GEOLOGICAL SURVEY CIRCULAR 256 

SEDIMENTATION IN SMALL RESERVOIRS 

ON THE SAN RAFAEL SWELL, UTAH 

I 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary 

GEOLOGIC4L SURVEY 

W. E. Wrather, Director 

GEOLOGICAL SURVEY CIRCULAR 256 

SEDIMENTATION IN SMALL RESERVOIRS 

ON THE SAN RAFAEL SWELL, UTAH 

By N. J. King and M. M. Mace 

Waahiugton, D. 0.,1958 

Free on application to the Geological Survey, Washington 25, D. C. 

CONTENTS 

Introduction ............................................ . 

Page 

1 

1 

2 

2 

2 

4 

4 

Precipitation .......... -.--............................ . 

Field work ......................................... . Sedimentation ......................................... .. 

Acknowledgments" .............................. .. Trap efficiency ............ - ........................ .. 

Physiography ......................................... . Compilations ... " ............................. _" ... .. 

Geology_,,, ............................................ . Rates of sediment production ...................... . 

Sedimentary rocks .••......•••...........•.......• Summary and conclusions ...•.••.....••.••..•.••.•.. 

Alluvium ............................................ . Literature cited .......... - .......................... .. 

Igneous rocks ............. - ...................... .. 

Structure~······-··· .. ············•·•······•···•····· 

Figure 

Table 1. 

2. 

3. 

4. 

5. 

6. 

ILLUSTRATIONS 

1. Physiographic map of San Rafael Swell. Utah ..................................................................... . 

2. Index map of Utah. showing San Rafael Swell and location of precipitation stations and study 

reservoirs .................................................................................................................. . 

3. Seasonal precipitation for stations on the San Rafael Swell ................................................... .. 

4. Seasonal precipitation and intensity relationships ............................................................... .. 

5. Occurrence-intensity relationship of summer storm precipitation .......................................... . 

6. Relation of.trap efficiency to capacity-area ratio ................................................................ .. 

TABLES 

Generalized section of rock formations in the San Rafael Ewell ................................................. .. 

Station index and summary data. for weather stations on San Rafael Swell ................................... . 

Size-frequency data for daily rainfall stations ........................................................................ . 

Data from reservoirs and drainage basin studies .................................................................... . 

Comparison of sediment production from various rock types .................................................... .. 

Comparison of sediment-production rates between New Mexico-Arizona and San Rafael Swell. 

Utah. for different rock types ............................................................................................. . 

Page 

6 

14 

14 

15 

19 

21 

21 

Page 

3 

7 

8 

10 

12 

15 

Page 

5 

6 

9 

16 

18 

19 

SEDIMENTATION IN SMALL RESERVOIRS 

ON THE SAN RAFAEL SWELL, UTAH 

By N. J. King and M. M. Mace 

INTRODUCTION 

Movement of seqiment from upland areas and eventually into main drainages and rivers is by no means 

through continuous transportation of material from the 

source to the delta. Instead it consists of a series of 

intermittent erosional and depositional phases that present a pulsating movement. Hence, sediment carried 

off upland areas may be deposited in lower reaches or 

along main drainages if an existing combination of factors tend to effect deposition. During this period actual sediment movement out of the basin may be relatively small. Following any change in existing conditions, however, these unconsolidated alluvial fills may 

be subjected to rapid removal; thus. for a limited 

time, abnormally high sediment production rates occur 

until the deposits are either removed or another cycle 

of deposition is started. 

To the extent that movement of sediment into the rivers is followed by eventual deposition in downstr.eam 

reservoirs, the logical way to arrest such accumulation of sediment is to effect deposition on upland areas. 

At present the only feasible method of reducing this 

sediment load is to delineate the areas of largest sediment contribution and to devise some treatment program aimed at reducing erosion in these areas. 

A program of erosion control has been proposed for 

the San Rafael Swell. which forms a part of the upper 

Colorado River drainage in southeastern Utah, through 

construction of erosion-coatrol structures. Studies on 

the rate of sedimentation in a number of small reservoirs have been undertaken as a means of determining 

the origin and movement of sediment in areas underlain by various types of sedimentary rocks. Studies of 

this nature have a distinct advantage in that the influence of precipitation, vegetation. soil types, slope, 

land use. and other less important factors are more 

easily related to erosion within a small area where individual features are not too complex. 

Measurement of sedimentation rates in small reservoirs offers the only feasible source for data of this 

type. As streams tributary to the reservoirs are 

strictly ephemeral. flow occurs at unpredictable periods and only in direct response to precipitation. Direct sampling of streamflow is thus impractical because of the great expense involved. 

Essentially, the method used in the study was to select a group of small reservoirs that were constructed 

by the Civilian Conservation Corps in the period 1936-

42. Individual study units were chosen with particular 

attention given to the availability of original surveys and 

records, condition of dam and spillway, effectiveness 

as a sediment trap. physiography, and geology of drainage basin. Resurveys were made to determine the amount of sediment trapped in each reservoir and this 

amount prorated over the period required for deposition, in each case the life of the reservoir. Hence, the 

annual average sedimentation rate could be determined 

for a given set of conditions. Further· analysis of these 

data permitted adjustment of the rates of reservoir sed'-

imentation for the loss of sediment during reservoir 

spill, to obtain an estimate of the rate of sediment production from the drainage area. The primary objection 

to this method is that without records of water-level 

changes no relationship c·an be determined between runoff and sediment movement. 

All reservoirs were surveyed by use of plane table 

and alidade, and as no permanent reference points had 

been established at the time of construction, elevations 

were adjusted to correspond as closely as possible to 

those used in the original surveys. Contours below 

spillway level were run at intervals of 2 feet or less 

on all reservoirs, not only to determine the present 

area-capacity relationship, but also to provide a check 

on the accuracy of original surveys. Reservoirs containing water too deep to wade were sounded from boats 

and spot elevations located on the plane table. Bureau 

of Land Management maps were used to determine the 

location, date of construction, and other pertinent data 

related to each reservoir. Monuments were left at each 

reservoir to facilitate resurvey. 

Local detailed geologic studies on coal measures in 

Upper Cretaceous rocks have been made by several 

writers (Andrews and Hunt, 1948; Baker, 1946; and 

Gilluly. 1928). A few local soil classifications are 

available, but no soil map of the entire area has been 

made. Because detailed information on soil was not 

available and as a close relationship apparently exists 

between soil and the rock from which it was derived, 

study units have been classified on the basis of the underlying geologic formations. 

Field Work 

Field work on which this report is based was started 

June 19, 1950, by a party consisting of the senior author and M. D. Whittier. engineer aid, under the supervision of H. V. Peterson, staff geologist, Salt Lake 

City, Utah, and R. W. Davenport, chief of the Technical Coordination Branch, Water Resources Division, 

Washington, D. C. The junior author joined the party 
2 SEDIMENTATION IN SMALL RESERVOIRS ON THE SAN RAFAEL SWELL 

July 5, 1950, and continued with the work until October 

5, 1950, when the study was completed. 

Acknowledgments 

Special appreciation is acknowledged to the Bureau 

of Land Management for its loan of the original reservoir surveys. Especially helpful was Conway Parry, 

conservationist, Bureau of Land Management, who 

materially aided the writers in the selection and initial 

location of the reservoirs to be studied. 

PHYSIOGRAPHY 

Physiographic features within any given area constitute an integral part of the study of the process of erosion and movement of sediment and cannot, therefore, 

be adequate-ly covered in a general description. Thus, 

individual features such as land forms, character of 

the drainage pattern, slope of the terrain, type of vegetative cover, and other less important factors may 

reflect separate or combined influence on runoff and 

transportation of sedimE:.nt. Although no attempt was 

made to evaluate the influence of individual features, 

a discussion of the general features of the San Rafael 

Swell is necessary to show its relationship to the overall area with which this report is concerned. 

The San Rafael Swell lies in southeastern Utah within 

the boundaries of the Colorado Plateaus province. As 

the name implies, the structure is a huge swell or 

dome whose greater axis trends northeast for almost 

70 miles and whose minor axis is about 30 miles long. 

It is bordered on the east by the Green River Desert, 

on the south by the Henry Mountains and the Water.-

pocket Fold, on the west by the Wasatch Plateau, and 

on the north by the Book Cliffs. A generalized physiographic map of the area (fig. 1) has been reproduced 

from a physiographic map of Utah prepared by the Department of Geology of Brigham Young University, 

Provo, Utah. 

Differential erosion of the sedimentary rocks has 

carved the surface of the San Rafael Swell into a topography that is both rugged and picturesque presenting 

such forms as steep-sided mesas and buttes, long 

sloping cuestas, and wide strike valleys that contrast 

with numerous narrow canyons as much as 1, 800 feet 

deep. The total relief within the area considered is 

about 8, 200 feet; the altitude ranges from about 12~ 300 

on the mountains west of the area to 4, 090 feet at 

Green River on the east margin. 

The most~prominent topographic feature of the San 

Rafael Swell is a series of odd-shaped sandstone forms 

called "the Reef", that encircles an area in the heart 

of the Swell about 40 miles long and 15 miles wide, 

locally known as "Sinbad. " These fantastically eroded 

forms are the remnants of an outcrop of massive 

cross-bedded sandstone that is much more resistant 

to erosion than the underlying shales. Cuestas and 

hogbacks, formed by resistant beds in the strata overlying this sandstone, encircle this belt of rugged to-:. 

pography exposing progressively younger formations 

toward the margins of the Swell. 

Intervening between the "Reef" and the Book Cliffs 

is the high conglomerate-capped face of Cedar Mountain, which has a relief of about 1, 500 feet. Between 

this mount~in and the high escarpment of the Book 

Cliffs to the north lies an elongate valley carved in 

shale that parallels the Book Cliffs for many miles and 

is known by many local names. In the area in which this 

report is chiefly concerned, however, it is known as 

Price River Valley and Clark Valley. A counterpart to 

this valley lies on the west side of the Swell in front of 

the eastward facing escarpment of the Wasatch Plateau 

and is called Castle Valley. 

Price River Valley and Castle Valley were formerly 

mantled by a gravel "apron" that sloped gently away 

from the cliffs. The present remnants of this apron or 

pediment appear as gravel-capped benches whose 

smooth surfaces present a distinctive contrast with the 

rugged shale surface in the intervening areas. 

According to Gilluly ( 1928) the runoff over the entire 

San Rafael Swell ultimately reaches one or another of 

four perennial rivers that either actually cross the 

Swell, as in the case of the San Rafael River and the 

Muddy River, or have tributaries that drain portions 

of it. The runoff from the east, south, and west portion 

of the Swell and from most of Sinbad eventually reaches 

either the San Rafael River or the Muddy River. These 

two rivers have, as tributaries, many smaller creeks 

and washes that afford only ephemeral flow, but form 

a network that drains the greater part of the Swell. The 

northern and northeastern parts are drained through 

Cottonwood Wash and Humbug and Summerville Draws 

to Price River, and through Saleratus Creek and Cottonwood Springs Wash to Green River. 

The vegetation of the region is typical of that found 

on the Colorado Plateaus. The higher parts of the region support pinyon, yellow pine, and juniper, whereas 

only the latter is found at intermediate altitpdes. Rabbitbrush, greasewood and sagebrush are common in the 

region, particularly on bottom lands. These lowlands, 

however, are devoid of trees except for groves of cottonwoods along streams, occasional cottonwoods at 

springs, and orchard and shade trees in the various 

towns. Prickly-pear cactus is found in scattered localities, and several varieties of grass are present al- · 

though not in sufficiently large amounts to permit heary 

and frequent pasturage. Residual soils are very thin 

or not present at all, and the only productive soil materials are found on alluvial fills in the valleys. 

GEOLOGY 

In any area topographic and surficial features such 

as slope of terrain, drainage pattern, alluvial fills, and 

soils are mainly determined by the balance between 

climate and the character of the underlying rock. Logically then, in one small locality over which the climate 

is essentially uniform, any difference in erosional 

features may be ascribed to original differences in 

geologic features. Hence, as soils and alluvial fills 

within any drainage basin are generally derived from 

rock formations that underlie that drainage basin, their 

characteristics are limited by the chemical and physical properties of the source materials. Residual· soils 

and weathered outcrops bear an even closer relationship to the local geology than their transported counterpart, because fine particles have not been carried away 

in suspension during the stage of transportation. As 

no soil classification has been completed in this area, 
5 0 

I I I I I I 

GEOLOGY 

mo 

20 Miles 

I 

Figure 1. -Physiographic map of San Rafael Swell, Utah. 

3 

uo• 
4 SEDIMENTATION IN SMALL RESERVOIRS ON THE SAN RAFAEL SWELL 

the writers believe that owing to this close relationship 

between surface materials and the underlying rocks, 

formations of different lithology will be covered by 

surface mantles that should exhibit different erosional 

characteristics, This method of classification may not 

be limited to the San Rafael Swell, but may be expanded 

to inc.lude the surrounding region, which is underlain 

by younger Tertiary formations that are not represented in the area studied. The following description of 

rocks that underlie the area emphasizes this difference 

in lithology as related to the respective surface 

mantles. 

Geologic information was obtained from Gilluly's 

(1928) report on field examinations of individual drainage basins. Gilluly's geologic map covers the west and 

central part of the San Rafael Swell; the east and southeast parts are covered by Baker (1946) and Hunt and 

others (1952). A brief description of the rocks present in the area, together with the soil types that result from the weathering of these rocks, follows. 

Essentially the area is underlain by four main types 

of rocks which, listed in order of decreasing areal extent, are shale, sandstone, limestone, and conglomerate. In the process of weathering, these rocks break 

down to form soils or surface mantles of varying characteristics depending on the parent rock. Shales disintegrate into clays and fine silts; sandstones, into individual sand grains; conglomerates, into pebbles with 

minor amounts of sand and silt; limestones, into limestone fragments and limy clays. The last two rock 

types are, however, of minor importance to this study 

because of their limited area of outcrop and relative 

resistance to erosion. 

Although the distance and method of transportation 

of the constituent materials of soils generally alter 

their resemblance to the parent rock, erosion throughout the Swell has kept pace with or exceeded the rate 

of soil development such that existing thin soils or 

weathered mantles are very closely related to the parent rock. The exception exists in the alluvial fills 

along the stream channels that drain areas underlain 

by several formations of different lithology. 

Rocks underlying the area are grouped for this report as follows: 

Sedimentary Rocks 

Sedimentary rocks in the San Rafael Swell range in 

age from Permian to Recent. Permian and Triassic 

rocks c;:rop out within the Sinbad area. The Wingate 

and Navajo formations of Jurassic ( ?) age and the 

Todilto limestone member of the Kayenta formation of 

Jurassic (?)age form the reef that surrounds Sinbad. 

Because no reservoirs were constructed in drainage 

pasins underlain by these formations, there are no 

erosion data available for this area. 

Later Mesozoic strata comprise nearly all the remainder of the area, and extensive outcrops of Cretaceous shales and sandstones occur along the northern 

and western margins of the Swell. In Sinbad, the 

Shinarump conglomerate crops out in a narrow band 

that encircles the limestones and sandstones of the 

Moenkopi formation, Kaibab limestone, and Coconino 

sandstone, that together underlie most of the area. 

Outside the Sinbad area conglomerate beds several feet 

thick are found only in the Morrison formation, and 

limestone occurs as thin lenses in the Morrison formation and as one bed about 20 feet' thick near the base of 

the Carmel formation. 

A generalized description of the formations is given 

in the geologic column, and erosion characteristics 

peculiar to each formation will be discussed under 

" Rates of sediment production. " 

Alluvium 

As erosion within any area is confined mainly to the 

surface mantle, the character and extent of all surface 

materials is of primary importance to any erosion 

study. The balance between surficial features and erosion, therefore, depends not only on the character but 

also on the amount of surface material that can be 

readily transported. These surface materials may be 

classified as residual, weathered in place; or transported, weathered elsewhere and carried to the present 

location. 

Although residual soils or mantles make up the surface cover over a large part of the San Rafael Swell, 

erosion has removed most of the readily erosible material leaving only thin residual mantles or bare bedrock outcrops. Locally these soils may contribute 

large amounts of sediment to streams that drain the 

area, but in general the contribution from this source 

is not great. 

Deposits of transported materials are the large contributors of sediment to the streams draining the area. 

They may be classified as colluvium or alluvium, dependent upon the method of transportation. Colluvium 

is sediment that has moved downslope by gravity and 

has been deposited near the toe of the slope. Alluvium 

is sediment that has been transported by streams. For 

the purpose of this study, however, no distinction will 

be made as most so-called colluvial slopes have been 

at least partially modified by the action of wind and 

water. 

Although residual soils and colluvium provide a 

seemingly inexhaustible source of sediment, an even 

greater source exists in the form of alluvial fills along 

the innumerable tributaries and main drainage channels. 

The unconsolidated sediment in these fills has little inherent resistance to erosion, and any channel flow 

readily transports the maximum amount of sediment 

possible under the given hydraulic condition. This lack 

of cohesion, especially in fills derfved from sandstones, may be attributed to the absence of suitable 

binding materials such as clays. Such reasoning, however, cannot be applied to fills derived from shales as 

these deposits consist primarily of particles in the clay 

and silt size. As may be expected, fills of this nature 

prove to be fairly resistant to erosion as long as they 

remain completely dry. Once the particles are wetted, 

however, they readily become dispersed and lQse almost all inherent resistance to erosion. These deposits are most susceptible to erosion during periods of 

high precipitation and runoff when erosional forces are 

most active. For the present, however, greatest emphasis is placed on the fact that nearly all alluvial fills 

are easily eroded and thus may be subjected to rapid 

removal. 
'GEOLOGY 5 

The actual extent of alluvial fills within the study 

areas could not be determined with any degree of accuracy as most areas are e~ther not gullied or existing 

gullies have not been cut to bedrock. Generally,; fills 

ranged from less than 1 foot in width and 1 foot in depth 

near the heads of small rills to a maximum width of 

several hundred feet and a maximum depth of 10 to 15 

feet along the larger drainage channels. Owing to their 

erratic nature it is hardly possible to describe them in 

any general summary. Table 4 includes specific data 

relative to their character and extent within any study 

area as well as the amount of gullying. 

completely dissected extensive fills bordering the 

drainage for a distance of almost 20 miles upstream 

from Green River, Utah. This is but one example of 

main channel erosion; many other examples of similar 

gullying can be observed along Cottonwood Wash, and 

Humbug and Summerville Draws or along almost any of 

the larger tributaries of the San Rafael, Price, or Muddy Rivers. Hence, extensive alluvial fills not only border the main drainage channels but they are being cur-· 

rently removed through active channel erosion in the 

form of gullying. 

Alluvial fills along the main drainage channels Igneous Rocks 

throughout the San Rafael Swell are often extensive, 

ranging from several hundred feet to more than a mile 

in width and averaging at least 200 feet in depth. Observations along Saleratus Wash revealed a central gullied channel at least 100 feet wide having vertical walls 

as much as 30 feet high, which are constantly being 

undercut as channel meandering diverts flow from one 

bank to the other. Smaller tributary channels, often as 

deep but not as wide as the main channel, have almost 

A number of sills and dikes occur in the southwestern 

part of the Swell but have no appreciable effect on the 

erosional features of the formations within the area. 

One reservoir (No. 1) located within this ~rea of past 

igneous activity has a small outcrop' of diabase within 

its drainage basin. The diabase sill in this drainage 

basin probably exerts little influence other than restricting the surface area of erodible materials. 

Table 1 -Generalized section of rock formations in the San Rafael Swelll 

System Series Group and formation Character Thicknc&S (feet) 

Quaternary Alluvium and terrace gravel Sandy clay, sand, and gravel in alluvial fans; terrace gravel on benches 

along streams. 

Unconformity 

Mancos shale Gray marine shale; sandy beds in lower part, rather persistent sandstone 

members about 200 feet and 600 feet above the base. 

4,000t 

Cretaceous Upper 

Cretaceous Dakota (?) sandstone Conglomerate; coarse and fine sandstone, in places quartzitic; gray and 0-55 

greenish clay • .. nconformity 

Upper Morrison formation 

Clay and shale, variegated, dominantly green-gray, maroon, and 

mauve; gray sandstone and conglomerate, very lenticular, massive 

and cross-bedd~d. especially abundant toward the base, where they 415~847 

Jurassic form the Salt Wash sandstone mefnber; subordinate thin lenticular 

limestones; gypsum locally at the base; in the northern part of the 

area a conglomerate 250 to 350 feet below the top. 

nconformity 

Summel'Ville Thin-bedded, chocolate-colored sandstone, earthy red-brown sandstone 125-331 

formation and shale, some gypsum, and a little limestone in some sections. 

ll. 

6 Curtis formation Greenish-gray conglomerate, shale, and gray thick-bedded sandstone. 76-252 

Jurrasic Upper and middle ~ ~nconformi.7 Thin-bedded red shale and sandstone at the base; thick, massive red- Ill 

Jurassic .::! Entrada sandstone brown earthy sandstone above, poorly bedded, weathering in rounded 265-844 .. forms. ex: 

s:l 

., .. Dense limestone and buff and red sandstone at the base; dominantly 

Carmel formation red and green shale, thin sandstones, and thick beds of gypsum toward 170-650 

the top. 

(?) §' -Unconformity (?) 

), Navajo sandstone Tan to light-gray massive cross-bedded limy sandstone, with a few thin 440-540 

g limestone ~enses. 

Jurrasic (?) Lower ~ Kayenta formation Red-brown sandstone, green and red shales, shale conglomerate, irreg- 44-240 

Jurassic u ularly interfingering and channeled, 

s:l Wingate sandstone Buff to tan and dark-gray massive cross-bedded limy sandstone, with a 360-400 i3 few thin lenses of limestone. Usually stained red by wash. 

Unconformi Green and red micaceous sandstones and thin red-brown shale; limestone 

Upper Chinle formation conglomerate variegated marl; all lenticular, channeled and 141-225 

Triassic interfingering Triassic 

Ctoss-bedded lenticular conglomerate, sandstone, clay, and shale, 

Upper Shinarump conglomerate interfingering. Much silicified wood. Quartz and chert pebbles in the 7C-178 

Triassic conglomeratic portions 

Unconformi Greett-gray pyritic shale; gypsiferous green and red shale; red micaceous 

ripple-marked sandstone; gray to buff sandstone; and red sandstone, 

Lower Moenkopi formation Very limy throughout, with a. thick, persistent gray marine limestone 735-850 

Triassic and sandstone member, the Sinbad limestone member 140 to 200 feet 

above the base. 

~"conformity 

Kaibab Light-gray to cream-colored cherty limestone; some oolite, somewhat 0·85 limestone sandy in places; equivalent to only uppermost part of typical Kaibab. 

Permian White to buff, sugary, friable to hard, massive cross-bedded quartz 

Coconino sandstone sandstone, on uneven grain. Some grit toward the base; the lowest 715 

40 feet largely limestone. Base not exposed, May include much 

more than typiCal Coconino sandstone, 

1 Gilluly, James, 1929, Geology and oil and gas prospects of part of the San Rafael Swell Utah, U.S. Geol, Survey Bull.806-C. 
6 SEDIMENTATION IN SMALL RESERVOIRS ON THE SAN RAFAEL SWELL 

Structure 

The structure of the area is relatively simple, The 

Swell is an elongated, asymmetric domal fold whose 

axis trends north by east. The central Sinbad area is 

encircled by a high ridge or "hogback" of Jurassic sandstone. From this ridge outward to the Book Cliffs and 

Wasatch Plateau escarpments, the Swell has a steplike 

appearance. Clark and Castle Valleys are cut in monoclinal structures and parallel these escarpments. A 

number of small folds and faults are present in the 

area but have no relevance to this study. 

PRECIPITATION 

Thus far this discussion has been limited to erosion 

as indirectly influenced by the slope of terrain and lithology of formations that underlie the area. Active 

erosion is apparently set off by precipitation that is abnormal in relation to conditions existing in the area. 

Actually the ideal precipitation with respect to the erosion problem would be the maximum amount that could 

fall without exceeding the rate of infiltration. Precipitation exceeding this amount causes runoff and incites 

erosion, whereas precipitation less than this amount 

deprives the soil of sufficient moisture to support a 

protective vegetative cover. Hence, dry years tend to 

denude the soil mantle of its vegetative cover and thereby lower its resistance to erosion, whereas wet years 

with high-intensity storms actually cause runoff and associated erosion. Precipitation data for stations on the 

San Rafael Swell have been analyzed in an attempt to 

compare precipitation records during the Ufe of the 

reservoirs with the long-term average for the area and 

then adjust known rates of sedimentation to long-term 

rates. 

Although no precipitation data are available near the 

center of the Swell, excellent long-period records have 

been kept at six stations along the outer margin. These 

records, as reported by the U. S. Weather Bureau in 

the "Monthly Climatological Data" for Utah, cover an 

average period of 43 years and show the average annual 

precipitation range from a ma~imum of 12. 91 inches at 

Hiawatha to a minimum of 5. 23 inches at Hanksville. 

Available data would indicate that the average annual 

precipitation near the center of the area is about 6 to 7 

inches. An index map showing the location of the stations is given as figure 2, and a summary of climatological data is given in table 2. 

Analysis of temperature records available for the 

same stations shows that above-freezing temperatures 

may be expected froi:n April to October inclusive. The 

year was divided into a 7-months summer season (AprilOctober) and a 5-months winter season (NovemberMarch). Although occasional winter storms cause offseason runoff, because of the high altitude, the bulk of 

winter precipitation falls as snow. Winter runoff, 

therefore, is so infrequent that for the purpose of this 

report only summer precipitation will be considered. 

Runoff from snowmelt is recognized as a possible contributor especially to channel erosion, but unfortunately 

data are not available to determine its actual effect. 

Although records covering 32 to 54 years are available for individual stations those records before 1930 

are incomplete for several stations and do not supply 

the necessary data for detailed storm analysis. Nevertheless, the seasonal precipitation data were plotted 

for the entire record period {fig. 3) and the long-term 

seasonal averages compared to the averages for the 

period 1930-50 {see extreme right-hand bars on fig. 4). 

The close comparison between these averages for individual stations would indicate that the period 1930-50 

may be considered representative of the long-term period. As the maximum age of the study reservoirs is 

14 years, the period 1930-50 will determine the precipitation pattern not only over the life of the reservoirs, but also for the period immediately prior to their 

construction. Many methods may be used to assemble 

available precipitation data for the area, but for the 

purpose of this report, methods were limited to {1) a 

determination of average daily intensities; (2) total 

summer precipitation in excess of the estimated rate 

of daily infiltration; (3) frequency of occurrence of 

storms of a given magnitude. 

Average storm intensity and seasonal distribution 

of precipitation over the San Rafael Swell are remarkably consistent. About two-thirds the annual precipitation occurs from April to October inclusive. Typical 

convection-type storms provide most of this, with daily 

totals ranging from a trace to a maximum of 2. 16 inches. Averages of daily precipitation for the six stations show that 80 percent of the days with precipitation had amounts less than 0. 25 inch, whereas 7 percent 

had over 0, 50 inch per day and only 1 percent experienced over 1. 00 inch per day. The number of days with 

precipitation greater than specified amounts is listed 

for individual stati011s in table 3, 

Summer storms are generally of the convectional 

type; they cause precipitation of small magnitude but 

of sufficiently high intensity to exceed the amount 

readily absorbed by the soil. Runoff, therefore, is a 

function of a number of variables such as soil development, vegetative cover, and land slope and generally 

represents only a small part of the total precipitation. 

An evaluation of rainfall-runoff relationship from precipitation data is further complicated because amounts 

recorded as daily totals usually fall within a few hours, 

The question arisestas to the minimum amount of daily 

rainfall that will produce some surface runoff. Comparisons· between precipitation and runoff 

Table 2. -Station index and summary data for U. S. Weather Bureau stations on San Rafael Swell, Utah 

Summer precipitation w~n:;r f-~it3~on Location An mlll tE moera ture Annual orecioitation A • 1-0ct 31 

Station County Latitude Longitude Elevation Yrs. I.Ma;. Min, Average Yrs. Max, Min. I Average Max, Min.· Average Max, Min. Average 

(feet)· Record ("F) ("F) Record (inches) (inches) (inches) (inches) (inches) (inches) 

Emery Emery 38"56 111 "14 6,260' 48 99 .. a 45.8 49 16.84 O.ii4 '7.55 13.42 0.45 5.12 8.59 Trace 2.36. 

Castle Dale Emery 39"13 111 '01 5,600 43 104 -35 45.6 4EI 17.05 3.29 8.69 12.4Ei 1,42 5,50 '7. 67 o. 72 8.03 

Hiawatha Carbon 39'29 111 '01 7,230 28 95 -18 45.0 32 22.09 6.66 12.91 17. 5~ 3. 80 8.81 7.29 1,48 4.30 

Price Carbon 39"36 110"49 5,500 34 108 -31 48.6 I~ 19.55 4.47 10.24 15,33 1. 75 £.36 5,98 1. 74 3,48 

Green River Emery 39"00 110'09 4,087 48 112 -42 52,5 12.12 3,11 6.10 9.4C 0.37 4,03 4, 79 0,35 1,96 

Hanksville Wayne 38'25 110°41 4,456 36 112 -35 52.1 3S 9.14 2,24 5,23 8.50 0,82 3. 76 4.57 Trace 1. 58 
PRECIPITATION 

fro-···- r-···' '--;1-~:,;-~.: : t i . iz\ t \ l....... r"i ( ~ . ... l· tl ... 

"r.-.1 

---- ) r· F.--

•

. } . ) L I ~- ~ ··:--.. ( l I IJ ·. -./··"' / .\ 

"· ... J / .· :', c 13 :Al . .J ·.. R ,;:- -a- c .......... ; ........... w·~-. ' 

.. ../ ,~· ~ I I ;.. -·· ·-.··-· ~.: j'.: \ .,.4 ·· - ·· .. ..r-- ... ~RI•CI ; :· ·"'i , '. \ - - ... - ,! i\ ./ ~ •• .r-· f{'~: .J. '\ '- --· ·.......-···......._. I_ f.: \ '..- '1\ • . : t .f.4 1--f---: 

---

...... v I t 

\ : / ) s I } 

7 

o PRECIPITATION STATION 

EWIERY 

CASTLE DALE 

HIAWATHA 

PRICE 

lltiEft RIVER 

MANICIVILLE 

Y""5 STUDY RESERVOIR 

L IVY CREEK IEftCH 

2. SAND liNCH 

I. RED TANK 

4. CINDERELLA 

5. WIDIE 

1. SUMMIT 

7. SALERATUS 

a. MILLER'S 

I. WINTER'S 

10. SCHOOL SECTIOft 

II. SPIDER 

12. DUIOUT 

Ul. CLARK VALLEY NO. I 

14.. CLARK VALLEY NO.2 

11. SOUTH IRIEN RIVER 

I 

UTAH 

Figure 2. -Index map of Utah. showing San Rafael Swell and location of precipitation stations and study reservoirs. 
8 SEDIMENTATION IN SMALL RESERVOIRS ON THE SAN RAFAEL SWELL,' UTAH 

20 EMERY EMERY 

0 ~~~~--~----~----~~--~----~-----L----~----~--~ 

1900 1910 1920 1930 1940 

CASTLEDALE, EMERY COUNTY.- Elevation, 5,500 feet 

20 

o ~- ··· .... ·· ·· ...... ...- \ ....... ~ ...... ·····-... ~ ....... ···· .......... ~ ...... y. .... ·····......... ~ 

1900 1910 - 1920 1930 1940 

20 

HIAWATHA, CARBON COUNTY.- Elevation, 7,300 feet 

A 

1950 

~ 

1950 

00 ~---+----~----+--A-A~~'~4~~~~{\~~----~-=~~"~\~~~~£H ~ 1o IV .. \/IY .. y 1\~J~····~ \!vv .. ~.~\ Z ·. ~~ .... · · ......... · .. v_ ...... •••• ....... ••• •••••• ••••• ••• ......... •• ·-..·· • 

·· .... ·· -. 

z OL---~~--~----~-----L----~----~----L---~----_.----~ - 1900 i910 1920 1930 1940 1950 

z 

Qp ~R~IC~E~,~C~A~R~B~O~N~C~O~U~N~T~Y~._-_E~I~e~va~t~io~n~,~5~,5~0~7~fe~e~t--~-----r----~----~ ~ 20 ... 1-

0:: 

(3 

LIJ 

g: 10 

..... · ·· ................ -... ... ······· .... : .... ··· ... v ·· ... ·· · .............. ~··':':':': .......... ··· 

0~----~--~~--~----~----~----~-----L----~--~~----~ 1900 1910 1920 1930 1940 1950 

~G_R~E~E_N~R_I_V~E_R~,~E~M~E~R~Y_C~O~U~N~T~Y~.~-~E~Ie~v~a~t~io_n~,~4~.,~0~8~7~f-ee_t __ -r----~----~ 20 ... 

10~-4---~---4----+IA---~~~+----~~~~-+---~ 

o~· 0~ .. ~ •"··· ... ~ ·-... Y: ... -~······ ..... ·····1.~~-./'·~-.... ~-..... """-. ~-.:.-.. --' ... :.:.-:-s 

1900 1910 1920 1930 .1940 1950 

HANKSVILLE, WAYNE COUNTY.- Elevation, 4, 200 feet 

20 

I 

NO RECORD, --J...J--------.1 

\ 1\, _ . r "~ 10 

o ~--~..-~v_ .. ·· ____.\v_· ... _· .....>..:_ ... · _._·-~_ .. ··..:.:.::.······......:· .t.:.:a.:8_........ _ .............1.v_..... _ .... ~_-. .L...~_ ...... _· •• ........... ~--···~~···_.;. 1·;...~.;~~..:.:;,;, .... ·_······ ...... ·-.. 

1900 1910 1920 1930 1940 1950 

Figure 3. --Seasonal precipitation for stations on the San Rafael Swell. 
PRECIPITATION 9 

Table 3. --Size-frequency data for daily rainfall stations on the San Rafael Swell, Utahl 

30 14 25 27 13 i9 26 19 28 21 24 34 12 17 15 25 20 26 ~2 30 22 0.25 14 2 9 8 4 5 16 10 10 7 7 23 5 7 5 6 3 13 3 7 5 .so 5 - 4 2 1 3 4 2 3 4 12 2 2 1 2 2 6 1 1 1 • 75 2 - 1 1 1 - 1 - 2 2 4 1 - - 1 - 3 1 - - 1.00 1 - 1 - 1 - - - - - 1 1 - - - - - 3 - - - Castle Dale (altitude, 5,500), Emery CountyZ 

43 27 31 29 21 32 34 35 44 44 66 57 14 13 11 0.25 11 4 8 6 2 4 12 7 7 8 8 19 1 5 4 .50 8 1 2 3 1 2 4 3 1 4 9 1 3 .75 3 2 1 2 3 1 1 1.00 2 

Hiawatha (altitude, 7, 300 feet), Carbon County 

76 54 66 69 52 82 79 77 92 61 52 62 48 71 56 74 60 56 44 53 37 0.25 15 6 7 9 5 9 17 13 12 16 14 21 6 11 8 14 9 12 11 16 7 .50 5 - 2 3 2 3 6 s 5 4 5 11 2 8 3 2 4 4 3 8 3 • 75 3' - 1 1 1 2 2 1 3 1 2 4 1 2 1 - 2 2 3 3 1 1.00 1 - - - - 1 1 1 - - 1 2 1 2 1 - 2 1 1 1 - Price (altitude, 5, 507 feet), Carbon County 

39 23 25 22 16 24 21 21 57 34 44 63 26 32 39 52 44 47 33 50 31 0.25 13 5 12 6 3 8 6 6 10 11 10 14 1 2 6 4 5 6 6 13 3 .50 3 1 5 3 - 3 3 2 4 6 2 5 - 1 2 2 - 3 2 6 - • 75 1 - 2 1 - 3 2 1 1 2 2 1 - 1 - 1 - 1 2 4 - 1.00 - - 1 1 - - - 1 - - 2 1 - - - - - - 1 1 - Green River (altitude, 4, 087 feet), Emery County 

37 22 25 28 17 27 36 35 53 41 48 77 40 57 50 61 44 43 34 55 5 0.25 6 1 5 8 1 4 4 2 4 5 5 12 5' 6 5 5 4 4 6 7 - .so 4 - 2 - - 1 1 - 1 1 - 5 2 1 2 1 - 2 1 - - .75 1 - 1 - - - 1 - 1 1 - 2 1 - - - - - - - - 1.00 1 - - - - - 1 - 1 1 - 1 - - - - - - - - - Hanksville (altitude, 4,200 feet), Wayne c;ounty 

41 28 27 14 9 12 18 14 0.25 7 3 6 9 2 6 6 6 .50 4 - 4 2 1 - 2 4 • 75 - - 3 1 - - 1 2 1.00 - - 2 - - - 1 1 

• No record. t For period April to October inclusive. 

at a number of locations in the southwest suggest that 

precipitation recorded as 0. 25 inch or more per day 

will produce flow on soils derived from shales, whereas 0. 50 inch per day or more may be required to produce appreciable flow on sandy soils. 

Once the amount of precipitation per day necessary 

to cause runoff has been approximated for any area, it 

follows that a quantitative measurement of rainfall in 

excess of this amount should segregate years in which 

erosion has been most active. Figure 4 shows the total 

seasonal precipitation for each station compared with 

the proportion that falls at rates exceeding various 

amounts per day. Following this reasoning, the greatest average runoff would have occurred in 1941, 

whereas almost no runoff would have occurred during 

1931 and 1934. 

A determination of average storm intensity together 

with the amount of precipitation available for runoff 

may provide some relationship between the number-of 

days with precipitation and the intensity of that pr.ecipitation, but it gives no indication of the seasonal recurrence of storms of a given magnitude. Hence, in order 

to show the proportion of precipitation available for 

runoff, occurrence-intensity curves have been developed for individual stations; these express the percentage of summers that have total precipitation in excess 

of various amounts per day (fig. 5). Interpolation from 

these curves permits rapid determination of the 

expected recurrence of runoff once the rate of infiltration is known for an area. For example: 

18 

5 

1 

-

--

20 

4 

1 

-

-

38 42 18 * 18 26 46 55 45 62 35 4 8 5 - - 7 4 7 6 8 1 4 1 - - 2 2 4 3 3 

- 2 1 - - - 1 2 2 - - - - - - - 1 - - z No record, November 1942-C>ctober 1948. 

For a drainage basin near Price where the rate 

of infiltration is about 0. 25 irich per day, the 

(P-0. 25 inch) curve taken from the Price station 

shows that about 100 percent of the summers will 

have runoff; 75 percent will have runoff in excess 

of 1. 00 inch; 43 percent will have runoff in excess of 2. 00 inches; and 7 percent of the summers 

will have runoff in excess of 4. 00 inches. 

2 

2 

1 

1 

Similar results may be obtained for any drainage unit 

once the average rate of infiltration is determined. 

A general summary of the 6 stations shows that, although only two storms exceeding 2. 00 inches per day 

occurred in the 21 years of record, storms of 0. 25 inch 

per day or greater occurred during 100 percent of the 

summers, storms of 0. 50 inch per day or greater occurred during 83 percent of the summers, and storms 

of 1. 00 inch or greater occurred during 29 percent of 

the summers. 

As may be expected, precipitation of the magnitude 

of 0. 50 inch or more per day results from convectiontype storms that vary greatly in intensity over short 

distances. This fact, coupled with the different infiltration rates for various types of surface mantles, accounts for the apparent difference in the amount of runoff between even small adjacent drainages. Minor differences in runoff may, however, be accompanied by 

a much greater variation in sediment load, because the 

carrying capacity of any stream increases exponentially 

with an increase in the volume and velocity of runoff. 
EMERY. EMERY COUNTY- Elevation. 6.260 feet 

15 .. 

10 

r- r- rn n n , n n n n ,...., nn n 

5 

0 fl" no n ~ II~ nbJ nU n & II~ II~ rl ~ n • II n~ n~ llbJ nbl nW n llbJ IIU nW n fl 

1930 1931 193·2 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 194 7 1948 1949 1950 AVERAGE 

1930- so J90L~so . 

~ CASTLE DALE, EMERY COUNTY- Elevation, 5,500 feet 

:&:; 15 

0 

z 

~ 10 

z 

Q 

1-

~ 5 

a: 

~ 0 

ff 

20 

15 

10 

5 

·a 

r'" 

,..... 

7: n ...... , ...... n n ...... -fi ,-, 

n ~ nU II 77, II ~ no llbJ 11 ~ II ~ II 7, II 7: n ~ nbJ I• NO RECORD ~1 II~ nU II II 

1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 AVERAGE 

1930-50 1900-.50 

HIAWATHA. CARBON COUNTY- Elevation. 7.300 feet 

0 TOTAL SEASONAL PRECIPITATION 

1WINTER- NOVEMBER I TO MARCH 31 IZl 'TOTAL PRECIPITATION OVER 0.25 INCH PER DAY. 

~TOTAL PRECIPITATION OVER 0.50 INCH PER DAY J 1 SUMMER- APRIL I TO OCTOBER 31 .TOTAL PRECIPITATION OVER 0.75 INCH PER DAY 

.. 

I r'" 

- i r-

.· ~ ri ~- n n ..... n ~ n n ,., nf?, ·n 7. nn ·,., n" ~- -n -n I 

~i ~ i nU II ~ I ~ II ~ II~ ~ ~ II ~ II ~ II~ II ~ II ~- i -~ II~ -~ -~--t~ i ~ J I II I r;. ·j 

1930 1931.19321933 1934 1935 1936 1937 1938 1939 1940 1941. 1942 194319.44-19451946 T947:-·t948.1949· 1950 AVERAGE .. - .. 

. 1930-50 1917 -5_0 . 

..... 

0 

Ul 

tr:l 

~ ..... 

~ 

tr:l 

2: 

o-3 

> 

o-3 

0 

2: 

z 

Ul 

~. 

> 

t"' 

t"' 

!::tl 

tr:l 

Ul 

tr:l 

!::tl 

<: 

0 

S3 

Ul 

0 

z 

o-3 

ttl 

tr:l 

Ul 

> z 

!::tl 

> 

txj 

> tr:l 

t"' 

Ul 

~-

tr:l 

t"' 

t"' 
PRICE. CARBON COUNTY - Elevation. !5507 feet 

15 

10 r- rr- r- r-r-

...., n n~ n ,...., n n n n~ n ,n n ..... n~ n n 

5 

II ~ nU II ~ II ~ lllJ ~ II~ i II ~ II ~ II i II ~ lin ~W ~II~ flU II ~ n i II I LJ II II 0 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 AVERAGE 

~ GREEN RIVER, EMERY COUNTY- Elevation, 4087 feet 

1930-50 1911-50 

~15 

z ~------------------------------------------------------------------------------------------------------------------, 

~I0~-----------------------------------------------------------------------------------------------------------------------------------------------4 

z 

Q 

~ 5 ~--44--------------------------------------------~~-++-----------------------------------++----------------~ ta.. 

0 w 0 I I I W I I I I I I 'JP I I yn I I I I I I Kff I I- I I ...... I I 1M! I I fiR I I VZJ I I W I I - I I ((ft I I .. I I W I I 'CI! I I f11 I I lUI I I [(I! Q ,.., I I I I I I I I I 

-~ 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 AVERAGE 

. 1930-50 1900-50 

HANKSVILLE. WAYNE COUNTY - Elevation, 4200 feet 

15 0 TOTAL SEASONAL PRECIPITATION ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0TO~L PRECIPIT~IDN OVER Q25 INCH PER D~ 

'

WINTER- NOVEMBER I TO MARCH 31 ~TOTAL PRECIPITATION OVER 0.50 INCH PER DAY 

10 SUMMER-APRIL I TO OCTOBER 31 • TOTAL PRECIPITATION OVER 0.75 INCH PER DAY 

5~-++-------++-HH----------------------~---------------~------i 

0 1 ;930 ;9

1

3i 1932 1933 19

1

M 1935 1936 1937 1938 1939 194'0 i94i ;9

1

42 1943 1944 1945 ls46 1

1

9

1

47 1

1

9

1

48 1949 i95o 1 ~~~R~~~ I 

1 

1930-50 1910-50 

Figure 4. --Seasonal precipitation and intensity relationship. 

'"0 

~ (') 

::a 

...... 

1-j 

> 

1-j 

0 

z 

..... 

..... 
*-(P-X):: TOTAL PRECIPITATION, APRIL I TO OCTOBER 31, IN EXCESS OF X INCHES PER DAY 

18 fMERY, 1

EMERY COUNT~.-Eiev. ~260 fti 8 CASTLE DALE, EMERY COUNTY.-Eiev. 5500 ft. 8 HIAWATHA, CARBON COUNTY. Elev. 7300 ft. I I I I I I I I 

16 r---,_--~----+---~--~ 16 16 

14 14 14 

(/) 

~ 12 12 12 

(.) 

z 

~ 10 10 10 . 

z 

0 

i= 8 

i:! 8 8 

a: 

~ 6 6 6 

0:: 

il.. 

4 4 4 

2 2 2 

0 0 0 

0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 

PERCENT OF SUMMERS 

~ 

N 

UJ 

trl 

t:l 

~ trl 

z 

1-3 

> 

1-3 

0 

z 

z 

~-

> 

t"' 

t"' 

::tl 

trl 

UJ 

trl 

::tl 

< 0 

~ 0 

z 

1-3 

tt: 

trl 

UJ 

> z 

~ ~ 

> trl 

t"' 

UJ 

:s 

trl 

t"' 

t"' 
~ 

* (P-X)= TOTAL PRECIPITATION, APRIL I TO OCTOBER 31, IN EXCESS OF X INCHS PER DAY 

PRICE , CARBON COUNTY.- Elev. 5507 feet 18 I I I I I I 

16~--;----+----+.---~--~ 

14~--4----+----~--4---~ 

en 

~12 

0 

~ 

~10~~+---~---t--~--~ z 

0 

~ - I I 8 I al~ I I !:: 

a. 

0 

~61 I f'a< 

a. 

4 I '< I I I ..,lo I 

2 I \'t" ...,i'"' -,.,.. w:O'"'!: I I 'd 

GREEN RIVER, EMERY COUNTY.- Elev. 4087 ft. HANKSVILLE, WAYNE COUNTY, Elev. 4200 ft. 

181 I I I I I 18 

16 I I I I I I 16· 1-----+------+----+---+---t 

14 14~--;---~----+----r--, 

12 

10 

8 

6 

4 

2 

0 

0 20 40 60 80 

PERCENT OF SUMMERS 

100 

12r----4-----+------r----4----~ 

10~---~----+------~---~---~ 

81-----+---~---+--~--~ 

_6J ~....a:- I I I I 

4 I , -1 .. ,. t I >< - I I I 

2 I I I- f'"bcE'"'' I I 1 ::occ: I 

20 40 60 80 100 

Figure 5. -Occurrence-intensity relationship of summer storm precipitation. 

'"1:1 

::0 

tr:l 

('"'! 

't"ij 

~ ~ 

0 

z 

...... 

tA) 
14 SEDIMENTATION IN SMALL RESERVOIRS ON THE SAN RAFAEL SWELL 

Hence, sediment-production rates at different reservoirs, although representative of the same period, may 

be entirely different. Because of the complete lack of 

precipitation data at the individual study areas, no effort has been made to compensate for this effect, but 

where the life span of reservoirs has not been synchronous, some may include wet or dry years that others 

do not, an effort has been made to adjust known sediment rates to a long-term estimate of sediment 

production. 

SEDIMENTATION 

Although the ideal objective of any erosion study 

would be to evaluate the effect of individual factors 

contributing to the movement of sediment, such a program is far too complex even in small drainage basins. 

Their combined effects, however, can be readily determined by measuring the actual movement of sediment 

out of any basin. By comparing sedimentation rates 

over the same period in different reservoirs, marked 

anomalies may be correlated with: differences in certain erosional features. Hence, an indirect method may 

be used in the attempt to evaluate the relative importance of individual features so that necessary adjustments can be made when determining the long-term 

rates of sedimentation for different areas. 

A group of representative reservoirs was surveyed 

with the assumption that they trapped a high percentage 

of sediment transported during periods of runoff. A 

quantitative measurement of sediment in each reservoir 

was then prorated over the life of the reservoir to yield 

the average annual sediment production for a drainage 

basin of known size. Theoretically such a method of 

determination should provide accurate results, but actually three important factors must. first be taken into 

consideration, ( 1) inaccuracies in original surveys, (2) 

loss of sediment during reservoir spill, (3) effect of 

abnormal seasonal precipitation when reservoirs are 

not synchronuous in age. 

Most original reservoir surveys were made before 

construction as part of an over·all range-development 

program. Hence, the original calculated capacity at 

spillway level includes the added volume provided by 

excavation of fill for the dam. During construction, 

however, the calculated capacity may have been altered 

somewhat by supplying some of the fill from above 

spillway level or by not establishing the spillway control at the exact predetermined elevation. Nevertheless, such inaccuracies are perhaps the least important sources of error as adjustments can generally be 

mad~ following a second detailed survey of the reservoir, dam, and spillway. 

Of far greater consequence are errors resulting 

from loss of sediment during reservoir spill. Coarse 

particles carried into a reservoir rapidly settle to the 

bottom following a reduction in stream velocity. Clay 

or silt particles, however, have such a low settling 

velocity that they remain in suspension long enough to 

be carried out of the reservoir during periods of spill. 

Owing to increased spilling as the reservoir is filled 

with sed~ment, the relative amount of sediment lost 

during spill increases progressively. The trap efficiency or percentage of sediment caught in a reservoir must be expressed as a function of the average 

particle size carried into the reservoir, the reduction 

of velocity between the inflow channel and the spillway, 

the depth or distance particles must settle, and the 

amount of spill. The general effect of the first three 

factors could be determined in the field, but no record 

of the amount of spill is available, except as the relative amount could be judged from the appearance of the 

spillway. Adjustments for this loss, therefore, have 

been made by estimating the trap efficiency of individual 

reservoirs from measured field data and applying this 

correction to measured sediment-production rates. 

Trap Efficiency 

Examination of the rates of sedimentation shows an 

obvious and appreciable variation with geologic type. 

In order to eliminate this effect in a study of the trap 

efficiency, the several rates of sedime'ntation were expressed in ratio to the average for their respective 

geologic or lithologic groups. These were further classified as to their relative amounts of spill according to 

the information given in column 16 of table 4. Although 

trap efficiency is dependent on both the character of the 

sediment and on reservoir size and design, the amount 

lost will be in proportion to the total amount of spill. 

Because the amount of spill depends largely on the capacity of the reservoir in relation to the size of the 

drainage area, data on their capacity per square mile 

of drainage area or C/ A ratio are included. 

Reservoir 

no. 

9 

11 

12 

Average 

2 

15 

Average 

5 

6 

7 

8 

14 

Average 

Average 

3 

4 

10 

13 

Ratio of rate 

of sediment production 

to group average 

Nebligible or no spill 

Little spill 

Some spill 

Large spill 

0.37 

3.5 

.4 

0.76 

• 90 

---:-83 

0,42 

•. 56 

• 63 

.80 

.24 

.67 

--:-55 

0.11 

• 55 

.24 

• 12 

--:25 

Acre-feet per 

square mile 

55.6 

126 

38.8 

73.5 

32.2 

30.2 

~ 

15.8 

10.5 

25.5 

18.3 

23.8 

9.7 

17.3 

19.8 

13.9 

7.6 

3.8 

"'11.3 

The above table shows a very significant decrease in 

rate of reservoir sedimentation and the value of C/ A 

with amount of spill. This would indicate that about 75 

percent of the sediment passes over the spillway and 

only 25 percent is retained in those reservoirs noted as 

having "large spill.;, Therefore, the following adjustment for trap efficiency to the observed data seemed 

necessary. 
SEDIMENTATION . ·, ! 15 

Negligible or no spill ••••••••••••••.•••• · •••••••••• ~...... 1. 00 

Little spill.................................................. . 80 

Some !?Pill.. • •• • • • • • • •• • • • • • • . • • • . . . • • . • • . • • . • • • . • • . • • • • • • • • • 50 

I..arge spill.... .••••••. ••••••••.. •.. ••••••••••••••••••.•.•.•• • 25. 

The relationship of these factors to the value of C/ A 

is shown on figure 6. 

A minor decrease in relative rate of sedimentation 

with increase 'in drainage area may also be noted. This 

apparent effect may reflect ·a general tendency to the 

inclusion of flatter valley and plains areas with increase in drainage area. It may also indicate a temporary aggradation or accumulation of sediment in the 

channels below the steeper headwater areas. In an infinitely small basin practically all sediment movement 

is out of the basin. whereas in a very large basin considerable sediment movement may·take place within the 

basin. but the amount depends on the extent to which 

channel and flood plain deposition or degradation. sedimentation or erosion is taking place during a particular 

period of observation. Although sediment-production 

rates from small areas tend to be higher than those 

from larger areas. no adjustments have been made because their significance in this study is small and probably transitory. 

A third variable and perhaps the most difficult to evaluate is the effect of abnormal seasonal precipitation 

when the age of reservoirs is not synchronous. As 

noted previously the rate of sediment production is dependent on the occurrence and amount of runoff. Hence. 

reservoirs may be expected to show considerable variation in the amount of sediment caught annually be:-

cause of the marked differences in seasonal precipitation. Figures 4 and 5 show the great disparity not only 

in the total seasonal rainfall. but in the manner of occurrence. Simultaneous records for the different stations show that local variations during the same summer are often larger than season to season variations 

at any one station. Few generalizations. therefore, can 

100.1 0.2 0.50 1.00 

TRAP EFFICIENCY 

Figure 6. -Relation of trap efficiency to capacity-area· 

ratio. 

be made for the entire area. Nevertheless. the summer 

of 1941 was so abnormally high in both total seasonal 

precipitation and runoff that reservoirs constructed 

before 1941. should show somewhat .high~r sediment 

production rates than reservoirs constructed after 

1941. Sediment-production rates at the two reservoirs 

constructed since 1941 were adjusted in accordance· 

with records at .the nearest precipitatio.n station. , 

All available information from previous surveys and 

publications. together with measured and computed data 

obtained from field examinations. are summarized in 

table 4. Accompanying explanatory notes show the 

source of information and methods of comp).ltation for 

individual columns. The final results of the study are 

shown in columns 21 and 22. which express the adjusted 

annual sediment production in acre-feet per square mile 

of drainage basin for the life of the reservoir and for 

the long-term period respectively. 

Compilations 

Source of information and methods of computation 

used in setting up table 4: 

Columns 1 and 2. Self explanatory. 

Columns 3, 11. and 17. Location. original capacity. 

and age of structures from Bureau of Land Management 

files. Locations are described by quarter sections. 

Column 4. Reservoir altitudes determined by aneroid 

barometer. 

Column 5. Drainage areas including re~ervoir area 

planimetered from areal photos. scale 1 inch•1 mile, 

except for .reservoirs 1. 5, 6, 10, 11. 13, and 14, which 

were determined in the field. 

Column 6. L~ngths of drainage basins determined 

either from areal photos or by field measurements. 

Column 7. Maximum relief as determined by aneroid 

barometer. 

Column 8. Geology from field notes. 

Column 9 •. From field observations. 

Column 10. Reservoir areas at spillway level as 

determined by planimete'ring plane table maps. 

Column 12. Present capacity of reservoir at spillway 

level as computed from the area-depth relationship. 

measured in the field. Contour intervals of 2 feet or 

less were obtained for each rese.rvoir below s'pillw.~y 

level. · 

Column 13. Average life capacity of reservoir with 

respect to drainage area. Average of columns 11 and 

12 divided by column 5. 

Column 14. Obtained by· subtracting column 12 from 

column 11. Minor adjustments have been made to allow 

for obvious inaccuracies in original surveys. 

Column 15. Percentage of original storage now occupied by sediment. 

Column 16. Reservoir spill as indicated by the nature 

and amount of spillway erosion since construction. 

Column 17. Life of reservoir. 

Column 18. Appraisal of records based on trap efficiency of reservoir, accuracy of original survey. and 

length of record. 

l";olumn 19. Sediment productibn per square mile. 

Column 14 divided by column 5. 

Column 20 .. Annual sedi:tllent produ.ction rate per 

square m~le •. Column 19 "d.ivided by column 17 .. 

·Column 21. Annual sed.iment-production ra,te adjusted 

for drainage area effect and trap-efficiency effect according to the method given on pages 14-18. 
16 SEDIMENTATION IN SMALL RESERVOIRS ON THE SAN RAFAEL SWELL 

Table 4. -Data from reservoir and drainage 

USGS B.L.M. Location Altitude 

no.· Project (feet) a 

no 

(1) (2) (3) (4) 

1 110 SW'i, sec. 35, 6,580 

T. 23 S., R. 5 E. 

2 176 NEt, sec. 17, 4,900 

T. 22 S., R. 8 E. 

3 177 NEt, sec. 16, 5,000 

T. 22 S., R. 8 E. 

4 25 SEt, sec. 26, 5, 890 

T. 20 S., R. 8 E. 

5 9 NWt, sec. 35, 6,000 

T. 19 S., R. 10 E. 

6 63 SEt, sec. 24, 6, 375 

T. 19 s. I R. 11 E. 

7 93 swt, sec. 8, 5,280 

T. 19 S., R. 13 E. 

8 76 SEt, sec. 7, 5,820 

Ti 18 s. I R. 10 E. 

9 67 SW 4, sec. 27, 5,720 

T. 17 S., R. 10 E. 

10 215 swt, sec •. 2, 5, 870 

T. 16 s. I R. 9 E. 

11 74 SEt, sec. 19, 5, 590 

1'· 15 s. I R. 12 E. 

12 128 SE4, sec. 5, 5,670 

\· 15 S., R. 12 E. 

13 154 NE4, sec. 30, 5, 705 

T· 14 s., R. 12 E. 

14 160 SE4, sec. 23, 5,900 

I· 14 s.' R. 12 E. 

15 3 SE4, sec. 20, 4,250 

T. 21 S., R. 16 E. 

Reservoir-continued 

USGS Present C/A no. capacity Acre-ft 

(acre-ft) (sq miles) 

(1) j_U) (13) 

l 2.40 15.8 

2 6. 81 32.2 

3 1.934 19.8 

4 4. 55 13.9 

5 7.47 10.5 

6 12.06 25.5 

7 6.80 18.3 

8 2.85 23.8 

9 6.02 55.6 

10 16.49 7.6 

11 11.29 126.0 

12 4.95 38.8 

13 2. 31 3~ 8 

14 3. 96 9. 7 

15 6.03 30.2 

a Altitudes determined by aneroid barometer. 

b Including adjacent drainage cut into reservoir. 

c Sediment in upstream structures included. 

Sediment 

Acre-ft 

t14) 

2. 80 

2. 50 c • 094 

16.4 

1. 8 

14.10 

6. 2 

.95 

• 16 

19.1 

7. 7.1 

.18 

2.9 

1. 02 

a. 5 

d Bulk of sediment deposited on fiat before t-eaching reservoir. 

(In column 18 symbol g • good, 

Drainaae basin 

Area Length 

(sq miles) (miles) 

(5) (6) 

0.24 0.76 

• 25 • 75 

• 10 .65 

.92 1. 25 

.80 1. 60 

.75 1. 45 

• 54 1. 30 

.14 • 64 

.11 • 62 

3. 45 3.40 

• 12 • 79 

b • 13 • 60 

.99 1. 85 

• 46 1. 53 

• 34 • 74 

Percent of 

original Spilling 

capacity 

(15) (16) 

53.8 Some 

26.9 Little 

4. 6 Large 

78.3 Large 

19.4 Some 

53.9 Some 

47.7 Some 

-25.0 Some 

2. 6 Negligible 

53.7 Large 

40.6 None 

3. 5 Negligible 

55.6 Large 

20.5 Some 

58.5 Little 
SEDIMENTATION 17 

basin studies on San Rafael Swell, Utah 

f = fair, and p • poor] 

Drainage basin-Continued Reservoir 

Maximum Geologic Erosional Surface Original 

relief symbol characteristics area capacity 

(feet) a (acres) (acre-ft) 

(7) . (8) (9) (10) (11) 

f75 Kmn Major guliying, 1.49 5.20 

minor sheet 

erosion. 

325 Jcus Sheet erosion. 3.35 9. 31 

150 Jecu Sheet erosion. • 66 2.02 

525 Kmn Major gullying 1.92 20.95 

sheet erosion. 

260 Jca Minor gullying 1. 82 9.27 

sheet erosion. 

1,500 Jmscueca Major gullying 4.97 26. 16 

sheet erosion. 

50 Je Minor gullying 3. 06 13.00 

sheet erosion. 

75 Jm Sheet erosion. 2.34 3.80 

120 Jm Sheet erosion. 2.06 6.18 

500t Kmn Major gullying. 8. 62 35.59 

125 Kmn Major gullying. 2.64 19.00 

65 Kmn Sheet erosion. 1. 79 5. 13 

215 Kmn Major gullying. 2.29 4.21 

250 Kmn Minor gullying .99 4.98 

sheet erosion. 

190 JmKmn Major gullying 2. 51 14.53 

minor sheet 

erosion. 

Length Total Ann. sediment Ann. sed. prod. Estimated longof Accuracy sediment production adjusted for term annual sedirecord of production for period trap efficiency ment production 

(years) records (acre-ft/ sq mile) {acre-ft/sq mile} (acre-ft/ sq mile) (acre-ft/ sq mile) -(1'7) (18) (19) (20) (21) .(22) 

12 g 11.66 0.97 1.9 1. 9 

11 g 10.00 .91 1. 1 1. 2 

7 f .94 • 13 • 5 • 5 

14 p 17.82 1.27 5. 1 5.0 

13 g 2. 25 • 17 .4 .4 

13 f 18.80 1.45 2. 9 2. 9 

12 g 11.48 • 96 1. 9 2. 0 

12 g 6.78- • 56 1. 1 1. 0 

13 g 1. 45 .11 .11 • 1 

10 p 5. 53 • 55 2. 2 2.2+ 

8 g 64.25 8.03 8.0 4.0+ 

12 g 1. 38 • 12 • 12 • 2 

11 p 2. 93 .2~ 1. 1 1.5+ 

11 g 2.22 • 20 .4 .4 

12 g 25.00 2.08 2.6 2.6 
Table 5. -Comparison of sediment production from various rock types 

Area of Length Estimated long- Average annual 

Group clas- Geologic formation drainage of term annual sediment produc- sification Reservoir from which soils basin record sediment pro- tion for group (by lithology) no. are derived. (square miles) (years) duction drainage area 

Symbol drainage area (ac-ft / sq mi) 

(ac-ft / sq mi) 

M 

I 9 Conglomerates and siliceous 0.11 13 o. 1 Q.l e,; shales of Morrison formation - 0 r:: Ji'ri .... as blllll 

r::..., 

0 12 Ferron sandstone member of . 13 12 .2 r:: ()~ 

.. Ill Mancos shale. Kmn 0.3 Q.l •.-1 

r:: Ill 0 Q.l 5 Limestone of lower Carmel .80 13 • 4 ti M Ill 

Q.l .. Q.l formation. Jca e Ill r:: ..... Q.l 0 

.J~ti 14 Gravel-capped pediment on . 46 11 .4 

Mancos shale. Kmn 

Ill 3 Sandstones of Entrada and • 10 7 • 5 Q.l 

r:: Curtis formations Jecu 

s 

Ill 

"0 2 S~ndstones and shales Curtis . 25 11 1.2 1.2 r:: 

as and Summerville formations. Ill 

..2 Jcus 

,Q 

as 7 Entrada sandstone. ·~ Je • 54 12 2.0 ~ 

8 Shales of upper Morrison for- • 14 12 1.0 

mation. Jm 

13 Pediment and Mancos shale. . 99 11 1. 5-t 

Kmn Ill 

"0 

Q.l 1 Lower Mancos shale. ,Q Kmn • 24 12 1. 9 

e Morrison, Summerville, and :; 

Ill Curtis formations. 2.6 

~ 6 Entrada sandstone, snales and . 75 13 2.9 

gypsum beds of upper Carmel ~ 

0 formation. Jmscueca Ill 

"0 

r:: 10 Mancos shale. Kmn 3.45 10 2. 2t as 

Ill 4 Mancos shale. Kmn . 92 14 5.0 Q.l 

~ 15 Mancos shale. Kmn Shales of • 34 12 2.6 til Morrison formation. Jm 

11 Mancos sha1e. Kmn • 12 8 4.0+ 

Remarks 

All structures except no. 12 a:re located in areas underlain by resistant bedrock with little or no soil 

mantle. Sediment contribution 

mainly from sheet erosiqn, gullying 

negligible. No deep accumulations 

of fill. Reservoir no. 12 located on 

Ferron sandstone with good soil 

cover. Sediment contribution from 

sheet wash which is minimized by 

sagebrush and grass covers. 

Reservoirs located in strike valleys 

cut in Curtis and Entrada sandstones. Soils are thin or absent 

with little ~egetation. Many outcrops of bare sandstone form a 

large part of the drainage basins. 

Residual mantles of fine sand range 

in depth from a trace to 6 feet. Erosion mainly from sheet wash;gullying negligible. 

Reservoirs located in wide shale valleys characterized by deep alluvial 

fills that are extensively gullied. 

Thin residual soils support limited 

vegetative cover of salt sage, grass 

and cedar trees. Shales weather so 

rapidly that rock itself is susceptible to sheet erosion. Interbedded 

shale and gypsum beds that underlie the drainage basin of no. 6 

weather similar to shale with about 

the same rate of sediment 

production. 

.-

00 

tl.l 

tzj 

t::l 

eE 

J::!:t 

z 

~ 

> 

~ 

0 

z 

z 

til 

~ t" 

~ 

~ til 

tzj 

!:tl 

< 0 

5d 

til 

0 

z 

~ 

:I: 

tzj 

til 

> z 

~ I%J 

> tzj 

~ 

~ tzj 

~ 

~ 
RATES OF SEDIMENT PRODUCTION 19 

Column 22. Estimate of long-term sedimentproduction rate based on data on column 21, length of 

records, and accuracy of records. 

RATES OF SEDIMENT PRODUCTION 

As noted previously, erosion within any drainage basin is confined mainly to the surface mantle, the character of which is determined by the lithology of the 

rocks that underlie the area. Hence,· drainage basins 

underlain by different types of rocks 'should exhibit 

marked differences in rates of sediment production. 

Table 4, column 22, shows that such a disparity does 

exist after corrections have been applied for differences in precipitation and for errors in quantitative 

measurements. The. disparity, therefore, is largely 

attributed to differences in topographic relief or in geology as reflected by the character of the surface materials. Referring again to table 4, column 7. topographic relief has seemingly little direct effect on the 

rate of erosion as the two basins that have the highest 

and lowest rate of sediment production have the same 

maximum relief. 

To examine the effect of geologic forces, the study 

reservoirs have been grouped in table 5 according to 

the lithology of the formations that underlie the drainage basins with each group arranged in increasing order of sediment-production rates. Such a classification shows a striking difference in the amount of sediment derived annually from each of the three groups. 

The third group, which consists mainly of shales, contributed almost twice the amount derived from friable 

sandstones and almost eight times the amount derived 

from conglomerates, limestones, and resistant sandstones. This classification has, however, the disadvantage that rates of sediment production cannot be assigned to formations that consist of several types of 

rock unless the area of outcrop can be determined for 

each rock type. It does have the advantage that a study 

of this nature can be expanded and possibly compared 

with similar studies in other areas~ One such study in 

New Mexico and Arizona (Hains, Van Sickle, and Peters·on, 1952) shows a remarkably close comparison 

of sedimen~roduction rates· from similar rock types 

in similar climatic environments. The comparisons 

are shown in table 6. 

Table 6. -Comparison of sediment-production rates 

between New Mexico-Arizona and Sari Rafael Swell, 

Utah,for different rock types 

Conglomerates, 

Location limestones Friable 

and resistant ~andstones Shales 

~andstones 

New MeJ~:ico- 0,2-0.3 1.1 1.6 

Arizona 

San Rafael 0 30 1.2 2.6 

Swell, Utah 

The lower sediment-production rate from sha.les in 

New Mexico and Arizona can be attributed to insufficient adjustment for sediment loss during reservoir, 

spill. Actually trap efficiencies were not computed so 

that adjustments could only be approximated from field 

observations. To the extent that a much higher percentage of the clay particles derived from shales would 

be lost during spill than the coarser partie les in groups 

1 and 2, a closer agreement would be expected in the 

first two groups than in the third. 

Unfortunately these studies have been limited to a 

fe.w widely scattered basins in the intermountain region 

and little data on sediment-production rates are available for distinct lithologic groups •. The comparisons 

shown in table 6, however, encourage further expansion 

of studies of this kind. 

Although rates of sediment production have been 

shown to fall into three large groups, depending on the 

lithology of the rocks underlying the drainage basin, 

considerable variations exist within each group. Undoubtedly these smaller variations may be attributed to 

local conditions relative to slope, type of vegetative 

cover, grazing use, variations in rainfall,· and other 

minor features. In the main, however, the ultimate 

controlling factor seems to be the amount and availability of erodible sediment within a drainage basin. As the 

amount varies greatly in area·s underlain by-different 

kinds of rock, these phases of the problem · must be 

considered for each rock type, (1) the rate of weathering of the rock itself, (2) the character of the weathered 

material as reflected by inhereat resistance to erosion 

or suitability as a medium for plant growth, (3) the amount of weathered material existing as residual soils 

or as alluvial fills that were deposited in old stream· 

channels during some previous period and are now a.-

vailable for removal because of some change in environment that has lead to erosion rather·than deposition. 

Weathering of the hard, well-indurated conglomer·-

ates, limestones, and resistant sandstones of the first 

group is so slo.;, under climatic conditions existing on 

the San Rafael Swell that individual particles can be. removed by wind or water almost as rapidly as the: rock 

weathers. In localities where thin soils have. developed, 

however, a grass cover now retards sheet erosion to 

the extent that further soil development is taking place. 

Another factor that reduces sheet erosion on this rock 

type is the high infiltration rate afforded- by the large 

average openings between the coarse particles in the 

weathered mantle or soils. Limited sheet·erosion combined with an almost complete lac.k of gullying due to 

the absence of alluvial fills in the stream channels apparently accounts for the low rate of sediment production from this rock group. 

Unfortunately, the actual area underlain by rocks of 

this group could not be determined, but a rough approximation would set the relative amount at 15 percent of 

the total area. Conglomerates of the Shinarump formation underlie a portion of the Sinbad area, but otherwise 

conglomerates are limited to the Salt Wash sandstone 

member of the Morrison formation and to the wellcemented gravel cappings on the pediment remnants 

extending from the Wasatch Plateau escarpment and 

Book Cliffs. Resistant sandstone members of the Moenkopi formation underlie a large part of the Sinbad and 

some resistant sandstones are found in the Mancos 

shale and as thin lenses in the Morrison formation. 

Limestones of the Moenkopi formation underlie the remainder of Sinbad, but otherwise limestones occur only 

as thin lenses in the Morrison formation and as one bed 

about 20 feet thick near the base of the Carmel 

formation. 
20 SEDIMENTATION IN SMALL RESERVOIRS ON THE SAN RAFAEL SWELL 

In contrast to the resistant rocks of group 1, the 

friable sandstdnes of group 2 weather rapidly to produce surface mantles consisting primarily of rounded, 

individual sand grains with only .minor amounts of silt 

and clay. Locally, sandy soils derived from this group 

are protected by an adequate vegetative cover, and 

sheet erosion is negligible, but once this cover becomes deteriorated, particles are easily moved downslope. Many bare sandstone outcrops together with local evidence such as hummocks and plant pedestals attest to the relative importance of sheet erosion on 

rocks of this type. 

Rapid movement of s.urface materials into the 

stream channels with subsequent deposition of at least 

a portion of this sediment has resulted in relatively 

deep alluvial fills. Similar to their residual counterpart, alluvial fills derived from sandstones have very 

little resistance to erosion unless protected by an adequate vegetative cover. Once this cover is destroyed 

and gullying starts, fills of this nature are rapidly excavated producing abnormally high sediment rates until 

the fills are either removed or another cycle of deposition is effected. Unfortunately, no reservoirs we.re 

located in areas characterized by gullying in alluvial 

fills derived from sandstones of group 2, and no data 

are available for sediment-production rates from these 

areas. Nevertheless, it is the writers' contention that, 

if reservoirs had been located in these areas rather 

than' in areas characterized by sheet erosion, much 

higher rates would have been obtained for group 2. 

Although actual measurement of sediment-production 

rates from sandstones of group 2 was limited to basins 

underlain by the Entrada and Curtis formations, it is 

believed that these rates should be generally applicable 

to areas underlain by the Wingate and Navajo sandstones. Under this assumption nearly 35 percent of the 

surface area of the San Rafael Swell would be classified in this group. 

Although about one-half the surface area of the San 

Rafael Swell is underlain by rocks of groups 1 and 2 

this study would indicate that together they supply only 

about one-fourth of the total sediment contribution 

from the basin as a whole. The remainder of the sediment must, therefore, be derived from rocks of group 

3. Field examinations in areas underlain by shales 

tend to confirm such a deduction, as rocks of this type 

are characterized by active sheet erosion that has locally dissected the surface into a miniature badland topography drained by well-developed gullies. The absence on these areas of a well-developed soil together 

with many plant pedestals and hummocks suggests that 

the removal of surface material by sheet erosion is as 

rapid today as it has been in the past. Nevertheless, 

shale outcrops with a slope of 30° or greater have a 

thin veneer of a few inches of weathered shale at the 

surface, and the more gentle slopes of 10° or less are 

often weathered to a depth of 1 to 2 feet. This would 

indicate that, although removal of the surface materials has almost kept pace with weathering, erosion has 

not been sufficiently active to remove all of the loose 

particles, and a seemingly unlimited amount of sediment could still be supplied· by sheet erosion on rocks 

of group 3. 

A more ready source of sediment exists, however, 

in the form of alluvial fills along the many channels 

that drain the,extensive shale outcrops. Sheet erosion 

during the past has moved :;ediment:into the· channels 

and deposited it the:re as unco.nsoli.dated fills that were 

stable only so long as the upper surface was protected 

so that gullying could not start. At present most of 

these fills are in the stage of being removed, which may 

explain the abnormally high sediment production rates 

from these areas. 

Rocks of group 3 consist primarily of shales of the 

Mancos and Morrison formations, .which underlie most 

of the surface area on the west, north, and northeast 

portions of the Swell. Shales and gypsum beds of the 

Summerville, Carmel, and Chinle formations, although 

characterized by abnormally high sediment production 

rates, are only of minor importance because of their 

limited area of outcrop. Characteristics of the different formations are listed in further detail in table 1. 

Thus far the study has been limited to a discussion of 

the rates of sediment movement from small drainage 

basins at the heads of tributary channels on the San Rafael Swell. Although detailed data on sediment movement from the entire area are not available, some general comparisons can be made between the relative 'sediment movement from the small study areas and from 

the San Rafael Swell as a whole. 

Measurements of the sediment load transported by the 

San Rafael River were made in the 1947-48 and 1948-49 

water years at Tidwell Ranch, near Green River, Utah, 

and give some indication as to the actual amount of sediment moved off the central, middle, and western portions of the Swell. These records are not representative of the long-term period, as precipitation during 

both years is below average for stations within this 

drainage. Nevertheless, they bring out a favorable 

comparison between the sediment movement into small 

upland reservoirs and the actual sediment movement 

out of the entire basin. The measured annual sedimentproduction rates from an area of 1 square mile varies 

from 0. 30 acre-foot for group 1 to 2. 6 acre-feet for 

group 3. On this basis the total annual sediment movement from the 1, 690 square miles of drainage area 

above Tidwell near Green River would be· expected to 

range from a minimum of 500 acre-feet to a maximum 

of 4, 400 acre-feet. Actual measurements show 360 

acre-feet and 1, 140 acre-feet respectively for the water 

years of 1947-48 and 19;48-49. The lower than expected 

actual sediment production for the 2 years of record 

may be attributed to below average precipitation during 

both of those years, but based on only 2 years record, 

it seems hardly possible to formulate any definite 

cone lusions. 

Further analysis of the 2 years of record shows that 

about half this total sediment movement occurred during 

the period of normal spring runoff from March to June. 

As the major part of this runoff is supplied by snowmelt 

on the Wasatch Plateau and only a minor amount comes 

from snowmelt on the Swell, the bulk of this sediment 

must be derived from the Wasatch slopes or from the 

main channel of the river where it crosses the ·Swell. 

Field observations on tributary channels as they cross 

Utah State Highway io near the foot of the Wasatch escarpment showed clear streams with low-sediment concentrations. Therefore, the major source of sediment 

at least during spring runoff probably is from deposits 

in and bordering the main channel of the San Rafael 

River. Following this conclusion, future studies on the 

San Rafael Swell should be mad~ with particular 
LITERATURE CITED 21 

emphasis on ( 1) the amount of erodible sediment in and 

bordering the main channels, (2) the manner of occurrence and character of the materials, and (3) the 

amount of sediment stored in the main channels following periods of runoff in response to local summer 

storms that are of sufficient magnitude to cause movement of sediment into the main channels, but not out 

of the basin. 

SUMMARY AND CONCLUSIONS 

Measurement of rates of sedimentation in a group of 

small reservoirs on the San Rafael Swell, Utah, offers 

a possible method for evaluating the relative production of sediment from areas having distinct, but different, erosional characteristics. Once the relative 

importance of erosional features such as slope~ vegetation, land use, geology, and precipitation can b~ de-. 

termined, areas of greatest potential sediment productiGn can be delineated. This is a requisite fo~ any future program of land treatment in the Colorado,:fUver 

basin designed for sediment abatement. 

After adjustment for sediment loss during reservoir 

spill and for drainage area effect, measured rates of 

sedimentation in selected small reservoirs were con:-

verted into long-term estimates of sediment production. Suitable adjustments were made for differences 

in precipitation, condition of the drainage a:rea, and 

other factors influencing sedimentation during the life 

span of the reservoir as compared with conditions that 

might be considered normal. 

Reservoirs were divided intq three groups based on 

the lithology of rocks underlying their respective 

drainage basins. Listed in order of increasing sediment production, they are (1) conglomerates, limestones, and resistant sandstones, (2) friable sandstones, and (3) shales and gypsum beds. The difference in the amount of sediment produced from the various groups is striking, being two to eight times as 

much from the shales and friable sandstones as from 

the resistant rocks of group 1. Variations within 

groups were comparatively small, undoubtedly reflecting the effects of slope, vegetative cover, and land 

use, but no definite relationship could be determined 

from the available data between sediment production 

and the latter named factors. 

On the basis of group averages, the annual sediment 

production from the San Rafael Swell would range between 0. 3 acre-foot per square mile for areas underlain by the rocks of group l to 2. 6 acre-feet per square 

mile for areas underlain by the rocks of group 3. 

Hence, the average annual sediment production from 

the San Rafael River drainage basin above Tidwell, 

Utah, an area of about 1, 690 square miles, would be 

expected to range from a calculated minimum of 5.00 

acre-feet to a maximum of 4, 400 acre-feet. Sediment 

records for the 1947-48, 1948-49 water years, show 

360 acre-feet and 1, 200 acre-feet respectively based 

on a conversion factor of 1, 500 tons per acre-foot. 

Further analysis of these records shows that about onehalf of this amount occurred during May and June as a 

result of normal spring runoff. But as nearly all spring 

runoff is supplied by snowmelt in the mountains, the 

bulk of this sediment must be obtained from the main 

channel ra.ther than from tributary channels. These 

figures show what differences can be expected between 

upland rates of sediment removal at a particular period 

and the concurrent rate of transportation of sediment 

at downstream points of major streams. 

These reservoirs have been monumented for repeat 

surveys, and it is recommended that further studies 

include the main channels on the San Rafael Swell with 

particular emphasis given to ( 1) amount of sediment in 

the form of old alluvial fills that are now available to 

erosion, (2) manner of occurrence, and (3) amount of 

sediment stored in main channels following runoff in 

response to high-intensity summer storms. 

LITERATURE CITED 

Andrews, D. A., and Hunt, C. B., 1948, Geologic map 

of eastern and so~thern Utah: U. S. Geol. Survey 

Oil and Gas Investigations, Preliminary Map no. 70. 

Baker, A. A., 1946, Geology of the Green River 

Desert-Cataract Canyon region, Emery, Wayne, and 

Farfield Counties, Utah: U. S. Geol. Survey Bull. 

951. 

Gilluly, James, 1928, Geology and oil and gas prospects of part of the San Rafael Swell, Utah: U. S. 

Geol. Survey Bull. 806-C, p. 69-130. 

Hains, C. F., Van Sickle, D. M., Peterson, H. V., 

1952, Sedimentation rates in small reservoirs in the 

Little Colorado River basin: U. S. Geol. Survey 

Water-Supply Paper 1110-D, p. 129-155. 

Hunt, C. B., and others, 1952, Geologic map of the 

Henry Mountains region, Utah: U. S. Geol. Survey 

Oil and Gas Investigations, OM 131. 

Ilft'.-DUP. UC., lASH., D.C. 14141 .. ,