basin (fig. .9), large quantities of water have 

seeped through and issued fully a mile and a half to the south, forming a small pond from which, at times, .a rushing torrent flows by 

Antelope Springs and into the Rio Grande. About the south end of 

the reservoir, however, no signs of leakage appear in the great landslide mass; the surface is apparently entirely unmodified. The water 

appears at the south margin of this landslide mass not from a single 

exit, like an underground stream, but probably from general seepage 

through this huge mass of material. About half a mile below the 

reservoir there is a small lake or pond in the midst of the landslide 

masses (fig. 9), and the .waters in this small lake have risen and 

fallen with the waters in the reservoir. It is clear, therefore, that 

the seepage is quite free as far south as this lake, and the stream 

already referred to makes it also very apparent that the seepage is 

free still farther south and, indeed, throughout the mass.

During September, 1915, the waters were being withdrawn from 

this reservoir. Exploratory work wa& being carried on at the south 

margin to determine the nature of the material where the leakage 

was taking place. This work did not seem to yield significant results,
TJ. S. GEOLOGICAL SURVEY BULLETIN G85 PLATE III

XTA MARIA RESERVOIR FROM THE NORTH.

Bristol Head is at the left. The landslide mass blocking the reservoir at the south appears 

just beyond l,he water. The dam appears in the foreground. At the left in the foreground glacial drift is exposed.

B. SANTA MARIA RESERVOIR DAM FROM THE NORTH.

A spillway appears near the left end of the dam. The control gate is regulated from the 

little house on the dum.

C. OUTLET OF THE SANTA MARIA RESEKVOIH.
IT. S. GEOLOGICAL SURVEY BULLETIN 685 PLATE IV

A. LOWER END OF MOSCA RESERVOIR, SHOWING DAM AND, AT THE LEFT 

ABOVE THE RESERVOIR, THE FRONTAL MORAINE.

S. MOSCA RESERVOIR DAM AND SPILLWAY, SHOWING THE GORGE BELOW THE 

RESERVOIR SITE IN THE FOREGROUND. C. TUNNEL OUTLET AT MOSCA RESERVOIR. SHOWING THE CRIBBING PUT IN AFTER THE FIRST BREAK OCCURRED
SANTA MARIA RESERVOIR. 2£

and could not be expected to, for the nature and composition of a 

landslide mass can not be judged as well from a few small openings 

as from the surface. Tunnels and pits might be driven at a hundred places, sand the only result could be to ascertain that the mass 

is heterogeneous detrital material that has fallen from the cliff at the 

east. It is composed of large and small angular blocks, rocks crushed 

in the falling and intermingled with some silts and sands. It may 

even have within it vast quantities of forest growth, which were 

enveloped in the sliding of the rocks from the mountain, and it may 

vary in composition greatly within short distances. It is safe to 

estimate its thickness at 500 feet, and its areal extent is shown in 

figure 8. Much of the torrential wash just south of this landslide 

area is interpreted as a mantle over other landslide material.

A serious problem confronts the engineer here, for the south end 

of the reservoir, where seepage is so generally taking place, has a 

very irregular outline. The covering of this slope with clay has been 

considered. One difficulty appears at once in the lack of an abundant supply of clay near at hand, and if-this were done it would be 

very expensive. This reservoir presents a very interesting question. 

If the waters will not stand at the height desired, why is it that the 

lake waters have been held at all ? It seems possible that the lower 

portion of the landslide mass may be more dense; perhaps the greater 

pressure has filled in more of the spaces. Possibly after the waters 

began to accumulate in this depression behind the landslide mass 

they leaked freely through to the south for many years. This leakage or seepage, however, passing through 2 miles of material, may 

have finally, with the help of rainfall and the waters seeping through 

the ground from above, sealed up with fine materials the passageways 

that had been used effecting a sort of automatic puddling. Through 

this natural process the lower portion of the landslide mass may have 

become almost if not quite impervious. It is true that before the 

reservoir project was undertaken there was some water flowing 

southward toward Antelope Springs. Those interested in the reservoir are justified in hoping that as the waters continue to seep 

through they may again seal up the underground routes and thus 

make the landslide mass at the south end a satisfactory barrier. It 

was believed by those in charge of the reservoir that the leakage during 1916 was less than during the preceding years. If the leakage 

continues for a term of years without ever causing a real break 

through to the south it seems that the same processes that made piossible the lake may make possible the larger reservoir.

In June, 1916, when this site was revisited, the water was much 

higher in the reservoir than during the preceding September, and 

yet the outflow to the south was much less, and the water in the small
26 RESERVOIR SITES IN SAN JUAN MOUNTAINS; COLO.

lake in the landslide mass just south of the reservoir had not risen 

with the rise of the water in the larger basin. Perhaps the ground 

was still frozen beneath the surface at that time early in the season, 

but if not, the reduction in the amount of seepage would seem to 

have removed all fear of serious trouble from that source. There 

are no other serious difficulties associated with this project.

MOSCA RESERVOIR.

By J. FEED. HUNTEB.

The dam of the Mosca reservoir project was built in 1913-14 to 

impend and store the flood waters of the Beaver Creek basin for

EXPLANATION

Lava flows or other

volcanic formations near

or at the surface

FIGURE 10. Map of a part of the Creede quadrangle-, showing the location and topographic relations of the Mosca reservoir and the distribution of the loose or unconsolidated formations within the area shown.

the irrigation of lands in San Luis Valley. It is 2 miles from the 

junction of Beaver Creek with the South Fork of the Rio Grande and 

5.6 miles in an air line west of south (about 7 miles by road) from 

the town of South Fork, on the Creede branch of the Denver & Rio 

Grande Railroad. (See fig. 10.) The dam is built across a narrow 

gorge at the lower end of a long stretch of meadow land known as 

Beaver Creek Park and is of sufficient height to back water for approximately 1£ miles. The valley for this distance has a flat allu-
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PIGDBE 11. Sketch map of Mosca reservoir project and environs, s

formations. (The exact location of-the core 
28 RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO.

At the entrance of this canyon is the dam, which is of concrete, 

reinforced by batter masonry on the downstream side, and is 85 feet

high at its breast and 210 feet long 

at the top. It is built between walls 

of considerably shattered volcanic 

rock of the variety known as quartz 

latite. However, west of the dam 

this rock continues for only a short 

distance, giving way to. unconsolidated glacial debris as indicated in 

the profile section of figure 12. (See 

also PL IV.) This volcanic rock 

forms a knob between the canyon and 

and the extensive deposit of sand and 

gravel which rests against and on it, 

filling a former channel in the harder 

rock to an unknown "depth. The outlet of the reservoir is by a channel 

driven, through this knob of latite 

in a northwesterly direction for approximately 40 feet.

The conditions can be best understood from the sketch of the dam 

and its environs (fig. 11) and the 

profile section across Beaver Creek 

just below the reservoir (fig. 12). 

These drawings are intended to present the relations graphically rather 

than to afford accurate maps of the 

locality. It should be understood 

that the contact between the volcanic 

rock and the gravels in the profile of 

figure 12 is hypothetical except for 

the points where it meets the surface. 

The Mosca dam project is of especial interest as a demonstration of 

the inefficiency of a barrier of a 

certain geologic type to retain water. 

Several attempts have been made to 

fill the reservoir, but in each attempt 

the porosity of the barrier hill has 

led to leaks and to serious washouts. 

The first of these developed around the 

southwest end of the dam (see PI. IV, C), and later ones resulted in a 

large washout along the road 1,500 feet to the northwest. (See fig. 11.)
\ 

MOSCA RESERVOIR, 29

Although the physiographic features that led to the selection of 

the dam site are rather obvious, the peculiar geologic conditions that 

make it undesirable and have resulted in its failure to retain water 

are apparent only after more detailed study. It is the purpose of this 

section to describe briefly the character of the formations and their 

structural relations; also to discuss the causes of failure and certain 

suggested remedies.

The bedrock in the vicinity of the Mosca dam is entirely of one 

kind, which may be best classified as a quartz latite.. This is the 

prevalent type of rock in the region and has an extensive distribution along the lower slopes of Beaver Creek and throughout the 

length of the South Fork of the Eio Grande. It is white or pinkish 

gray, is of easy fracture, and consists of scattered crystals of feldspar, biotite, augite, and orthoclase, less than 2 millimeters in diameter, set in a rather, dense and obscure groundmass composed chiefly

of an indistinctly polarizing aggregate, feldspar microlites, triclymite, and ferritic material. The lava has picked up a few angular 

fragments of andesites and other rocks in the process of its eruptiofc, 

and where weathering has taken place small quantities of calcite, 

chlorite, and limonitic minerals have been developed. This latite 

has been poured out as molten lava, cooled, buried by younger flows, 

and later expo'sed by stream erosion.'

The rock knob at the southwest end of the dam and the immediate 

rock mass at the other end of the dam. have been interpreted as of 

landslide origin. This would account for the hillside topography 

shown in Plate V, #, and for the shattered condition of the dock at 

both ends of the dam. A serious leakage has occurred around the 

northeast end of the dam.

A close examination of the nature of the morainal deposits west 

and north of the bedrock knob shows that the faith which has manifestly been reposed in them is hardly warranted. A small cut west 

of the clam and above the road reveals its porous, unconsolidated 

character and shows that it consists of irregular stratified sand and 

fine gravel.

A better exposure has resulted from the washouts along the road 

some 1,500 feet to the northwest. Here nearly 50 feet of section 

shows alluvial layers of irregular, cross-bedded coarse and fine sands, 

grits, gravels, and conglomerates.
30 RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO.

Partial section of material exposed in the largest washout from Mosca reservoir.

Feet.

1. Coarse conglomerate 10

2. Rather fine, homogeneous, uncousolidated sand of darkbrown color, saturated with water______________ 10

3. Black sandy member with thin layers of volcanic pebbles,

largely black glass, becoming coarser at base________ 15±

4. Sandy member, similar to No. 2, with thin lenses of grit and 

fine gravel > ___ _ __ 10

The pebbles of the conglomerate are all of volcanic origin, such as 

might be derived from the Beaver Creek basin, are well rounded, 

and range in size from fine grit to boulders 10 inches in diameter. 

The coarser boulder beds show more irregular bedding and were 

doubtless deposited by torrential stream wash, whereas the sandy 

members are even bedded and suggest delta or pond deposits. No 

striations could be discovered on the boulders.

An examination of the hill above the dam reveals at once its

. morainal origin, for the country west of the dam for a distance of

half a mile, included within the 8,900-foot contour as shown on the

Creede topographic map. is of abnormal configuration,. is poorly

drained, and is covered with a thick mantle of boulders and wash.

Save for a small V-shaped area of till on the west wajl immediately 

north of the latite knob, the waters of Beaver Creek now travel 

through a canyon of latite as far as the road bridge at the 8,600-foot 

contour crossing. At that place the morainal debris comes down to 

the creek and for 120 to 150 yards makes the south side of the stream 

channel. Below this a wall of latite rises gradually downstream. 

From the west end of the reservoir to this point, just below the 

bridge, an earlier, premorainal stream probably flowed through a 

fairly straight channel, now filled with glacial material. It was by

way of this same route that the underground seepage waters from 

the reservoir found their escape. Below this point the earlier 

Beaver Creek channel coincides with that of the present stream.

The unconsolidated deposits here mentioned are believed to represent the accumulations of debris gathered by great glaciers descending from points high in the adjacent mountains of the Beaver Creek 

basin. After the retreat of the glaciers and after the South Fork 

had established itself in a deepened channel, Beaver Creek commenced the task of cutting another channel, releasing its impounded 

waters, and cleaning its valley of the vast accumulations of debris. 

In this process it seems to have found its easiest path to be coincident 

with its earlier channel except in a stretch below the Mosca dam as 

far as the road bridge. Here, by reason of the immense accumulations, it was forced to cut an entirely new course through the detrital 

material and into the harder volcanic rocksy forming the present
MOSCA EESERVOIK. 31

gorge. This work was the more easily clone by reason of the immense 

amount of unconsolidated gravel and boulders available in the upper 

course of the basin, which increased the abrasive power of the stream.

Many of the principal facts relating to the Mosca reservoir were 

established on the ground; other details have been acquired from 

fragmentary reports of persons who have knowledge of the project. 

With all the data so far available it is possible to construct an incomplete history of the project.

On the completion of the dam, in the summer of 1914, the head 

gate was closed and the filling of the reservoir commenced. The 

rising water brought an increasing pressure against the very porous 

gravel barrier which is really the major feature in the damming of 

the valley. Immediately the water began to seep around the latite 

knob at the southwest end of the dam, as indicated in figure 2 the 

knob which projects for about 90 feet above the valley floor through 

the accumulations of sand and gravel.

Before the reservoir was two-thirds full the fine till began to give 

way and to wash out so that it became necessary to construct cribbing filled with broken rock along the upstream limb of the Vshaped contact of the latite and gravel. (See PI. IV, C.) A core 

wall about 120 feet in length has been built in a westerly direction 

from bedrock at the contact of the moraine with the latite. This 

contact is said to dip 45° along the line of the wall.

When the water was allowed to rise in the reservoir a second time 

a much larger leak developed at a point some 1,500 feet from the 

reservoir, on the slope 100 yards south of the road bridge. This 

leak proved to be even more serious than the first and quickly developed into a disastrous washout. The water escaped so rapidly and 

abundantly that the soft, unconsolidated sands and gravels are 

described as having been catapulted out of the hill, leaving a ravine 

as much as 20 feet deep and being washed down £he slope to the 

valley bottom to form a large torrential fan. Although the beds 

at once became saturated and seepages appeared over nearly the 

entire slope, the bulk of the water is reported to have come out at 

about 20 feet below the bottom of the reservoir. This leakage 

proved so great and serious that it was necessary to reopen the head 

gate at once and relieve the pressure from the head of water in the 

reservoir. For several months during the winter of 1914-15 no effort 

was made to use the dam. Ins the spring of 1915 an attempt was 

made to puddle the reservoir west of the dam but without success.

The essential causes of the failure of the Mosca reservoir to retain 

water are clearly the presence of the buried channel and the unusually high porosity of the sands and gravels that fill it. This porosity is shown by the fact that at the time of the examination the
32 RESEBVOIR SITES IN SAN JUAN MOUNTAINS; COLO.

exposures in the vicinity of the larger washout were saturated to a 

high degree and the sands and gravels were still in the process of 

draining, although the water was out of the reservoir save for the 

normal flow of Beaver Creek through it. Furthermore, the surface waters have no doubt taken advantage of the porosity and looseness of the material and have established more or less definite subterranean channels, as is indicated by the number of permanent and 

periodic springs scattered here and there over the area. The intakes 

of the subterranean channels are represented by smaller, partly 

choked openings near the base of the gravel embankment of the 

reservoir a short distance west of the latite knob. Flow lines of sand 

and debris into these holes indicate the passage of water out of the 

reservoir. In view of these conditions it is not surprising that when 

the water rose in the reservoir, exerting an unusual pressure against 

the morainal barrier, the gravels at once became saturated, subterranean channels were established, and the breaks resulted. It seems 

clear that the morainal deposits, in this locality at least, are not competent to retain a large head of water.

In view of the large sum of money already expended the further 

utilization of the project became a very serious question with the 

promoters, stockholders, and engineers. It is not within the province of this report to offer an opinion as to the feasibility of diminishing the leakage to such an extent that the reservoir could be used. 

However, it is very evident that to attempt to hold water in the reservoir without reinforcing the morainal end of the dam would be 

simply to invite disaster. Indeed, from past experience it is not improbable that the entire hill between the reservoir and the washout 

would be tunneled by the rush of water, 'and possibly the buried 

channel would be exhumed. Puddling of reservoir levees and embankments has been found practicable in many places but is only 

questionably applicable to this reservoir because of the absence in the 

vicinity of silt, clay, or fine shaly material with which to line the 

embankment. Revetments of masonry, concrete, macadam, asphalt, 

or logs have been suggested but should be adopted only after careful 

estimates by the engineers as to their cost and practicability. It 

would be highly advisable as a preliminary step to make a careful 

survey in an effort to ascertain the lowest point in the reservoir at 

which leakage begins. From the meager geologic examination so far 

made it seems probable that the leaks may have originated in the 

floor of the reservoir itself. If this is found to be the case, a revetment of the embankment alone would not be sufficient, but it would 

have to extend for greater or less distances out upon the floor of the 

reservoir, amounting to many hundreds of square yards.
TERRACE RESERVOIR, 33 

SUPPLEMENTARY NOTE BY W. W. ATWOOD.

When the writer visited the Mosca reservoir in June, 1916, a group of engineers were at work trying to prevent the leakage. On the east side of the dam, 

where the leakage had been very great, they were drilling holes at intervals of 10 

feet and to the depth of 90 feet. Farther south the interval between the holes 

was to be 20 feet. Into these holes they were forcing cement under a pressure 

of 40 pounds, in the hope that the cement, spreading through the fracture*! 

rock, would seal up all openings. The holes were drilled from the roadway, and 

90 feet took them nearly to the bottom of the reservoir.

Another project was also under way. A tunnel had been driven into the 

moraine a little to the nprth of west from the cribbing near the west end of 

the clam. This tunnel had been driven 180 feet, and it was proposed to take 

it 135 feet farther. The base of the tunnel is 13 feet below the reservoir gate 

and about 7 feet above the base of the reservoir. At intervals of 6 feet perforated iron pipes were driven from the roof of the tunnel upward through 

the Sands and gravels, in the hope that the waters seeping into the moraine 

would enter the small holes in the pipes and be led off by the tunnel into the 

stream below the dam, thus preventing the seepage from passing through the 

moraine and washing it away.

Early in the season, before this work had been completed, the gate was 

closed and the water was allowed to rise in the reservoir. Before it reached 

the desired height a serious leak occurred near the east end of this tunnel. It 

appeared that not all the escaping water was accommodated by the pipes then 

in place, and' that some ran directly into the tunnel. This water washed with 

it sands and gravels, and a large mass fell and broke down that end of the 

tunnel. The engineers concluded that the timbering must be made exceedingly 

strong and that the work must be carried much farther into the hill before the 

water was again allowed to rise. It was their intention to complete the work 

thus outlined and in the spring of 1917 to make another trial. The device 

put into the moraine was called a ".drip curtain."

TERRACE RESERVOIR.

The Terrace reservoir lies in the valley of Alamosa Creek, in the 

foohill belt of the San Juan Mountains in Conejos County, and may 

be easily reached by road from Monte Vista or Alamosa. (See 

PL V, A.) The "terrace," or broad alluvial land that receives the 

water from this reservoir, is a torrential fan spread out in the San 

Luis Valley beyond the base of the mountains. From the apex this 

fan slopes radially to the northeast, east, and southeast. Near the 

apex the slope is 75 feet to the mile. Farther east, lower on the fan, 

the slope is 10 feet to the mile. This fan form is admirably adapted 

to irrigation, as the main supply canal may be brought to the apex 

of the fan and lateral canals constructed to the right and left so as 

to supply waters over the entire area of the deposit. Near the apex 

the alluvial material is coarse and in places bouldery, but farther 

from the mountains it is finer and very suitable for agriculture. 

Thousands and thousands of sheep that spend their summers high 

on the mountains are brought to these lowland fields for the winter

65633° 18 Bull. 685 3
34 RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO. ^

and fattened. Thousands of hogs are here fattened for market. 

During the winter horses and cattle that have been on the open 

range during the summer are brought into the irrigated fields and fed. 

The dam is 5 miles above the mouth of the canyon. It is an immense earth dam, reported to be the largest in the United States. It

 ^V- :'* *'' <'; "/ ^ v v.

 * /» '" . '- " ' t> . «* «

 "-^L-*.' % '/v'»' '. **/ *' - 4-~ ' »"!«' *".« ''» .. ^^Si'/.---.-^'-*.-;-:^ : " . ' - 

 - - 

FIGURE 13! Sketch map of the lower end of the Terrace reservoir, showing the general 

distribution of the frontal moraine that determined the location of the dam and .reservoir site. 1, Lower-opening of tunnel; 2, 3, 4, seepage vents. Contours have been 

taken from a map in the office of the State engineer of Colorado.

is 165 feet high, and its length at the base along the course of the 

stream bed is 1,075 feet. A tunnel driven through hard rock (see 

fig. 13) is 1,000 feet long, 7 feet high, and, on .the average, 12 feet 

wide. The discharge, under a 70-foot head, is estimated at 1,300 

cubic feet per second.

The dam is constructed in a narrow rock gorge just downstream 

from a broad, open portion of the valley. (See Pis. VI and VII.)
U. S. GEOLOGICAL SURVEY BULLETIN 685 PLATE V

A. TERRACE RESERVOIR.

The dam is at the right, and the eurth filling and spillway are in the middle ground beyond the 

water. The low hills at the left form a part of the moraine arid rest upou a lava How.

MOSCA RESERVOIR SITE.

This view shows the location of the dam, the cribbing where (he iirst break occurred, the landslide 

mass at the left end of the dam, the great moraine at the right eud of the dam, aud the narrow 

gorge below the reservoir site.
U. S. GEOLOGICAL SURVEY BULLETIN 685 PLATE VI

A. TERRACE RESERVOIR DAM FROM THE NORTH.

S. CREST OF TERRACE RESERVOIR DAM.

The gate iu the tunnel is controlled from the house on the dam.

C. TERRACE HESEHVOIR DAM FROM BELOW.

The spillway here shown was used during the construction of the dam.
TERRACE RESERVOIR. 35

In figure 13 the general topography and geologic conditions of the 

area surrounding the site of the dam are given. The glacial moraine 

at the north, east, and south indicates that for some time the front 

of a great alpine glacier .ested at just this place in the valley of Alamosa Creek. While the ice was present the drainage must have 

found an outlet in part from beneath and in part from the surface of

CREST

FIGUKB 14. Cross section of the earth dam of the Terrace reservoir, based on data 

furnished by R. I. Meeker, engineer in charge, 1916.

the glacier. The heavy morainic deposit completely filled the j>reglacial route of the stream, and as the ice retreated the water issuing 

from the melting glacier found a route at the .south margin of the 

valley and there cut a narrow gorge in rock (PI. VII, 0). This 

gorge, due to a disarrangement of drainage caused by the glacier and 

the moraine it left, was chosen for the site of the dam because of the 

firm rock walls on either side. In constructing the- dam a concrete 

baffling wall was firmly cemented to the sides of the 

gorge, and a concrete core 

wall 70 feet high was also 

constructed. (See figs. 14 

and 15.) The earth material used was chiefly of 

glacial origin, and a gigantic puddle was formed 

in the central portion of 

.the gorge about the core 

wall, so that an abundance 

of fine material might 

settle in that portion of the 

proposed dam.

A lower earth dam or filling was constructed a little to the northeast and above the former channel of the stream. Through this filling a spillway has been provided.

This reservoir draws upon a large drainage area. Alamosa Creek 

and its tributaries head°far back in the high mountain area to the 

west, and there is usually an abundance of water available. At points 

2, 3, and 4 on figure 13 large leakages, which appear as springs on the

FIGWRE 15. Cross section of the gorge in which the 

earth dam of the Terrace reservoir has been 

placed, based on data furnished by R. I. Meeker, 

engineer in charge, 1916.
36 RESERVOIR SITES IN SAN JUAN MOUNTAINS/ COLO.

loose glacial debris, but when seen in the summer of 1916 the water 

was issuing in a remarkably clear condition. Those in charge of this 

reservoir report that the seepages began soon after the water was 

first allowed to rise and have occurred during each succeeding season. The leaks suggest the danger of large subterranean routes

through the glacial material, perhaps at the base of the drift and 

on the rock surface, but as they have continued for several years 

without producing any serious damage there seems to be no immediate reason for fear.

LA JARA RESERVOIR.

The La Jara reservoir is on La Jar a Creek, in Conejos County, 

and may be reached most easily from Monte Vista. It was constructed in 1909. The geologic conditions of the area surrounding 

the lower end, where a dam has been placed, are shown in figure 16.

At the north there are 

dense lava flows forming the gentle slopes 

of this foothill region, 

but at the south there 

is a heavy frontal moraine of an ancient 

glacier. The ice appears to have advanced from the high 

mountain region a 

little to the south of 

west and, on reaching 

this lowland country, 

deployed northward

so that La Jara Creek 

was forced to flow between the margin of 

the ice and the lava 

hills. On the retreat

of the ice the stream cut a narrow gorge in the margin of the 

moraine, where the present outlet of the reservoir is placed.

Two dams were constructed of the surface debris of glacial origin 

and the weathered volcanic rocks. One is 759 feet long and 51-| 

feet high. The other and more northern one is 23^ feet high and 

.495 feet long. The slope of these earth dams is 3 to 1 on the upper 

side, facing the reservoir, and 2 to 1 on theu lower side. Their crests 

are about 15 feet above high water and 12 feet wide. A spillway has 

been provided through a low trough just north of the glacial moraine, by a route that was probably used by the stream when the 

ice was present. The estimated area tributary to this reservoir is

FIGURE 16. Sketch map showing geologic conditions of 

the area Immediately surrounding the lower end of 

the La Jara reservoir, based in part on a map' in the 

office of the State engineer of Colorado.
U. S. GEOLOGICAL SURVEY BULLETIN 685 PLATE VII

A. GENERAL VIEW OF TERRACE RESERVOIR DAM FROM THE SOUTH.

The rock walls of the canyon appear, in the midde ground is the spillway used during the construction, the house from which the gates are controlled appears on the dam, and the Une of hills in 

the distance next below the sky Une is a portion of the great frontal moraine.

S. OUTLET OF TERRACE RESERVOIR 

TUNNEL.

C. POSTGLACIAL GORGE BELOW 

TERRACE RESERVOIR.
V. 8. GEOLOGICAL SURVEY BULLETIN 685 PLATE VIII

A. LA JARA RESERVOIR DAM. 

Moraiiiic material appears in the foreground.

B. LA JARA RESERVOIR. 

The moraine shows in the foreground. The distant hills are composed of lava flows.

C. TUNNEL OPENING IN THE LA JARA DAM.
RESERVOIRS ON CLEAR CREEK. 37

50 square miles, and the claim is made for the full capacity of the 

reservoir, estimated at 20,000 acre-feet. The water is taken from La 

Jara Creek and all its tributaries, including Lost Creek, and may be 

used for domestic purposes, irrigation, power, or any other beneficial purposes.

This reservoir has become the property of the Terrace Keservoir 

Co. and is used as an " exchange reservoir." Water is taken from 

Alamosa Creek, and in return the La Jara water is carried into that 

creek a little farther downstream to satisfy old priorities. This exchange is permitted by a State statute.

There is some seepage through the dam, out it has never become 

dangerous. The reservoir as constructed will hold much more water 

than is available. To the south of the great frontal moraine, and 

from a quarter to half a mile south of the reservoir, a number of 

large springs have developed, and those who are familiar with the 

history of this project report that these springs have appeared since 

the damming of La Jara Creek, and that they are affected by the 

height of the water in the reservoir. This would indicate that the 

great morainic mass is not sufficiently compact to hold all the water 

collected in the reservoir to the north of it. The water presumably 

sinks through to the base of the glacial debris and follows the rock 

surface beneath.

Plate VIII gives good general views of the reservoir, of the larger 

dam where the outlet is located, and of the massive moraine south of 

the reservoir:

PBOPOSED RESERVOIR IN CLEAR CREEK.

The upper portion of Clear Creek is in the north-central part of 

the San Cristobal quadrangle. A short distance above the point 

where the stream turns toward the southeast to flow to the Rio 

Grande the valley is constricted, as shown in figure 17.

The constriction is due in part to rock, but the rock is overlain by 

glacial debris. Upstream the valley is broad, open meadowland. 

Before construction work is begun a detailed study of the geology 

at and near the dam site should be made. The clearing away of the 

glacial debris and all loose rock and the construction of the spillway in solid rock should make this project safe so far as leakage is 

concerned. The drainage area that furnishes. water to this creek, 

however, is relatively small.

PROPOSED RESERVOIR ON SOUTH FORK OF CLEAR CREEK.

The South Fork of Clear breek is nearly due west of Santa Maria 

Lake, in the central part of the San Cristobal quadrangle (fig. 17). 

The upper part of the valley is broad and open, and its floor is an 

extensive meadowland. Near the point where the stream turns southeastward the valley is rather narrow, and the narrow portion has
38 RESERVOIR SITES IN SAN JUAN MOUNTAINS, COLO.

been thought of as a possible site for a- dam, above which the waters 

would collect on the present meadows. This is a very unfortunate selection for a reservoir site, as the deposits at the proposed site are 

of glacial and landslide origin. It is extremely doubtful whether 

that material would hold in more than a low head of water. The 

fate of the Mosca reservoir, in the Creede quadrangle, should be a

EXPLANATION

Lava flows or other

volcanic formations near

or at the surface

FIGURE 17. Map of a part of the San Cristobal quadrangle, showing the location and 

topographic relations of the proposed reservoir sites in Clear Creek and South Fork of 

Clear Creek and the distribution of the loose or unconsolidated formations within the 

area shown.

sufficient warning against undertaking a project under geologic 

conditions such as exist near the mouth of the South Fork of Clear 

Creek. Furthermore, the surplus waters of that fork are exceedingly 

small, and the project would certainly not justify large investments. 

Before construction work is begun a detailed geologic study should 

be made of the land immediately adjoining the proposed site for the 

dam. \

\