Sedimentation Rates 

in Small Reservoirs in 

the Little Colorado 

River Basin

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1110-D

Sedimentation Rates 

in Small Reservoirs in 

the Little Colorado 

River Basin

By C. F. MAINS, D. M. VAN SICKLE, and H. V. PETERSON

CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES, 1948-51

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1110-D

Measurement of sedimentation 

in stock reservoirs as a possible 

means of determining origin and 

movement of sediment

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1952
UNITED STATES DEPARTMENT OF THE INTERIOR 

Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. C. - Price 40 cents (paper cover)
CONTENTS

Page

Abstract. _______________________________.__________ 129

Introduction._ ____________________________________________________ 129

Acknowledgments -_---__--_______________-_____-_-_____-____-_ 132

Physiography.____________________________________________________ 132

Defiance Plateau._____________________________________________ 133

Black Creek valley..._________________________________________ 133

Manuelito Plateau___________________________________________ 134

Geology__________________________________-__--_-_-___-___ 135

Sedimentary rocks._______-__________-_________-_-__________:i__ 136

Permian rocks____________________________________________ 136

Triassic rocks_-___---__-____-__-____---_____-__-_-__--_-__ 136

Jurassic rocks.____________________________________________ 137

Cretaceous rocks-__-__-______________---_---___-__-_--____ 137

Tertiary rocks______-_______._____________________________ 137

Igneous rocks _________________________________________________ 137

Structure...-------.---------___--__-_--__----_---___--------- 137

Precipitation. _ _ ___________________________________________________ 138

Method of measuring sedimentation.________________________________ 141

Rates of sediment production._______________________________________ 142

Description of individual reservoirs__________________-_______________ 147

Index__________________________________________________________ 155

ILLUSTRATIONS

Page 

PLATE 7. Geologic map of parts of Arizona and New Mexico showing

location of reservoirs surveyed_______________________ In pocket

FIGURE 17. Map showing forest types of vegetation__________________ 131

18. Summer precipitation near Fort Defiance, Ariz_____________ 139

19. Magnitude of summer storm precipitation, Fort Defiance,

Ariz.________________________________________ 140

in

SEDIMENTATION RATES IN SMALL RESERVOIRS IN 

THE LITTLE COLORADO RIVER BASIN

By C. F. HAINS, D. M. VAN SICKLE, and H. V. PETERSON

ABSTRACT

Measurement of the rate of sedimentation in a group of small reservoirs 

located in land-use District 18 of the Navajo Indian Reservation, Arizona-New 

Mexico, suggests a possible method for determining the origin of sediment and 

estimating the rate of sediment movement from headwater areas underlain 

by various geologic formations. Land-use District 18 lies within the Little 

Colorado River basin. The area is underlain by sedimentary rocks, which range 

in age from Permian to Late Cretaceous and in character from coarse, wellindurated sandstones and conglomerates to soft fine-textured sandstones of 

aeolian origin and soft friable shales.

Measurements of sediment trapped in 35 reservoirs distributed over areas 

underlain by these formations show a striking difference in rate of annual sediment movement. Groups of reservoirs located on shale and soft sandstone 

trapped on an average 2 to 8 times as much sediment per square mile of drainage 

area as other groups located on well-indurated sandstone and conglomerate. 

In individual reservoirs the contrast is even more striking, the movement ranging from less than 0.1 acre-ft annually per square mile of drainage area in 

reservoirs underlain by conglomerate to more than 2 acre-ft in reservoirs located 

on the Chinle shales. The differences appear to be due mainly to characteristics 

of the underlying rock, although it is recognized that other characteristics of 

the drainage basin, such as soil, slope, cover, and land use, also have an influence.

To the extent that distinct differences in erosibility of the land can be attributed to geology, investigations of this kind offer a method for obtaining a ready 

inventory of the source and rates of sedimentation in areas where geologic 

mapping has been completed. The results would delineate areas of greatest 

erosion potential and thereby would be useful in devising plans of treatment 

aimed at abating erosion.

INTRODUCTION

Studies on sedimentation in a number of small reservoirs located 

on the part of the Navajo Indian Reservation lying along each side 

of the northern boundary between Arizona and New Mexico (see pi. 7) 

have been undertaken as a means of determining the origin and movement of sediment in areas underlain by various types of sedimentary 

rocks. Most of the reservoirs were constructed for storing water for 

livestock use, but some were installed as part of a land-treatment program designed to reduce erosion and stimulate reestablishment of

129
130 CONTRIBUTIONS TO HYDROLOGY, 1948-51

vegetation. In capacity they range from less than 1 acre-ft to a 

maximum of 65 acre-ft. The dams are all earth-fill with side spillways.

The formations on which the reservoirs are located have a wide 

distribution throughout the Colorado Plateau. Therefore, accumulation of sediment in the reservoirs surveyed makes it possible to 

estimate within fairly close limits sediment movement from other parts 

of the Plateau underlain by similar geologic formations. Additional 

information obtained gives some indication of the influence of soil 

types, vegetation, slope, land use, and other factors on the erosibility 

of a given area.

Surveys of reservoirs offer about the only feasible plan for obtaining information on sedimentation rates in arid country. Typical 

of such areas, the streams feeding the reservoirs are strictly ephemeral, 

with flow occurring at unpredictable periods. Direct sampling of the 

stream flow was therefore impractical because it was difficult to 

anticipate the time of flow and to assemble the equipment and personnel necessary to obtain the requisite number of representative samples.

Essentially the method used was to determine the amount of sediment trapped in the reservoirs and to prorate this amount over the 

life span of the reservoir. This gave an average yearly sedimentproduction rate for a drainage area of known size. This rate was 

further modified by comparing rainfall during the life of the reservoir with the long-term rainfall of the area, which gave a general 

average expectancy of sediment production from the basin. The 

method has the disadvantage, of course, that data on sediment movement during individual seasons or storms are unknown, and thus 

no relationship between runoff and sediment can be developed. It 

does, however, have the advantage that small drainage areas of 

distinctive physical characteristics can be studied.

The area in which the reservoirs are located, land-use unit, District 

18, of the Navajo Indian Reservation in Arizona and New Mexico, 

covers about 1,500 sq mi. It is a part of the Little Colorado River 

basin and exhibits many of the characteristics typical of the basin. 

In selecting reservoirs for study, an effort was made to obtain a 

distribution that would reflect the differences in erosion in various 

types of localities. Because detailed information on soil was not 

available and as there is an obviously close relationship between 

soil and the rock from which it is derived, division on the basis of 

geologic formations was considered both practical and desirable. A 

generalized description has been included of the chief physiographic 

and geologic features of the area, with particular reference to those 

factors that appear to have some effect on erosion, runoff, and climate.
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 131

EXPLANATION

R.18 W.

I08» 45'

109° 15' R.30E. R.31E

FIGURE 17. Forest types of vegetation. From map by Gregory.

A part of the geologic map of the area by Gregory,1 with some 

modification, is shown as plate 7. Figure 17 showing distribution of 

forest types of vegetation is also taken from Gregory's report.

1 Gregory, H. B., 1916, The Navajo Country: U. S. Geol. Survey Water-Supply Paper 

380.
132 CONTEIBUTIONS TO HYDROLOGY, 1948-51

Reservoirs containing water were surveyed by use of compass 

and pacing, dry reservoirs by plane table and stadia. Sediment 

deposits were probed, using %-in. steel bars, to determine depth. 

In nearly all reservoirs three or more contours were surveyed to obtain 

the area-capacity curve. Bureau of Indian Affairs records were 

consulted for location, date of construction, and the general history 

of the reservoirs.

Data obtained from the study indicate significant differences in 

erosion characteristics of areas underlain by various geologic formations. It is believed that an expansion of the study to other parts 

of the Little Colorado River basin would furnish needed information 

on the general erosion potential of the area and would focus attention 

on particular tracts where the rate of erosion is critical.

Field surveys for the studies were made during the summers of 

1946 and 1947 by C. F. Hains, hydraulic engineer, and D. M. Van 

Sickle, geologist, under the supervision of H. V. Peterson, staff 

geologist. The work was begun as a part of the general investigations 

of sedimentation being conducted by the Geological Survey under 

the Department of Interior's soil and moisture conservation program.

ACKNOWLEDGMENTS

Special acknowledgment is due members of the Navajo Indian 

Agency who made maps and records available and those who assisted 

in the field work. Especially helpful were R. E. Kilgore, conservationist, and John J. Schwartz, irrigation engineer, Window Rock, 

Ariz., and H. B. Coddington, conservationist, Mexican Springs, 

N. Mex.

PHYSIOGRAPHY

Erosion and movement of sediment within any given area, particularly in the semiarid West, are strongly influenced by such physiographic features as land forms; slope of the terrain; character of the 

drainage pattern, including both the main channels and the minor 

tributaries; type of cover, and possibly other less important features. 

Although no attempt was made in the studies to discriminate between 

individual features or to set up standards for determining their influence it nevertheless was obvious that erosion characteristics in certain parts of the area were attributable mainly to variations in physiography. Therefore, an effort has been made to describe the principal 

physiographic features of the area and to point out their relationship 

to the erosion problem.

The area studied lies in northwestern New Mexico and northeastern 

Arizona within the boundaries of the Colorado Plateau province. 

Flat-lying or slightly tilted strata, cut by deep canyons, with buttes and 

mesas rising abruptly from almost level, broad expanses, are the
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 133

general features of the province. Gregory 2 in describing the Navajo 

country divided it into several geographic provinces. The area studied 

includes parts of three subprovinces the Defiance Plateau, Black 

Creek valley, and the Manuelito Plateau.

DEFIANCE PLATEAU

The area studied lies along the eastern edge of the Defiance Plateau 

and extends eastward to the floor of Black Creek valley. A yellowpine forest grows at the higher altitudes; Gregory 3 points out that 

this forest is somewhat unique in that the ground under the trees is 

almost devoid of the normal forest litter and young pines are few. In 

open parks there is only a sparse growth of grass and brush. At lower 

altitudes the forest is of .the pinon- jumper type with a sparse cover of 

brush and grass occupying the spaces between the trees. Soils are 

generally thin and the valleys have no deep alluvial fills. The soils 

are predominantly sandy or gravelly, reflecting the characteristics of 

the parent rock.

BLACK CHEEK VALLEY

Black Creek valley extends from just south of the Sonsela Buttes 

to Puerco River and occupies the middle part of District 18. The 

valley as described by Gregory 4 varies widely throughout its course.

In the vicinity of Crystal its floor is flat and occupied by ephemeral lakes. 

Beginning at Hunters Point the valley narrows to one mile, then increases in 

width to Oak Spring, at which point it is replaced by a red-walled canyon, 600 

feet deep and less than half a mile wide, cut through the Defiance monocline. 

Below the canyon the valley gradually increases in width until it joins the 

Puerco.

Extending along the eastern edge of the valley the colorful sandstone cliffs rise abruptly 100 to 200 ft above the valley floor. Just 

south of Red Lake and again near Fort Defiance, volcanic rocks jut 

100 to 300 ft above the surrounding terrain.

Black Creek, the main stream in the valley, rises in the Chuska 

Mountains to the northeast. Its tributaries drain the eastern slope of 

the Defiance Plateau, the southwestern slopes of the Chuska Mountains, and the western slopes of the Manuelito Plateau. The channel 

has an average gradient of about 25 ft to the mile over its length of 

60 miles from the Chuska Mountains to Puerco River. Generally the 

flow is ephemeral, but a reach of several miles in the headwaters maintains a permanent stream. All the tributary drainages are comparatively short. Only a few, which are spring fed, are perennial, and 

these for short distances only.

2 Gregory, H. E., op. cit., pp. 22-49.

3 Gregory, H. E., op. cit., p. 35. 

* Gregory, H. B., op. cit., p. 32. 

983731 52 2
134 CONTRIBUTIONS TO HYDROLOGY, 1948-51

The small drainage areas that are tributary to Black Creek, especially from the east, have the largest sediment movement in the district. Gully erosion is extensively developed along almost all the 

streams draining the eastern slopes of the valley. This erosion can 

be attributed to a number of factors, the chief one being that these 

basins contain a deep alluvial fill mainly of fine sand derived from 

the adjacent sandstone and shale formations. This material is soft 

and noncohesive and therefore particularly vulnerable to erosion once 

the protective cover is depleted or destroyed by heavy grazing or in 

other ways. Steep slopes aid in the development of gullies. Intensive efforts have been made to stop the erosion in certain localities. 

The use of dams and spreader dikes in one of the side tributaries (nos. 

8 to 23 on pi. 7) is an example of such efforts.

Many of the dams were built as gully plugs intended only to reduce 

bank cutting and prevent further deepening of the channel; others 

were designed to divert water from the channel and spread it over 

adjacent areas, thus reducing crest flows and at the same time supplying water to range forage.

Channels of streams tributary to Black Creek on the west are generally carved in bedrock where they cross the Defiance monocline, so 

that the sediment load is small although the gradients are steep. On 

entering the main valley, however, the streams tend to gully, and the 

sediment load increases accordingly.

MANUELITO PLATEAU

The Manuelito Plateau occupies the area east of the Black Creek 

valley, south of latitude 35°45'. Of this geographic subprovince 

Gregory 5 says,

In this area the high points are remnants of horizontal sandstone beds and are 

usually sharply defined by cliffs on all sides. The valleys are broad, open, flatfloored washes, in many places trenched by narrow arroyos cut in material which 

covers the rock floor. The valley slopes are gentle, and gullies rather than hills 

impede progress.

The area drains southward into Puerco River and thence into Little 

Colorado River. All the streams are ephemeral. Gullies are numerous, although the gradients of the streams incised in the valley floors 

are generally flatter than those on the Defiance Plateau and in Black 

Creek valley. The alluvial fill is fine-textured, a mixture of sand, 

silt, and clay derived from the underlying sandstone and shale formations of Cretaceous age. It is evident that the easily erosible material 

in this area exceeds that in the Black Creek tributaries, but the measurements show that erosion and movement of sediment are less. Reasons for this inconsistency are discussed later.

5 Gregory, H. B., op. cit., p. 26.
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 135

Reservoirs 1 and 2 are outside the geographic subprovinees' designated by Gregory, but as they occur on the same geologic formation 

as that underlying the Manuelito Plateau, they exhibit similar sediment-movement characteristics. The two reservoirs are several hundred feet higher than the plateau area, but otherwise their drainage 

basins are comparable to those found on the Manuelito PlateaUi,

GEOLOGY

Axiomatically erosion in any area is confined mainly to the soil 

mantle. As the type of soil is in turn dependent on the rock from 

which it originates, there is a close relationship between erosion and 

geology. Exceptions to this rule occur in the study area in places 

where some of the surface rocks are so poorly indurated that they 

weather rapidly and break down into particles small enough to be 

readily transported by wind and water. Soils do not accumulate on 

such formations, and the relationship between geology and erosion is 

a direct one. Also it should be noted that there is close similarity between the geology of the study area and that of the surrounding 

region. Formations like those described are widely distributed over 

much of northeastern Arizona and northwestern New Mexico. Thus, 

it is logical to assume that methods of measuring erosion and movement of sediment in the study area are in some degree applicable to 

the broad region in which the geology is similar.

Geologic information was obtained mainly from Gregory's descriptions and map of the area 6 and from field examinations of individual 

drainage basins. As previously noted, a reproduction of part of his 

map is included as plate 7 of this report. Modifications have been 

made by grouping a number of formations having similar characteristics under one heading. A short description of the formations in the 

area and the soil types that result from weathering of these rocks 

follows.

Essentially the area is underlain by three types of rocks: conglomerate, sandstone, and shale. As these rocks weather, the type of soil 

formed varies according to the parent rock. Conglomerates disintegrate into pebbles and minor amounts of sand and silt; sandstone 

into individual sand grains; and shales into clays and fine silts. Residual soils closely reflect this composition, but transported material may 

deviate considerably because the finer constituents are carried away 

in suspension. In general, transported soils are coarser than their 

residual counterpart.

The reservoirs are underlain in part by residual soil and in part by 

transported soil. As the reservoirs themselves are located on stream

8 Gregory, H. B., The Navajo country : U. S. Geol. Survey Water-Supply Paper 380,1916 ; 

Geology of the Navajo country : U. S. Geol. Survey Prof. Paper 93,1917.
136 CONTRIBUTIONS TO HYDROLOGY, 1948-51

channels, the immediate upstream area nearly always is underlain by 

an alluvial fill of varying thickness. In some places most of the sediment entering the reservoirs comes from the deposit of transported 

soils; in others there is evidence that residual soils are the greatest 

contributors. Specific data about the individual reservoirs and their 

relation to the geology and segmentation rate are given in the latter 

part of the report.

The rocks underlying the area covered in this report are discussed 

in the following sections.

SEDIMENTARY BOCKS

The consolidated sedimentary rocks range in age from Permian to 

to Tertiary. Triassic, Jurassic, and Cretaceous rocks are more widely 

distributed in the eastern part of the area, whereas the older Permian 

strata are restricted to a narrow belt along its western edge.

PERMIAN ROCKS

The De Chelly sandstone member of the Cutler formation is the 

only formation of Permian age that crops out in this area. It is a 

light-red uniform-grained, relatively coarse textured cross-bedded 

well-indurated sandstone. The exposure forms a belt along the western edge of the Defiance monocline.

TRIASSIC ROCKS

The Moenkopi formation, which on the map is shown differentiated 

from the De Chelly sandstone member of the Cutler formation, is also 

exposed in restricted localities along the Defiance monocline. The 

formation consists of chocolate-red and banded arenaceous shales and 

thin sandstone beds. Because of their limited extent within the area 

of study, the beds have little, if any, influence on sediment production 

in the reservoirs examined.

The Shinarump conglomerate overlies the Moenkopi formation and 

consists of a relatively thin series of cross-bedded lenticular conglomerate and sandstone ranging in thickness from a trace to 100 ft. The 

pebbles are chiefly quartz, quartzite, and petrified wood. The Shinarump conglomerate forms a belt along the eastern edge of the Defiance 

Plateau.

The Chinle formation as used by Gregory in this area includes in its 

upper part a representative of the Glen Canyon group of Jurassic age. 

For the purposes of this report the authors have followed Gregory's 

usage. The formation underlies the floor of Black Creek and is exposed along the flanks of the valley at many localities. Only the shales 

and sandstone members of the formation were observed in this district, although the limestone conglomerate and the gypsiferous and 

calcareous members may also be present.
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 137 

JURASSIC BOCKS

Jurassic rocks are undifferentiated in this report. They include 

highly colored sandstones that form the prominent escarpment along 

the eastern edge of Black Creek valley. In general, the sandstones 

represent wind deposits and are uniformly fine grained. In weathering, the rock breaks down into a fine-textured noncohesive highly 

erosible mantle. The unit that overlies the sandstone consists of alternating beds of sandstone and shale with occasional lenses of gypsum. 

Like the underlying sandstone it weathers into highly erosible sandy 

mantles.

CRETACEOUS BOCKS

The Cretaceous formations, as mapped, include the basal Dakota 

sandstone, the Mancos shale, and the Mesaverde formation with included later Cretaceous formations undifferentiated. None of the 

reservoirs studied are located in areas underlain by either the Dakota 

sandstone or the Mancos shale. Both these formations occur in a 

narrow, relatively steep belt along the western edge of the Manuelito 

Plateau. Areas underlain by the Mancos shale are probably among 

the highest silt producers in the district, and reservoirs rapidly fill 

with silt, which probably accounts for the general lack of reservoirs. 

The Dakota is a well-indurated, conglomeratic sandstone containing 

occasional beds of shale. Because of its resistance and limited area! 

distribution, it is not an important contributor of sediment.

The Mesaverde formation and later Cretaceous formations (undifferentiated) crop out over most of the Manuelito Plateau. Characterized by alternating beds of shale and massive sandstone, they weather 

into mesas with relatively steep slopes separated by gently sloping 

alluvial-filled valleys. The mesa tops contribute little sediment, but 

the slopes and valleys are vulnerable to erosion.

TERTIARY ROCKS

Only two small areas of the Chuska sandstone are present. Both 

are located near the Sonsela Buttes in the northern part of the area. 

Gray and white fine- to medium-grained sandstone, with lenses of 

conglomerate, are typical of this formation. No reservoirs are located 

near the outcrops.

IGNEOUS BOOKS

Tertiary volcanic rocks consisting of tuffs, ash, and lavas are scattered throughout the area. Also in the area are a few small volcanic 

necks and dikes. No reservoirs are located within the volcanic region.

STErTTCTUBrE

Generally the structure of the area is relatively simple. The Defiance monocline extends northward along the western edge of the De-
138 CONTRIBUTIONS TO HYDROLOGY, 1948-51

fiance Plateau. Exposures of the Shinarump conglomerate and the 

undifferentiatecl Permian and Triassic sedimentary rocks form dip 

slopes along the monocline. The Black Creek valley has been carved 

in the soft Chinle formation at a lower altitude along the monocline. 

Eastward from the valley the dip flattens and the beds rise in steplike 

fashion to the Manuelito Plateau, which is capped by horizontal beds 

of the Mesaverde formation.

PRECIPITATION

According to available data the average precipitation of .the area 

under study is about 12 in. annually. Mean annual precipitation at 

individual points probably ranges from 10 in. to 16 in. depending 

upon altitude, exposure, and other factors. Few records of precipitation within the area are available. Therefore, all estimates used in 

this study are based on the records obtained at the station near Fort 

Defiance as reported for Arizona by the U. S. Weather Bureau in 

Monthly Climatological Data. The mean annual precipitation at 

the Fort Defiance station for the period 1931-47 was 12.94 in.

Precipitation records for 37 additional years at stations in the vicinity of Fort Defiance were located after the above-mentioned computations were completed. The additional records were obtained at Fort 

Defiance prior to 1906 and at St. Michaels for the period 1906-24. 

As they indicate only slightly higher averages than those shown for 

the annual and for the summer precipitation at Fort Defiance during 

the period used, it was not necessary to revise the computations.

Approximately half the annual precipitation occurs as summer rain; 

part of the rest is snow. Usually the snow melt evaporates or sinks 

into the ground, but occasionally high temperatures, at times accompanied by rain, causes a small amount of winter runoff. Infrequently 

a convection-type storm occurring in early spring or late fall produces 

runoff, and on rare occasions heavy general winter rains are the cause 

of major floods of wide extent.

Convection-type storms produce most of the runoff in small basins. 

This runoff usually is only a small part of the precipitation, the larger 

part being absorbed by the soil, where it is utilized by plants or is lost 

by evaporation. The amount absorbed by the soil is governed by the 

infiltration rate, and only precipitation in excess of this rate produces 

runoff. Summer storms, although they may be of small magnitude, 

are generally of high intensity, and therefore many of them exceed 

the infiltration rates and produce runoff. However, there is still 

considerable doubt as to the minimum amount of even intense rain 

necessary to cause runoff. Comparison of precipitation and runoff 

at a number of locations in Arizona has shown that summer storms of 

less than 0.25 in. per day usually do not produce any flow*
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 139

When the factors influencing the infiltration rate in a given area, 

are known, an estimate can be made of the amount of precipitation 

per day necessary to cause runoff, the runoff representing the excess 

of rainfall over infiltration. Figure 18 shows the total summer 

precipation at Fort Defiance compared with the proportion that falls 

at rates exceeding 0.50 in. per day. The assumption is made that 

in this case a 0.50 in. storm is necessary to produce runoff. It can 

be seen that under this assumption the greatest runoff would have 

occurred during 1945 and that no runoff would have occurred during 

1935, 1942, and 1943.

1

Totol precipitotion June to September, inclusive

Total precipitotion in excess of 0.50 inch per 

day June to September, inclusive

1 ^ I I

, i

I

. _ .

% % V I 1 I

I 

I

Ii

-

1931 1935 1940 1949 

FIGURE 18. Summer precipitation near Fort Defiance, Ariz.

In order to show the proportion of precipitation available for runoff, 

duration curves have been developed in figure 19 for the Fort Defiance 

station. These show the total summer precipitation in excess of various 

amounts per day. Daily precipitation was used, as this is the shortest 

interval record available. No effort was made to plot daily precipitation in excess of 1 in. because of the infrequent occurrence of storms 

of this magnitude, but a mental extrapolation of these curves will 

give some indication of the occurrence of the larger storms.

Only one storm producing more than 2 in. of rainfall per day was 

recorded during the 18 summers of record, but it will be noted from
140 CO1STRIBUTIONS TO HYDROLOGY, 1948-51

figure 19 that storms producing 1 in. or more of rainfall per day 

occurred during 50 percent of the summers, and storms producing 

0.5 in. of rainfall per day occurred during 85 percent of the summers. 

Naturally, the runoff from storms of these magnitudes varies within 

wide limits, but the variation in sediment load doubtless is much

Average for period 1945 47

(P>X) Total precipitation in excess of 

X inches per day

10 20 30 40 50 60 70 80 90 100

PERCENT OF SUMMERS ON RECORD 

FIGURE 19. Magnitude of summer storm precipitation, Fort Defiance, Ariz.

greater because the sediment-carrying capacity of any stream increases 

in greater degree than the magnitude of the storms and velocity of 

flow. Thus comparisons of rates of sedimentation at different reservoirs may not be entirely consistent, even where the measurements are 

obtained during synchronous periods, as relatively small local variations in the intensity and amount of precipitation in the area may be 

reflected by much greater variations in sediment movement. No pre-
Data from reservoirs impounded ty dams, dikes, and spreaders in District 18, near Fort Defiance, Navajo Indian Reservation, Ariss. and N. Mea.

[Accuracy of records: G, good; F, fair; P, poor]

usos

reservoir 

no.

(1)

1 

2 

3 

4 

5 

6 

7

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35

Indian Service 

reservoir 

no.

(2)

18A-247. 

18A-285 _ _

18A-312 

18A-41 

18A-42.. . ....

18A-313 

AAA-46-2... 

AAA-46-3. __ ..

AAA-46-12. 

AAA-43-14 ...

AAA-46-4 

AAA-46-5 ___.

AAA-46-1 ... .

AAA-43-13__._.-

AAA-43-16 

18-A-52 ... .

AAA-47 ...

AAA -43-12....,-

18-A-317.. 

AAA-43-5... .

18A-146-. ___ -

18A-145.. -

AAA-43-8 

18A-194 . 

18A-232.. .......

1-42 

Location

Latitude Longitude South 

(miles)

West 

(miles)

(3)

0 /

35 30 

35 30 

36 00 

36 15 

36 15 

36 15 

36 15

36 00 

36 00 

36 00 

36 00 

36 00 

36 00 

36 00 

36 00 

36 00 

36 00 

36 00 

36 00 

36 00

36 00 

36 00 

36 00 

36 00 

35 45 

35 30 

35 30 

35 30 

35 45 

35 45 

35 45 

35 45 

35 45 

35 45 

35 45

0 /

108 45 

108 30 

109 00 

109 00 

109 00 

108 45 

109 00

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

109 00 

108 45 

108 45 

109 00

7.88 

10.50 

1.00 

15.00 

13.00 

13.5? 

12.75

10.8 

10.8 

11.3 

11.0 

11.0 

11.2 

10.3? 

10.4 

10.5 

10.7 

10.6 

11.1 

11.2

11.2 

11.2 

11.9 

9.25 

6.85 

6.8 

12.7 

14.9 

11.6 

14.75 

.5 

.8 

10 

10.4 

1.6

0.62 

9.75 

8.50 

9.50 

10.50 

9.7? 

4.2

.1

.2 

.5 

.6 

.6 

1.6 

1.7? 

1.85 

1.76 

1.8 

1.7 

1.9 

2.0

2.0 

2.0 

2.6 

9.0 

5.05 

7.6 

6.6 

13.9 

10.1 

11.6 

2.6 

2.0 

4.9 

6.0 

2.1

Drainage basin

Altitude 

(feet)

(4)

7,150 

7,325 

7,650 

7,525 

7,550 

7,625 

7,425

7,150 

7,140 

7,150 

7,150 

7,125 

7,000

7,100 

to6, goo

6, goo 6, goo

6,800 

7,600 

6,650 

7,100 

6,400 

5,950 

7,650 

7,350 

6,950 

7,000 

7,425 

7,400 

7,000

Area 

(square 

miles)

(6)

0.2 

1.8 

1.1 

.4 

1.1 

«2.1 

.5

9.3 <«9.4 

1.2 

.2 

Dike 

 12.4 

/ Dike 

.3 

Dike 

 .07 

.7 <«1.2 

<«14.5

/.I 

Dike 

 15.1 

4.7 

.6 

.1 

2.1 

».l 

.2 

2.4 .5 

2.3 

2.3 

.15 

18.2

Length 

(miles)

(6)

0.3 

3.8 

1.1 

.8 

2.4 

«2.0 

1.1

6.2 

 6.3 

1.7

.8

<«7.6

.4

 .7 

1.4 

 »L7 

<«8.1

.5

<I9.0 

3.5 

1.3 

.3 

3.6 

.7 

.8 

2.9 

 1.3 

2.8 

2.0 

.6 

7.0

Maximum 

relief 

(feet)

(7)

75 

700 

150 

100 

150 

«500 

100

1,400 

 1,400 

1,000 

900

 « 1, 500

50

 100 

1,000 

* 1,000 

* 1,600

100

1,700 

400 

150 

50 

700 

50 

50 

300 

75 

500 

150 

100 

900

Geologic 

symbol

(8)

Kmv 

Kmv 

Trp 

Trs 

Trs 

Ju 

Trc

Ju+Km 

Ju+Km 

Ju+Km 

Ju+Km 

Ju+Km 

Ju+Km 

Trc 

Trc 

Trc 

Trc 

Trc+Ju 

Trc+Ju 

Trc+Km

Trc+Km 

Trc+Km 

Trc+Km 

Trp 

Trc 

Trc? 

Trc? 

Trc+Trs 

Trp 

Trp 

Trc+Ju 

Ju 

Kmv 

Kmv 

Ju+Km

Reservoir

Area 

(acres)

(9)

2.5 

5.5 

7.75 

4.58 

2.5 

1.9 

17.0

.44 

.82

.77

.2 

.5 

1.0 

1.6

2.8

2.2 

7.6 

2.2 

.23 

5.1

2.3

4.8

2.8

11.0

Original 

depth 

(feet)

(10)

8.0 

7.2 

18± 

10 

14.6 

7.9 

15

18 

19 

7 

7.1

17 

3.5 

12 

4 

6 

3.5 

7 

7

5

7.5

11

10.5 

19.5 

6+ 

7 

15

15 

10.5 

8 

26

1948 depth 

(feet)

(ID

3.9 

3.4 

17 

9.5 

10.6 

6.4 

14.2

3.8

1.7 

9 

3 

3.5 

2.25 

2 

0

4

3.5 

19± 

9± 

10.5 

17.5 

2+ 

6.8 

14

6.1

5

Capacity 

(acre-feet)

(12)

4.88 

11.4 

48 

17.7 

13.3 

5.6 

65

1.4

.16 

4.7 

.3 

.7 

.5 

.5 

2.8

4.8 

.6 

4.3 

64 

6.5 

1.2 

35 

2 

6.8 

23.4 

11±

2.7 

1.0

Sediment

Volume 

(acre-feet)

(13)

1.9 

7.8 

5.5 

.8 

3.2 

2.1 

4.0

17.5 

19.2 

.7 

.5 

.03 

28.3 

.02 

.7 

.05 

.05 

.4 

1.3 

3.5

1.0 

.6 

6.7 

.6 

3 

.02 

1.8 

.5± 

.3 

.6 

.04 

8.6 

1.0 

.4 

100=h

Percent of 

original 

capacity

(14)

28 

41 

10 

4.3 

19 

27 

5.8

109 

107 

117 

26

105 

11 

13 

14 

7 

45 

72 

65

17 

100 

61 

1 

32 

1.6 

5 

20± 

4.2 

2.5 

.3 

150 

27 

29 

110±

Water storage at survey

Depth 

(feet)

(15)

0.5 

1.2 

11.5 

5.5 

6.5 

.3 

10.6

0 

0 

0 

1 

0 

0 

0 

1.0 

0 

0 

1 

0 

0

1.5 

0 

1.0 

8 

7.7 

1.0 

6.5

3.8 

10.5 

0 

0 

2.1 

0 

0

Volume 

(acre-feet)

(16)

0.12 

4.0 

17.0 

.9 

4.6 

.11 

10.8

0 

0 

0 

.02 

0 

0 

0 

.02 

0 

0 

.1 

0 

0

1.0 

0 

.4 

6.2 

.9 

.04 

5.0

1.8 

11.2 

0 

0 

.27 

0 

0

Spilling

(17)

Negligible _ . _ -

Some most years. . 

Large most years. .

Some, once or 

more.

Small _ .........

Some most yearsLarge and diversions. 

Small _______

Negligible. .

Negligible 

Age 

(years)

(18)

22 

»12 

6 

13 

13 

5 

6

2 

2 

2 

2 

2 

5 

2 

2

"2 

»2 

»2 

2

5

5 

12 

2 

5 

5 

5 

*12 

12 

6 

5 

14 

'10 

5

Accuracy 

of 

records

(19)

F 

F 

G 

F 

F 

P 

F

F 

F 

P 

G

F

G

P 

P 

F 

P

F 

F 

P 

F 

G 

P 

F 

G 

O 

G 

F 

P 

G

Sediment production (acrefeet/square 

mile)

(20)

9.5 

4.3 

5.0 

2.0 

2.9 

1.0 

8.0

1.9 

3.9 

.6

2.6

5.3

2.4

\ L4 

.6 

2.1 

6.0

6.3 

.13 

5.0 

.2 

.9 

6 

1.5 

.25 

.1 

3.7 

.4 

2.7 

6.5

Annual 

sediment 

production 

for period ° (acrefeet/square 

mile)

(21)

0.43 

.36 

.9 

.15 

.22 

.2 

1.3

1.3 

1.1 

1.2

.7

.3 

1.0

.01 

2.5 

.04 

.17 

1.0 

.12 

.02 

.2 

.7 

.03 

.27 

1.1

Estimated 

long-term 

annual 

sediment 

producion ° (acrefeet/square 

mile)

(22)

0.5 

.5 

.7 

.2 

.25

1.1

1.1 

1.1 

1.0

.9

.01 

2.0

.15

.1 

.02

1 

.03 

.3 

1.2

o Sediment in upstream reservoirs included.

b Estimate based on information from local residents and relationship to other reservoirs.

c Includes adjacent drainage cut into reservoir.

 > Entire drainage; not adjusted for upstream dikes and diversions.

 Contributing area only. \ 

f Receives water from reservoir 20 through pipe and ditch. _ 

EXPLANATION OF COLUMNS

1, 2. Self-explanatory.

3. Location obtained from files of Bureau of Indian Affairs. Locations are given in miles south 

and west from the stated point of latitude and longitude on 15-min quadrangles.

4. Altitude at reservoirs obtained by aneroid barometer.

5. Drainage areas planimetered from areal mosaics, scale, 1 ln.«=l mile, except for reservoirs 1 and 

2, which were determined in the field.

6. Lengths of basins from area! mosaics or drainage maps.

7. Maximum relief estimated in the field or taken from reconnaissance topographic maps having 

a contour interval of 200 ft.

8. Geology from field notes and Gregory's geologic map (pi. 7).

9. Reservoir areas at spillway level, planimetered or computed from field surveys. 

10,11. From data obtained in the field.

12. Capacity at spillway level in 1948 computed from area-depth relationship as measured in the 

field. In most cases three contours were surveyed.

13. Obtained by probing sediment deposits.

14. Percentages above 100 indicate that the reservoir is full and that deposition is now taking 

place above spillway level.

15. From data obtained in the field.

16. Computed from area-depth relationship as determined from field surveys. It is representative 

of conditions found at time of examination.

17. From data obtained in the field.

18. Age obtained from files of Bureau of Indian Affairs.

19. Appraisal of results at individual locations based on trap efficiency of reservoir, accuracy of 

sediment measurements, and length of records.

20. Sediment production per square mile (column 13 divided by column 5). Where upstream 

reservoirs have spilled, the computation includes all deposition in the upstream reservoirs.

21. Sediment production per square mile per year for the period of record (column 20 divided by 

column 18).

22. Estimate of long-term sedimentation based on data in column 20, length of record, and accuracy of records. Explanation of the adjustments used in compiling these figures is given in the 

description of the individual reservoirs, pp. 147-153.

983731 52 (Face p. 141)

SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 141

cise data are available for measuring these local differences, and no 

effort has been made to compensate for them. However, where the 

life span of the reservoirs is not sychronous, general adjustments have 

been introduced in an effort to make the long-term estimates of sediment production more nearly comparable.

METHOD OF MEASURING SEDIMENTATION

As noted previously, sedimentation was measured by determining 

the quantity of sediment deposited in each reservoir. When the age of 

the individual reservoir is known, this measure can be converted into 

an average annual yield from a drainage basin of known size. If the 

annual rate is computed in this way, reservoirs of different life span 

would be expected to show considerable variation in the sediment 

caught because of differences in precipitation. Eeference to figure 18 

shows the great disparity, not only in the total quantity of summer 

rainfall, but in the recurrence. Thus in 1945,42 percent of the summer 

rainfall occurred in daily storms producing more that 0.5 in., whereas 

22, 30, and 30 percent respectively, occurred in the next 3 years.

Reservoirs constructed in the past 2 or 3 years, of which there are 

several, would not be expected to show as large sediment deposition as 

those in operation during 1945. An effort was made to adjust for this 

difference, but as sediment deposits laid down in individual seasons 

could not be identified, attempts at adjustment were found to be 

impractical, and in the final comparison reservoirs that have been in 

operation only 2 or 3 years have not been used except where they 

exhibit some well-defined sedimentation characteristic that is consistently a characteristic of the older reservoirs.

Another source of error in comparing sedimentation within the 

various reservoirs is the lack of data on the amount of spilling. As a 

large part of the sediment is carried in suspension by streams entering 

the reservoirs, a part of this load undoubtedly would not be trapped 

during any period of overflow, even though a reduction in velocity 

did occur in the reservoir reach. The efficiency with which a reservoir 

traps sediment diminishes progressively as the reservoir is filled. No 

record of the amount of spilling is available, but so far as possible, 

suitable adjustments have been made for this loss on the basis of 

information from local observers, remaining capacity of the reservoir, 

and evidence of small or excessive flow through the spillway. These 

points are covered in more detail in the description of individual 

reservoirs.

The accompanying table (facing page) contains information on the 

drainage-basin characteristics of the individual reservoir sites and 

measured and computed data obtained from the field examination. 

No particular system was used for numbering the reservoirs; they 

were simply numbered in the order in which they were surveyed.
142 CONTRIBUTIONS TO HYDROLOGY, 1948-51 

RATES OF SEDIMENT PRODUCTION

In the following table the study reservoirs have been grouped on 

the basis of the geologic formation from which the soils of the drainage basins were derived, and the groups have been arranged in order 

of increased amount of sediment production. The difference between 

the amount of sediment derived annually from the first three groups 

and that derived from the last two is striking. Areas where the soil is 

derived from the Chinle formation and the Jurassic rocks produce 

about five times as much sediment as those underlain by the De Chelly 

sandstone member of the Cutler formation, the Shinarump conglomerate, and the shales and sandstones^ of the Mesaverde formation.

A part of the difference may be due to conditions within individual 

drainage basins, such as slope, type and density of vegetative cover, 

grazing use, variations in rainfall, and other features that admittedly 

have a direct influence on erosion. However, it is evident that these 

features are not solely responsible for the variations as, with one or 

two exceptions, there is rather close agreement in the rate of sedimentation at individual reservoirs within each group, and yet there 

is wide divergence among the groups. Thus, reservoir 3, which shows 

a sedimentation rate several times greater than the others in the area 

where _ the soil is derived from the undifferentiated Permian and 

Triassic rocks, still has a sedimentation rate well below that of reservoirs where the soil is derived from the Chinle formation and the 

Jurassic rocks. A similar variation in sedimentation rate is apparent 

in reservoirs located on soils derived from the Mesaverde and later 

Cretaceous formations, but even the highest production, recorded at 

reservoirs 1 and 2, is only about half that of the lowest producers 

in the Jurassic and Chinle groups.

Further investigation into reasons for the difference reveals that 

probably the controlling factor in the sedimentation rate at each of 

the reservoirs is the availability of erosible material in the drainage 

basin. There are three phases of the problem: the rate of weathering 

of the rock itself, the resistance to erosion inherent in the weathered 

<material, and the amount of weathered material stored within the 

canyons and valleys during some previous period and now made available for removal because of some change in environment leading to 

greater and more rapid runoff.

It is obvious that hard, well-indurated rock weathers slowly under 

the conditions prevailing in the Navajo country. Removal of the 

weathered particles by water, and to some extent by wind, proceeds 

almost as rapidly as the rock weathers, and deep soil mantles are 

seldom, if ever, present. This is characteristic of a large part" of 

the Defiance Plateau, which is underlain by the De Chelly sandstone 

member of the Cutler formation and the Shinarump conglomerate.
Comparative sediment production from various geolo

Geologic formation 

(with symbol) from 

which soils are derived

Undifferentiated Triassic and Permian rocks, 

Trp.

Shinarump conglomerate, Trs.

Mesaverde formation 

and later formations, 

Kmv.

Undifferentiated Jurassic rocks, Ju.

Chinle formation, Trc.

Reservoir 

no.

3

24 

29 

30

4 

5 

26

27

1 

2 

33 

34

11

13

15 

19

32

35

7

25

Area of 

drainage 

basin 

(square

miles)

1.1

4.7 

.2 

2.4

.4 

1.1 

.1 

2.1

.2 

1.8 

2.3 

.15

.2

12.4

.3 

1.2

2.3

18.2

.5

Length of 

record 

(years)

6

12 

12

iO

13

5 

5

22 

12 

14 

10

2

5

2 

2

5

5

Estimated

long-term annual sediment 

production 

(acre-ft per

sq mile of

drainage area)

0.7

.01 

.1 

.02

.20 

.25 

.40 

.15

.5 

.5 

.03 

.3

1.1

1.1

1.0 

.9

1.0

1.2

1.1

2.0

Average

annual sediment production for group 

(acre-ft per

sq mile of

drainage area)

0.2

.25

.3

1.1

1.6

Reservoirs all located o

open grassy flats. S

accumulations of fill

Reservoirs 4 and 5 loc

with open patches o

Erosion limited to sh

are underlain by gra

glomerate.

Reservoirs located on 

with relatively deep 

tered juniper intersp

most localities. Gu

Most reservoirs located 

ering of sandstones a

cover sparse grass a

erosion on toterfluvi

Soils are flne-textured 

ment comes from sh

is extensive, as at 

Deep fills occuring o

gully erosion. The 
144 CONTRIBUTIONS TO HYDROLOGY, 1948-51

Even the valleys are devoid of deep-fill deposits, and many of the 

stream channels are eroded to bedrock. In addition to its shallowness, the mantle is very pervious, and high-intensity storms are required to produce surface runoff. This aids in sustaining a moderately heavy vegetative cover, which in turn impedes erosion.

Possibly the rate of erosion on the Defiance Plateau approaches 

the normal as nearly as any observed in the study area. The weathering and the removal of the weathered rock appear to have been in 

balance for a long period. It is believed that grazing has not been 

sufficient to change materially the rate of erosion over the area as a 

whole, although there are some indications that deterioration of the 

cover owing to excessive grazing has had some effect in places.

In contrast with the De Chelly sandstone member of the Cutler 

formation and Shinarump conglomerate, the sandstones of the Jurassic and the shales of the Chinle formation weather rapidly to form 

naturally deep fine-textured mantles everywhere except on the steeper 

slopes. Kemoval of material from steeper slopes and subsequent deposition in areas of gentler slope have resulted in deep alluvial fills 

within the valleys and along the narrow canyon floors. So long as 

this fill was protected by an adequate vegetative cover there was little 

tendency to excessive erosion, but once the cover deteriorated or was 

destroyed, both sheet wash and gullying developed, contributing to 

the high rates of sedimentation indicated at reservoirs in areas where 

the sands were weathered from these rocks.

Contributing further to this high sedimentation rate is the lack 

of any inherent resistance to erosion by the alluvial fills. The sandstones of the Jurassic rocks break down into minute, rounded individual sand grains. Because of a deficiency of clay and silt, which 

ordinarily act as a cement, these deposits are particularly vulnerable 

to erosion, both from sheet wash on unprotected surfaces and from 

bank cutting in gullies. Some of the deepest gullies observed have 

developed in deposits of this origin.

Disintegrated material derived from the Chinle formation is, in 

general, fine-textured, consisting mainly of clay and silt mixed with 

minor amounts of sand. Possibly because of the relatively high content of both halite and gypsum in the deposits, the particles at the 

surface appear to be highly dispersed and thus are easily attacked by 

running water. Another factor that adds to the high rate of silt 

production from areas underlain by the Chinle formation is the low 

resistance of the rock itself to erosion. The shale facies of the formation are particularly soft and highly vulnerable to gullying and sheet 

erosion, and development of badland topography on the steeper slopes
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 145

is characteristic, as exemplified by the spectacular badland erosion in 

the Painted Desert and along the rim of Paradise Valley. Thus, although the sandstone beds may be sufficiently indurated to resist rapid 

erosion, unprotected outcrops of the Chinle formation probably contribute at least as much sediment as does its residual soil mantle.

One of the surprising anomalies apparent within the groups is the 

comparatively low rate of sediment production from areas underlain 

by the Mesaverde formation and later Cretaceous formations. From 

general appearances it might be expected that these areas would have 

one of the highest rates, as there is usually an abundance of alluvial 

material available for transportation by water. The Mesaverde and 

later Cretaceous formations consist of alternating layers of shale and 

sandstone, which weather to a characteristic topography of low, rolling 

hills and sandstone-capped mesas, separated by shallow alluvial-filled 

valleys, many of which have been gullied almost their full length. The 

residual surface mantle is generally deep but compact; the mixture of 

clay and sand, derived respectively from the shale and sandstone members, forms a cohesive soil which appears to have an unusual resistance 

to sheet wash. The transported alluvium in the valleys shows a similar 

resistance.

Observation in areas underlain by the Mesaverde formation and 

later Cretaceous formations indicates that gully erosion is probably 

the largest source of sediment. The fact that gullying has not developed to any appreciable extent in the drainage basins of any of the four 

reservoirs considered in this group may account for the relatively low 

sediment production. Had the reservoirs been located in basins where 

gullying was extensive, it is likely that the results would have been 

entirely different.

In addition to developing the geologic relationships, comparisons 

were attempted based on such factors as size of drainage area, topography and slope, type of cover, and grazing use. However, insufficient data relating to these factors particularly on slope and condition 

of the vegetative cover prevented the establishment of dependable 

relationships, even though variations in these factors should have 

some influence on erosion.

On the basis of sediment produced from the areas underlain by 

various geologic formations, the total sediment production from District 18 has been estimated at 900 acre-ft annually. The areas underlain by the rock formations discussed herein constitute 91 percent of 

District 18. The sediment production from each is shown in the 

following table.
146 CONTRIBUTIONS TO HYDROLOGY, 1948-51

Estimated annual sediment production from District 18, Navajb Indian 

Reservation, New Mexico-Arizona

Geologic symbol (see pi. 7)

Trp_.

Trs.-

Kmv 

Jn

Trc..

Total........ ............................

Percent of 

area

14 

10 

36 

9 

22

191

Total area 

(sqmi)

210 

ISO 

540 

130 

330

"1,360

Sediment production (acreft per sq mi)

0.21 

.25 

.3 

1.1

1.56

Total sediment production (acre-ft)

45 

35 

160 

140 

530

910

' Remainder of District 18 is underlain by rocks for which no information on sedimentation rates is 

available.

An annual rate of sediment production of this magnitude, from an 

area of the size and character studied, appears inordinately high. 

Applying this rate to 22,100 sq mi area of the Little Colorado Eiver 

basin above the Grand Falls gaging station, the annual rate of sediment production would be about 13,000 acre-ft. The only actual measurements of suspended load 7 carried by the river were made at the 

Grand Falls station during the flood of July 6 to September 26,1931. 

The records show that approximately 5,500 acre-ft of sediment passed 

the station during this period, obviously an extraordinarily large 

movement. The inconsistency between this figure and the above estimate is explained by the fact that total sediment movement within 

a given basin is not necessarily a measure of the total sediment carried 

out of the basin through main drainage channels. The reservoir 

accumulations represent movement from small segments of the basin 

located mainly in the steeper and more dissected headwater areas 

of the tributaries. Had the sediment not been trapped in the reservoir, at least some would have been deposited in flatter reaches of 

the downstream flood plain. Only the part of the sediment that actually reaches the main channel through the principal trunk gully systems or other well-defined water courses can be carried out of the 

basin during flood periods. Moreover, the lower parts of river basins 

contain areas of flat slopes that may have somewhat lower rates of 

sediment production than headwater areas that contain suitable sites 

for small reservoirs.

It should be noted also that the summer storms that cause sediment 

movement in small areas usually are local in extent and do not produce sufficient runoff to carry the sediment any great distance. There 

is nearly always a marked recession in the flood peaks as they move 

downstream, and part of the sediment load is dropped as a result. 

This, of course, is the chief process whereby the deep valley fills have 

been formed. It is believed that if, in such an area as the Little Col-

' Howard, C. S., 1947, Suspended sediment in the Colorado River, 1925-1941: D. S. Geol. 

Survey Water-Supply Paper 998, pp. 144-145.
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 147

orado River basin, a storm occurred that was large enough to cause 

flow without marked diminution through all the tributaries, the total 

sediment movement would more nearly approach that shown by the 

reservoir measurements. For smaller storms the measurements simply 

give an indication of the potential sediment production for various 

parts of the basin.

DESCRIPTION OF INDIVIDUAL RESERVOIRS

No. 1. Reservoir 1 is known to have spilled, but probably not significantly, except during recent years when sediment has reduced 

storage capacity. Present source of sediment is largely from sheet 

erosion of residual soils, as the drainage area has not been gullied 

to any appreciable extent. The higher ridges have a pinon-juniper 

cover, but the lower slopes and valley floors are grass-covered. In 

some places there is evidence of overgrazing. When first constructed, 

the reservoir provided a nearly permanent supply of water, but since 

loss of capacity from sedimentation has occurred it goes dry for a 

part of each summer. The estimated long-term sediment production 

was raised slightly upward to compensate for losses over the spillway.

No. 2. Large amounts of spilling, especially in recent years, probably account for the lower rate of sedimentation in reservoir 2 compared with reservoir 1. Cover and other physical conditions are 

similar except that part of the basin has been logged and a small 

part is cultivated. Sheet erosion is apparent, particularly in the 

logged-over area, and a discontinuous gully system has developed 

along parts of the main channel. The long-term sediment production 

has been estimated to be the same as at reservoir 1, chiefly because 

of known losses over the spillway. Probably current sedimentation 

is somewhat higher but improvement is expected as the cover increases 

on the logged-over area. The reservoir now has little value for stock 

watering because of its shallow depth.

No. 3. Reservoir 3 is located on the Defiance Plateau. Heavy 

forests cover the ridges and slopes, and narrow grassy valleys border 

the stream channels. Type of cover indicates the highest precipitation 

in the area. The reservoir undoubtedly always contains water, derived principally from winter and early spring runoff. Most sediment 

obviously comes from sheet erosion, although some of the grassy areas 

contain a few shallow gullies. In general, the amount of sediment 

produced from the area seems large, considering the forested character 

of the drainage basin. Trap efficiency in the reservoir is high, as 

indicated by the large capacity per unit of drainage area, although 

some spilling has occurred. The long-term estimate of sedimentation 

has been reduced somewhat to compensate for the above-average precipitation during the life of the reservoir.
148 CONTRIBUTIONS TO HYDROLOGY, 1948-51

No. If wnd 5. Reservoirs 4 and 5 are located on lower parts of the 

Defiance Plateau. The drainage slopes are gentle, and the sandy to 

gravelly soils are derived from the underlying Shinarump conglomerate. The cover is sage interspersed with grass and fringed with forest. 

Erosion is limited mainly to sheet wash, but there are a few shallow 

active gullies. Deposition has partly filled some of the existing gullies 

located as much as 1,000 ft upstream from the reservoirs 5 or 6 ft 

above spillway elevation. Some spilling has occurred at both reservoirs, and the long-term estimates of sediment production were raised 

slightly to compensate for this loss.

No. 6. Reservoir 6 has been breached owing to overtopping of the 

dam, caused directly by inadequate spillway construction. As a result, the data have little significance because there is no means of estimating the loss of sediment carried by the stream or the amount removed from the reservoir as a result of the dam failure. Therefore, 

no estimate has been made of the long-term sediment production.

No. 7. Reservoir 7 is one of the most useful examined during the 

study because it is of adequate size and well adapted to the drainage 

basin. It is used as a stock reservoir. Spilling has occurred at times, 

but it appears that generally most of the sediment has been trapped. 

Soils in the drainage basin are usually fme-textured, being derived 

mainly from the underlying shales of the Chinle formation. Sediment production is largely from sheet erosion, as gullying is not extensive. The estimated long-term sediment production was reduced 

somewhat below the measured rate to compensate for the greaterthan-average precipitation during the 6-year life span of the reservoir.

Nos. 8-$3. Reservoirs 8 to 23 are considered together because of 

their location and the purpose for which they were constructed. Not 

all are reservoirs in the strict sense as some are dikes utilized to divert 

and spread water and they provide no storage. Their location in the 

drainage basin is shown by the inset on plate 7.

Ostensibily the construction in this area was aimed at arresting 

severe gully erosion. The dikes were constructed to divert water from 

shallower parts of the channel and spread it over adjacent flats; 

larger dams formed reservoirs on deeper reaches of the gully system 

thus providing considerable storage for water and sediment detention. 

Prior to being completely filled with sediment the larger reservoirs 

provided water for stock. Reservoirs 13, 21, and 23 have been in operation 5 years, the others only 2 years. According to local Indian 

residents, all the available storage filled with sediment within a short 

period after completion, but even so there has been little movement 

of sediment out of the area as it has been deposited either behind the 

dams or on the spreading areas.

Some of the worst erosion observed in the district occurs within 

the general area in which this group of reservoirs is located. A dis-
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 149

continuous gully, which started below reservoir 23, extends upstream 

for several miles. In the reach between reservoirs 21 and 8, unfilled 

parts of the gully range in width from 25 to 100 ft and in depth from 

7 to 40 ft. The tributary gully on which reservoirs 14 to 19 are located 

is somewhat shallower; the average depth being about 7 ft. The gully 

on which reservoir 10 is located is 7 ft deep at the dam but deepens 

to 18 ft at its junction with the parent channel.

The erosion above reservoir 13 is taking place in a narrow canyon 

that cuts eastward through the high escarpment formed by the massive 

sandstones of the Jurassic rocks, which crop out along the east side 

of Black Creek valley. Prior to the recent cutting the canyon was 

filled to a depth of 10 to 40 ft with fine-textured sandy alluvium 

derived chiefly from the adjacent sandstone cliffs. This filling left 

a flat valley an eighth of a mile to half a mile wide between the sandstone outcrops. Because of its noncohesive character, the fill material is highly erosible. Vegetation is the only protection against 

erosion, and where gullies once start they tend to cut to bedrock on 

the steeper grades. Directly above reservoir 13 the gully depth is 

controlled by bedrock; below that point the grade lessens as Black 

Creek valley is approached, and the gully becomes shallower until it 

finally disappears below reservoir 23. Side gullies have developed 

in tributary valleys in the same manner, their depth in most localities 

being accordant with that of the parent streams.

Doubtless much of the sediment stored in the reservoirs and above 

the dikes comes from bank cutting and from the upstream advancement of headcuts in some of the tributary gullies. It is evident, however, that part of it is caused by sheet erosion on the valley floor, but 

probably an even larger amount is contributed by direct flow from 

the steep sidehill slopes. Vegetative cover on the slopes consists of a 

scattered growth of juniper and pinon trees and some shrubs interspersed with areas of bare rock. The valley floor has a sparse to 

moderate covering of grass and low shrubs, which generally is not 

dense enough to prevent sheet wash.

The drainage areas above reservoirs 11, 13, 15, and 19 are eroding 

in the manner described above. Reservoir 13 is the only one of the 

group located on the main gully and sediment movement from the 

drainage areas above this point is considered to include the total 

deposition at all localities on the main stream channel during the 

period of record. The drainage area above reservoir 11 is not gullied, 

and silt production from the basin may be considered as indicative of 

normal sheet wash from valley floor and relatively steep sidehill areas 

are small compared with that above reservoir 13. In other respects 

the drainage basins are similar.

A significant feature of the findings is the close agreement of the 

rates of sedimentation found at each measuring point. The rates at
150 CONTRIBUTIONS TO HYDROLOGY, 1948-51

the four stations range from 1.0 to 1.3 acre-ft per square mile of 

drainage area per year. This variation is probably within the limits 

of error in measuring, so that the rate at each may be considered 

identical. As it might be expected that gullied areas would show 

greater movement of sediment than ungullied ones, the situation is 

somewhat anomalous and difficult to explain. The most logical explanation appears to be that, because of the highly erosible character 

of the sandy soil derived from weathering of the sandstones of 

Jurassic rocks, all flows that cross these soils, either on an unprotected 

surface or in gullied channels, carry practically a capacity load of 

sediment. On the assumption that runoff from equivalent areas is 

about the same, the indicated relation would hold. Observations in 

other localities having these same soil types show a similar high 

erosion rate, indicating that generally such soils can be classed as 

among the most erosible found in the whole area.

No. @4> Some spilling has occurred at reservoir 24 through an 

outlet pipe, but it is believed that only a negligible amount of sediment has escaped. A permanent spring maintains a good vegetative 

growth in the main drainage channel leading to the reservoir, thus 

reducing bank cutting to a minimum. The drainage area in general 

has a fair grass cover, which prevents excessive sediment movement. 

Erosion from the north fork of the drainage basin has caused deposition of a small fan in the channel near the outer edge of the reservoir, 

but this material did not invade the deeper storage. The sediment 

deposit was difficult to measure, owing to the indistinct contact between the newly deposited sediment and the original floor of the 

reservoir. The record is not accorded much weight on this account.

No. 25. Reservoir 25, located on the floor of Black Creek valley, 

has the highest rate of sediment production measured during 

the study. The drainage area reflects this condition; both sheet 

and gully erosion are developed to a greater degree than in the surrounding area. Part of this can be attributed to rather severe overgrazing, owing to the basin's central location near the Indian settlements. The reservoir is known to have spilled considerably, but 

there is evidence that most of the sediment load has been retained. 

The long-term estimate of sediment production from the area has been 

reduced somewhat from the measured amount to compensate for high 

precipitation during the period of record.

Nos. 26 and 27. The drainage basins of reservoirs 26 and 27 are adjacent, and both are located in areas underlain by the sandstones and 

shales of the Chinle formation. The small drainage area of no. 26 

is unusually flat. In addition, a coarser facies of the Chinle formation, which strongly resembles and may actually be a part of the 

Shinarump conglomerate, predominates throughout the basin.
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 151

Weathering produces a coarse, gravelly soil containing numerous 

pebbles and pieces of petrified wood, and therefore sedimentation 

from the basin is low. Reservoir 27 drains the top of a low mesa, 

which has a sparse cover of juniper and shrubs. The basin is underlain by coarse sandstone interspersed with conglomerate, and shallow 

gravelly and sandy soils are characteristic. No gullies have developed, but in places there are indications of sheet erosion. Along 

the channel floor there is considerable gravel, very little of which has 

been carried to the reservoir. The long-term estimate of sediment 

production in both reservoirs has been lowered somewhat to compensate for the above-normal precipitation prevailing during the past few 

years. Because of their basin characteristics both reservoirs have 

been grouped with other reservoirs located on areas underlain by the 

Shinarump conglomerate.

No. %8. Reservoir 28 is on the main channel of the Puerco 

below its junction with Black Creek and has thus been subjected to 

heavy flood and sediment movement from both streams. As these 

streams drain areas containing more than one formation, the total 

sediment deposition is not considered truly representative of any 

locality having characteristics related to a particular geologic formation, and the record, therefore, has not been used in making estimates 

of long-term sediment production.

Nos. 29 and 30. The adjacent basins draining to reservoirs 29 and 

30 are similar to no. 3. Both are located on the Defiance Plateau, 

which has a forest cover of ponderosa pine on the ridges and grassy 

flats along the channel flood plains. No. 30 had one of the lowest 

measured sedimentation rates, the obvious reason being its location at 

the lower edge of a long, gently sloping grassy flat, which has trapped 

much of the sediment carried by the stream. Soils in both basins 

are thin and sandy, containing in many places well-rounded gravels 

that are residuals from underlying conglomerates. Deep accumulations of fill are absent, and as a consequence few gullies have developed. 

Some spilling has occurred at no. 29, but it is believed that little if any 

sediment was lost through this overflow and that the measured rate 

of sedimentation is about normal for the long-term expectancy for 

the type of terrain found in this drainage basin. Reservoir 30 is 

located below a seepage area, and the shallow, gullied inflowing channel is bordered by sedges and grass, which indicates that the flow is 

probably permanent.

No. 31. Although the sedimentation in this reservoir was measured, 

it was afterward found that the greater part of the flow from the 

basin had been diverted into an adjacent area. Therefore, the apparent rates of sedimentation have no significance in making an 

estimate of long-term production.
152 CONTRIBUTIONS TO HYDROLOGY, 1948-51

No. 32. The drainage area above reservoir 32 is underlain by 

sandy soils similar to those found in the vicinity of reservoirs 8 to 23. 

Indicated rates of sediment production are somewhat lower than at 

the latter location, but this can be attributed to excessive spilling 

caused by the reservoir being completely filled with sediment.

Nos. 33 and 34. Reservoirs 33 and 34, located on the Manuelitb 

Plateau, have drainage areas underlain by sandstones and shales of 

the Mesaverde formation and later Cretaceous formations. Both 

basins have a cover of scattered juniper interspersed with grassy areas. 

Overgrazing is apparent, and generally the small valleys are deeply 

gullied; yet silt contribution from the two basins is low. This low 

rate of sedimentation can, in part, be attributed to the gentle slopes 

of the drainage basins, but apparently the soil itself must have an 

inherent resistance to erosion. The mixture of clay and fine sand 

that results from the disintegration of the underlying sandstones and 

shales seemingly forms a compact mantle from which particles are 

not easily detached and removed by water. Reservoir 33 shows an 

exceptionally low sedimention rate, which apparently is attributable 

only to the inherent resistance of the soil to erosion, as the basin differs 

otherwise but little from adjacent areas.

No. 35. Gage readings on water levels were begun at reservoir 35 

in April 1945 and were continued until the summer of 1947, when 

sedimentation had completely filled all available storage. Originally 

the reservoir had a large storage capacity and it had been hoped that 

runoff data provided by the gage readings would be useful in supplementary hydrologic studies on stock reservoirs. However, sedimentation occurred at such a fast rate that the original capacity survey, made at the time that the gages were installed, was obsolete after 

the first storm. No resurvey of the capacity was attempted, as the 

gages silted over rapidly. By analyzing the readings of water levels 

and the notes supplied by the observer regarding the level of the sediment line, it has been possible to make an estimate of sedimentation 

rate.

Owing to the lack of an accurate capacity curve and because there 

was considerable ungaged spilling from the reservoir, it has been 

impossible to compute runoff from the drainage basin. There are 

indications, however, that 150 to 200 acre-ft of runoff occurred during the latter part of July and the first part of August 1945. This 

flow deposited about 50 acre-ft of sediment. Additional deposition 

occurred during 1946 and storms occurring in 1947 completed the 

filling of the reservoir, covered a part of the spillway and left some 

sediment in adjacent fields. All gages are buried at present, and the 

station is no longer operative. Approximately 100 acre-ft of sediment is stored behind the structure.
SEDIMENTATION RATES LITTLE COLORADO RIVER BASIN 153

Examination of the drainage basin disclosed that the sediment was 

from several different sources. Much of the debris within the reservoir is coarse sand, which doubtless is the coarser part of the original 

soil remaining after the finer particles were carried off in suspension, 

the sorting having taken place both in the reservoir and in the channels leading to the reservoir. The drainage area contains numerous 

gullies and side rills, and caving of the gully banks obviously has 

provided a large part of the sediment. Sheet erosion is also visible. 

Headcutting of the main channels has now proceeded until it is 

approaching the drainage divide, so there is no longer any substantial 

contribution from this source. How much of this headcutting 

occurred during the period of record is unknown, as the position of 

the headcuts was not determined at the beginning of the studies.

Soils in the drainage area are derived mainly from weathering of 

the sandstone of the Jurassic rocks and have the same characteristic 

lack of cohesion and high erosibility as soils in the vicinity of reservoirs 8 to 23. Significantly the rate of sediment production is practically the same in both localities, thus strengthening the suggestion 

that such areas furnish a near-capacity load for any flow that crosses 

the soils, either on the surface or in channels.

INDEX

Abstract---.---.-.-...-.-.--....---------.-. 129

Black Creek valley................. 133.138,149-150

Chinle formation. __.. 136,138,142,144-145,149-150 

Chuska Mountains_______________ 133 

Chuska sandstone_____________ 137 

Colorado Plateau -..___-------.----.... 132

Cutler formation...__._._...--... 136,142-144

Dakota sandstone_______________ 137 

Dates of field surveys-.________ __.. 132 

De Chelly sandstone member._____ 136,142,144 

Defiance monocline-____-------- 133-134,136-137

Defiance Plateau... . .. 133,138,142-144,151 

District 18-__....__.___......__ 130,133

Erosion, measurement of..._________ 141 

rate of__._______...........__ 144

Fort Defiance... ......................... 133,138

Geologic history________________ 135-136 

Geologic map of the area. __ __ ______ pi. 7 

Glen Canyon group. _____________ 136 

Grand Falls gaging station...________ 146

Igneous rocks______________..__ 137

Little Colorado River...._____...___ 146-147 

Location of area_____________ 129-130,132

Mancos shale_________________ 137 

Manuelito Plateau.-...__.__. 134,137-138,152

Mesaverde formation___.. ___ 127,144-145,152 

Mexican Springs, N. Mex._________._ 132 

Moenkopi formation___._..________ 136

Navajo country.................... 129-131,133,142

Painted Desert. ______.._____ . 145 

Paradise Valley......................... 145

Precipitation, discussion of..______ 138-141 

Puerco River._____.__......._ 133-134,151

Bed Lake..._______.....__...___ 133

Reservoirs, capacity of ________ 132,141 

data pertaining to..________ 141 

description of ____.___. __ 135,147-153 

performance........_____...... 141

survey of._________________ 132

trap efficiency of_-...____-...- 141

Runoff, discussion of...:___________ 138-141

St. Michaels -..._..... ...... 138

Sediment, measurement of_______...-. 141

rates of production of________ 129- 

130,140,142-143,145-147

Sedimentary rocks____________ .. 136 

Shinarump conglomerate.________ 136 

Soils.. . 130

Sonsella Buttes ....-_______-_.... 133,137

Structure of area. _____________ 137-138

Vegetation.. ................. 130-131,133; fig. 17

Window Rock....__.__._____ 132

155

U. S. GOVERNMENT PRINTIRS OFFICE; 1952