Effect of Sediment 

Characteristics on Erosion 

and Deposition in 

Ephemeral-Stream Channels 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 352-C 

Effect of Sediment 

Characteristics on Erosion 

and Deposition in 

Ephemeral-Stream Channels 

By S. A. SCHUMM 

EROSION AND SEDIMENTATION IN A SEMIARID 

ENVIRONMENT 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 352-C 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 

Reprinted, 1962 

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

Washington 25, D.C. - Price 45 cents (paper cover) 
CONTENTS 

Abstract __________________________________________ _ 

Introduction ______________________________________ _ 

Acknowledgments __________________________________ _ 

Methods of investigation ___________________________ _ 

Areas of study _____________________________________ _ 

Sage Creek, S. Dak ____________________________ _ 

Sand Creek, Nebr ______________________________ _ 

Arroyo Calabasas, N. Mex ______________________ _ 

Bayou Gulch, Colo _____________________________ _ 

Medano Creek, Colo ____________________________ _ 

Comparison of sediment and channel characteristics 

between areas ___________________________________ _ 

Relation between sediment and channel characterPage 

31 

31 

32 

32 

33 

33 

41 

49 

53 

57 

61 

istics________________________________________ 61 

Summary of comparisons________________________ 62 

Deposition in ephemeral streams ____________ -------- __ 

Cross sections ______________ - ____ --------------- Slope _________________________________________ _ 

Vegetation ____________________________________ _ 

Causes of deposition ___________________________ _ 

Erosion in ~ephemeral streams ________________________ _ 

Channel incision _________________ - - - - - - - - - - - - - - - Bank caving __________________________________ _ 

Practical considerations _______________ --------------- References ________________________________________ _ 

ILLUSTRATIONS 

Page 

63 

63 

65 

67 

67 

68 

68 

69 

69 

70 

Page 

FIGURE 17. Index map showing location of study areas_____________________________________________________________ 33 

18. Index map of Sage Creek, S. Dak_____________________________________________________________________ 34 

19. Longitudinal profile and cross sections, Sage Creek, S. Dak _______________________________ --------------- 36 

20. Sage Creek, S.Dak., above section 8------------------------------------------------------------------ 38 

21. Sage Creek, S.Dak., below section 8------------------------------------------------------------------ 40 

22. Variations in channel and sediment characteristics, Sage Creek, S. Dak ___________________________ --- _----- 42 

23. Index map of Sand Creek, N ebr _________________________________________________________________ - --- _ 43 

24. Longitudinal profile and cross sections, Sand Creek, Nebr________________________________________________ 45 

25. Sand Creek, Nebr__________________________________________________________________________________ 46 

26. Aerial photographs of Sand Creek, N ebr _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 48 

27. Variations in channel and sediment characteristics, Sand Creek, Nebr_____________________________________ 49 

28. Index map of Arroyo Calabasas, N. Mex ___________________________________ :.. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 50 

29. Longitudinal profile and cross sections, Arroyo Calabasas, N. Mex ___________________________________ ----- 52 

30. Arroyo Calabasas, N. Mex_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 54 

31. Variations in channel and sediment characteristics, Arroyo Calabasas, N. Mex ________________________ ----- 56 

32. Index map of Bayou Gulch, Colo_____________________________________________________________________ 56 

33. Longitudinal profile and cross sections, Bayou Gulch, Colo_______________________________________________ 57 

34. Bayou Gulch and Medano Creek, Colo________________________________________________________________ 58 

35. Variations in channel and sediment characteristi~s, Bayou Gulch, Colo _______________________________ ----- 60 

36. Index map of Medano Creek, Colo____________________________________________________________________ 60 

37. Cross sections, Medano Creek, Colo___________________________________________________________________ 61 

38. Relation between width-depth ratio and weighted mean percent silt-clay _________________________ - ____ ----_ 63 

39. Relation between stream gradient and weighted mean percent silt-clay _______________________________ --___ 64 

40. Relation between width-depth ratio and weighted mean percent silt-clay___________________________________ 65 

41. Comparison of the longitudinal profile of Newlin Creek, Colo____________________________________________ 68 

TABLES 

Page 

TABLE 1. Precipitation at stations near the study areas ______________________________________________________ -- __ - 35 

2. Channel and sediment data, Sage Creek, S. Dak_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 35 

3. Channel and sediment data, Sand Creek, Nebr__________________________________________________________ 44 

4. Channel and sediment data, Arroyo Calabasas and Arroyo de los Frijoles, N. Mex_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 51 

5. Channel and sediment data, Bayou Gulch, Colo_________________________________________________________ 55 

6. Channel and sediment data, Medano Creek, Colo_______________________________________________________ 59 

7. Mean values of channel and sediment data_____________________________________________________________ 62 

8. Ratio of drainage area to channel length __________________________________________________________ -_-__ 68 

m 

EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 

THE EFFECT OF SEDIMENT CHARACTERISTICS ON EROSION AND DEPOSITION IN 

EPHEMERAL STREAM CHANNELS 

By S .. \. ScHUMM 

ABSTRACT 

This study of five semiarid valleys emphasizes the importance 

of physical properties of sediment in determining streamchannel shape and differences in the mechanics of erosion and 

deposition between areas. Prerequisites for selection of the 

five areas were a progressive decrease in the percent silt-clay 

in stream channels and banks, active aggradation or erosion 

within a reach of the stream channel, and nearly uniform 

lithology within each drainage basin. 

A comparison of the data obtained from each area demonstrates that in a drainage channel composed of fine-grained, 

highly cohesive sediment, deposition occurs on the sides of the 

channel as well as on the channel floor. The result is a reduction in the ehannel width-depth ratio across an aggrading reach. 

Vegetation seems to aid deposition by its rapid growth on 

recently deposited fine alluvium, but it is not the initial cause 

of aggradation. Bank caving yields only small amounts of 

sediment and, caved blocks are often nuclei for deposition along 

channel sides because of their resistance to disintegration. 

Degradation in the finer sediments is generally by upstream 

headcut migration. 

In contrast, those channels containing only small amounts of 

silt-elay are aggraded from bottom to top. No plastering of 

fine sediments on the banks oecurs. Less vegetation grows on 

these poorly cohesive, highly mobile sediments. Headcutting 

occurs only where the coarser sediments are capped by a layer 

of fine material. In general, a break in the longitudinal profile 

of this type channel is quickly removed by channel degradation. 

Bank caving seems to supply more sediment to the stream load, 

for the blocks of poorly cohesive alluvium disintegrate upon 

impact. 

Deposition causes marked ehanges in the ephemeral-stream 

channels. As the reaeh of maximum aggradation is approached 

channel gradient decreases, and depending on the amount of 

silt-clay in channel and banks, the width-depth ratio may increase or even decrease. In all ehannels studied percent siltclay increases and median grain size decreases with aggradation. At the lower end of an aggrading reach, the gradient 

steepens, but sediment is still fine and vegetation generally covers the entire valley floor. The reaeh of maximum deposition 

may migrate upstream with time, but renewed degradation on 

the downstream reach of steep gradient may cause a trench to 

cut through the valley plug, renewing through transport of sediment and runoff. 

The aggradation in the study areas is apparently a result of 

high sediment yields in the headwater parts of the drainage 

basins. Deposition in the channel occurs; however, where the 

rate of increase of drainage area per mile of channel length 

is low. In these reaches of small tributary contribution, water 

loss into the alluvium and the subsequent increase in sediment 

concentration causes deposition. 

The shape of stable cross sections, expressed as a width-depth 

ratio, is dependent on the weighted mean percent silt-clay in 

banks and channels such that width-depth ratio increases with 

decreased silt-clay in the alluvium. Gradient also shows an 

inverse relation to weighted mean percent silt-clay for these 

small streams of low annual discharge. 

It is suggested that the relation between channel shape and 

silt-clay can be used as a criterion of channel stability, for 

aggrading channels generally pl.!)t well above the width-depth, 

silt-clay regression line; whereas, degrading channels plot below the line. 

The study suggests that preventive conservation may be the 

most practical solution to some erosion problems, such as 

arroyo cutting. Deposition, if it is desired to fill a trenched 

channel, should be induced in reaches where conditions are 

most favorable for natural aggradation. Conservation measures 

should be modified depending on the character of the valley 

and its alluvium. Only certain critical reaches of a channel 

need be controlled to prevent erosion over larger areas. 

INTRODUCTION 

Recently there has been an increase in studies of 

stream morphology. Leopold and Maddock ( 1953) 

have discussed downstream changes in width and depth 

of a stream channel with increased discharge. Additional studies by Wolman ( 1955), Leopold and Miller 

( 1956), and Wolman and Leopold (1957) have tended 

to confirm Leopold and Maddock's conclusions under 

diverse conditions and have revealed 1nuch about 

stream-channel development. The writer in another 

investigation introduced a parameter for sediment type 

and stressed the importance of the effect of sediment 

type on channel shape (Schumm, 1960b). All the 

above investigations were restricted to stable channels, 

or at least channels that were not rapidly aggrading or 

degrading._ The present study concerns some small 

ephemeral-stream channels that are being actively 

aggraded or eroded. 

31 
32 EFFECT OF SEDIMENT CHARACTERISTICS 

A know ledge of the mechanics of deposition and 

erosion in semiarid valleys would not only be academically valuable, but could also provide the basis for 

conservation .measures aimed at restoring gullied valleys to their prior condition. Much has been written on 

the need for careful use of the semiarid valley floor, for 

it may be the only productive land within a drainage 

basin. For example, Peterson ( 1950), as a result of 

interviews with several early settlers, states that at the 

time of white settlement in the valley of San Simon 

Creek, Ariz., the valley floor was flat and unbroken. 

Large areas were covered with grass, luxuriant enough 

to be harvested for hay. Th~ creek itself was perennial 

throughout most of its length and lined with trees. 

During the 1880's, 50,000 head of cattle are said to 

have grazed the yalley. At present the valley floor is 

trenched from its confluence with the Gila River for 

nearly 70 miles upstream, although a reach of about 2 

miles is uncut a:bout 40 miles above the mouth. The 

stream is now ephemeral, and barren flats and miniature badlands border the gully. Other. valleys in the 

West have a similar history. 

Much thought has been devoted to the development of 

conservation measures; nevertheless, until the mechanics of sediment deposition and removal are better understood, truly effective conservation measures will not 

be realized. This report is a contribution toward the 

development of that necessary understanding of semiarid erosional and depositional processes. 

Many students of stream activity have recognized 

that the type of sediment transported by a stream 

greatly influences the characteristics of that stream. 

Therefore, in order to form general conclusions, areas 

of diverse sediment types must be studied. 

A study of the relation behveen sediment type and 

stream-channel shape (Schumm, 1960a) has shown 

that for stable stream channels containing less than 40 

percent gravel and cobbles, the channel shape expressed as a width-depth ratio (F) bears the following 

relation to a weighted mean value for the percent siltclay in bank and channel samples (M) : F=255 M-1·

08• 

Silt-clay is defined as the sediment passing through the 

200-mesh sieve, smaller than 0.074 mm. 

Areas which had a wide range in the percent siltclay in channel and banks were selected for study because it was believed that such differences might be 

fundamental with regard to the mechanics of sediment 

deposition and erosion. 

This report is composed of two main parts. The 

first contains a geographic description of each study 

area as well as a discussion of the changes in sediment 

and channel characteristics along each channel. In 

the second part the data collected in the five study 

areas are combined to show the relations between sediment type and channel character, and the causes of, 

and the differences in, erosional and deposition processes 

in the study areas. In addition, conclusions and their 

practical application are discussed. 

ACKNOWLEDGMENTS 

During the summer of 1957 the following assisted 

the writer in the field: William Krummel, C. T. Sumsion, and R. H. Taylor. Irene Lapke performed the 

hydrometer analysis of the finer sediment sa.mples. 

The writer wishes to acknowledge the constructive 

criticism offered by J. H. Mackin, M.G. Wolman, and 

J. T. Hack. 

John Rutter, superintendent, Badlands National 

Monument, and Philip Ward, ranger, Great Sand 

Dunes National Monument, ajded the fieldwork, and 

their courtesy and assistance are greatly appreciated. 

METHODS OF INVESTIGATION 

Five drainage basins were selected for study. Both a 

range in alluvial characteristics from area to area and 

active deposition or erosion within the channel were 

basic prerequisites. To eliminate as many variables as 

possible, areas were selected in which differences in 

lithology within a drainage basin did not affect the 

stream in any recognizable manner. This necessitated 

the selection of small drainage basins; the largest basin 

chosen had a drainage area of about 90 square miles. 

The reaches of each channel in which deposition was 

important generally could be readily identified, perhaps most easily by the recognition of recently deposited alluvium by its fresh a.ppearance and lack of 

vegetative cover. Downstream changes in channel 

shape and gradient and sediment character, also confirmed the location of aggrading reaches. These deposition criteria will be discussed in more detail under 

"Deposition in ephemeral streams." Unfortunately 

discharge da.ta were not available for each channel 

studied. 

The five areas selected, although scattered from 

southern South Dakota to northern New Mexico (fig. 

17), fulfilled all the basic requirements and were similar in annual precipitation and land use. Similar 

observations were made in each area. Where topographic maps of good quality, large scale, and small 

contour interval were available, the fieldwork was 

limited to the collection of data at many locations along 

the channel. Cross sections were chosen primarily to 

illustrate the changes occurring in the channel and on 

the flood plain as deposition or erosion became increasingly important in the downstream direction. Photo-
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 33 

r------------------1 i ·~ Is 0 u TH I I I i I i D A K 0 T Aj 

i el Sage Creek \ 

~-------------, I ! W. Sand Creek ,....---,1, 

I \ 

1 N E B R A S.- K A \ 

r------------1----, \ i i ·, 

f Bayou Gulch r------------------· 

i • 4 i I I I COLORADO, 

i .5 ! l Medarw Creek ! ,. ________________ J 

i 

i 

i 

i 

Arroyo Calabasas 

eJ 

j NEW MEXICO 

i 

i i ___________ j 

, r----· '--1 

200 0 200 MILES 

FIGURE 17.-lndex map showing- location of five study areas. 

graphs and sediment samples of channel, bank, and 

overbank deposits were taken, ·and the gradient of the 

stream was measured at each section. The loeations of 

the sections were marked on maps or aerial photographs. The longitudinal profile of the stream "'as 

surveyed along the aggrading reach when topographic 

1na.ps were not available. In brief, the downstream 

changes in channel character accompanying deposition. 

were documented by many surveyed cross sections of 

the channels, spaced closely enough to reveal the manner of channel filling. In addition, observations "\Vere 

made on the influence of vegetation on deposition. 

The width and depth of the drainage channels were 

especially difficult to cletennine: (a) where recent incision formed a steep-sided trench, in which all the runoff 

was contained; or (b) where a filled channel held only 

the smaller flows. In the first type, depth of water 

during the recent floods was estimated from observations of the height of scour or deposition on the banks. 

In the second type, depth was measured to the lowest 

elevation on the profile from the edge of the first permanent surface or bank above the channel floor. On 

figures 19, 24, 29, 33, and 37 the dashed line on each 

cross section shows the width measured at that section. 

Width was measured from one edge of a bank to the 

opposite, at a distance above the channel floor determined by channel depth. 

Samples of sediment were taken from the stream 

channel, its banks, and the flood plain. Samples of 

the surface inch of channel sediment were taken from 

10 to 15 points along the cross sections, depending on 

channel width. These samples were combined to give 

a composite sample of the channel alluvium. 

In the laboratory, a size analysis of each sediment 

sample was made. Since the author had found previously (Schumm, 1960) , that the median grain size of 

the sample alone is not the most important characteristic of the sediment, the cumulative grain-size curve 

was used to select other parameters of possible value, 

such as those used to determine the engineering properties of granular materials. Burmister (1952) has 

prepared tables which allow estimation of permeability, 

cohesion, and frost-heaving characteristics of a soil 

:from the grain size below which 10 percent of the 

sample is finer (Hazen's effective size, D10). Burmister'':S tables were used to describe in general terms 

the physical properties of each sample. 

The percent silt-clay in the sample, taken as that 

part of each sample passing the 200-mesh sieve, was 

also selected for comparison between areas. Burmister 

gives phJ'Sical reasons for selecting the 200-mesh sieve 

or 0.07 4 mm as the boundary between silt-clay and sand, 

for the soil becomes less well drained and capillarity 

increases as the percentage of material passing the 200-

mesh sieve increases; in addition, the 200-mesh sieve is 

the practical lower limit of sieving. In a recent paper 

Dunn (1959) demonstrates that the resistance of alluvium to tractive force is, related directly to the siltclay content of the sediment. Dunn, however, defines 

silt-clay as sediment finer than 0.06 mm. 

AREAS. OF STUDY 

For each of the five study areas a geographic description will be presented. This will be followed by 

a general description and a quantitative description of 

sediment and channel variations downstream. Comparisons of the five areas will be made in a later section. 

SAGE CREEK, SOUTH DAKOTA 

DESCRIPTION 

Location.-Sage Creek, a tributary to the White 

River, (figs. 17, 18) drains an area of about 92 square 

miles in the southeastern corner of Pennington County, 

S.Dak. The northern drainage divide is about 11 miles 

south of the town' of Wall, and the eastern divide is 

about 7 miles west of the town of Interior. The headwaters lie "\vithin the boundary of Badlands National 

Monument. That part of Sage Creek studied in detail 
34 EFFECT OF SEDIMENT CHARACTERISTICS 

R.15 E. R.16 E. 

EXPLANATION 

~"\..__ . __ __; ~_/ 

N 

\ T. 

2 

S. 

( 

NA TIONA.l 

~pS + -------, 

\ 

v~ 1 

Location of 

cross Section + 

P4 I - .... ~ ~ Location of ~ photograph station r- ___ _J 

Boundary of ) : 

.drainage basin 1 

~-- I 

( r------- _j 

\! 

\ 

\ 

\ 

\ 

Base from U.S. Geological Survey 

topographic maps and National 

Park Service surveys 

+ 

2 0 2 MILES 

T. 

3 

s. 

T. 

4 

s. 

FIGDRF. 18.-Index map of Sage Creek, S. Dak., drainage basin showing location of cross sections and photograph stations. 

is shown on two Geological Survey topographic maps, 

the Conata and Bouquet Table quadrangles. 

Annual precipitation.-The mean annual precipitation at Interior, based on a 10-year record, is 16.53 

inches (table 1). The greatest part of this mean annual 

precipitation, 13.97 inches, occurred during the period 

April through October. According to Thornthwaite's 

climate classification ( 1941), the Sage Creek area is 

semiarid. 

Vegetation and land use.-Most of the drainage basin 

is used as grazing land, although some alfalfa is grown · 

on that part of the basin that lies on the flood plain of 

the White River. Vegetation is of the short grass 

prairie type, although in the badlands vegetation is 

generally absent. 

Physiography and geology.-The headwaters of Sage 

Creek drain the precipitous badland escarpment known 

locally as the Wall. This escarpment, composed of 
Length 

EROSION A.i.~D SEDIMENTATION IN A SEMIARID ENVIRONMENT 

TABLE I.-Precipitation at stations near the study areas 

[U.S. Weather Bureau data] 

Mean precipitation (inches) 

Station of record 

Febmacy I Mwcch 

Interior, S. Dak _____ 

Fort Robinson, N ebr _ Santa Fe, N. Mex ___ 

Santa Fe (AP), N. Mex __________ 

Parker (9E), Colo ___ Blanca, Colo. _______ 

La Veta Pass, Colo __ 

Cross section Class 

l ___________ u 2 ___________ s 3 ___________ s 4 ___________ s 5 ___________ u 6 ___________ A 7 ___________ A g ___________ u 9 ___________ D lQ __________ A 1L _________ A 12 __________ A 13 __________ u 14---~------ u 

Mean_ --------

(years) Annual January April May June July August Sep. October Novemtember ber 

10 16.53 0. 45 0. 34 0. 78 1. 75 2.97 2. 64 2. 62 1. 74 1. 05 1. 20 0. 52 

40 17. 19 . 47 . 63 1. 00 2. 19 2. 79 2. 49 2. 07 1. 64 1. 51 1. 29 . 48 

40 14. 19 . 66 . 74 . 81 1. 07 1. 46 1. 19 2. 28 1. 90 1. 68 1.11 .71 

15 10. 80 . 32 . 42 . 54 . 67 .71 . 93 1. 80 2. 42 1. 29 . 99 . 23 

23 13. 42 . 34 . 36 . 69 1. 91 2. 21 1. 60 1. 83 1. 83 1. 05 . 75 . 56 

11 9. 20 . 10 . 25 . 17 1. 09 . 87 . 56 2. 20 2. 05 . 73 . 82 . 19 

30 20. 98 1. 13 1. 73 2. 54 2. 39 1. 78 1. 09 2. 19 2. 13 1. 27 1. 44 1. 69 

TABLE 2.-Channel and sediment characteristics, Sage Creek, B. Dak. 

[Class: A=aggrading; D=degrading; S=stable; U=unclass111.ed] 

Drainage Distance area between Median Hazen's Trask Silt-clay Channel Channel Width-

(square cross grain size effective size sorting (percent) Gradient width depth depth miles) sections D60 (mm) D1o (mm) index (feet) (feet) ratio (F) 

(miles) 

Channel sediment 

1. 65 -------- 0.04 0.00055 3. 63 74 0.0055 33 3.0 11. 0 

1.71 0.076 . 06 . 00036 11. 20 55 . 0055 16 7.0 2. 3 

3. 42 1. 36 . 06 . 001 1.42 68 . 0045 20 7. 0· 2. 9 

9.48 . 94 .12 . 009 4.61 40 . 0045 31 5.0 6.2 

13.08 2. 12 3.00 . 005 11. 80 29 . 0007 34 6. 5 5. 4 

14. 33 1. 21 . 10 . 007 5. 96 45 . 002 32 6.2 4. 9 

14. 59 1. 06 . 09 . 0001 13. 80 47 . 002 12 4.0 3.0 

18. 11 1. 32 . 034 . 0001 4. 45 86 . 002 22 8.0 2. 8 

18. 12 . .30 -------- ---------- -------- -------- . 0022 18 10.0 1.8 

45. 12 1. 36 . 067 . 0012 2. 14 60 . 002 21 3.0 7.0 

45. 40 . 91 . 032 . 0002 1. 63 92 . 002 15 3.0 5.0 

80. 25 1. 44 . 047 . 016 1. 29 78 . 0022 22 4. 0 5. 5 

80. 65 . 99 -------- ---------- -------- -------- -------- 12 2. 5 4. 8 

82. 84 . 38 . 044 . 004 1. 54 93 . 0017 32 6. 5 4. 9 

-------- -------- 0. 31 0.0037 5. 29 64 -------- -------- -------- --------

Bank sediment 

35 

December 

0. 47 

. 63 

. 58 

. 48 

. 29 

. 17 

1. 60 

Weighted mean 

silt-clay, M 

(percent) 

-------7 

7 

5 

4 

5 

6 

8 

-------6 

9 

7 

-------8 

-------

3 

9 

4 

4 

3 

1 

7 

7 

1 

8 

-

9 

2 ___________ -------- -------- -------- 0.028 

. 030 

. 028 

. 035 

. 032 

. 038 

. 031 

. 033 

. 027 

. 015 

. 007 

. 060 

0.0005 

. 0003 

. 0005 

. 001 

2.20 

2.24 

2.00 

1. 83 

93 - - - - - - -- - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - 3_- --------- -------- -------- -------- 4_ ---------- -------- -------- -------- 5_---------- ------ -- ------- - ---- ---- 6 ___________ -------- -------- -------- 7 ___________ ------------------------

8_- --------- -------- -------- -------- 10 __________ -------- -------- -------- 11 __________ ------------------------ 12 __________ -------- -------- -------- 13 __________ -------- -------- -------- 14 __________ -------- -------- --------

Mean ________________________ _ 

l ___________ -------- -------- --------

7 ___________ ------------------------ 13 __________ -------- -------- -------- 14 __________ -------- -------- --------

Mean ________________________ _ 

0.030 

0.040 

. 035 

. 007 

. 044 

0.032 

. 00032 

. 0013 

. 0007 

. 0006 

. 0013 

. 0045 

. 0002 

. 015 

0.0022 

1. 68 

2. 00 

2.68 

1. 91 

2. 13 

2. 00 

5. 10 

1. 19 

2. 25 

Overbank sediment 

0.0006 

. 0002 

. 0002 

. 0007 

0.0004 

3. 63 

1. 89 

5. 10 

1. 78 

3. 10 

93 

96 

90 

84 

83 

88 

90 

89 

79 

97 

81 

89 - -- ----- -------- -------- -------- --------

74 -------- -------- -------- -------- -------- 90 

97 

85 

87 -------- -------- -------- -------- --------
36 EFFECT OF SEDIMENT CHARACTERISTICS 

the sandstones, siltstones, and claystones of the White 

River group of Oligocene age, is being eroded rapidly. 

On many of the slopes a depth of 0.5 inch of material 

is eroded during 1 year (Schumm, 1956). Sediment 

eroded from the scarp must be transported at least 10 

miles across a gently sloping surface formed by the 

northward retreat of the badland scarp, to reach the 

White River. In spite of the gentle slope of this 

surface, about 20 feet to the mile, it is trenched by 

ephemeral streams. A few residuals or outliers of th~ 

main badland mass rise above it. 

Over almost the entire drainage area the White River 

group is exposed at the surface, although a geologic map 

of the area (Ward, 1922), which unfortunately fails to 

cover the entire drainage basin, suggests that the Pierre 

shale of Cretaceous age crops out in the lower part of 

the Sage Creek valley. However, since the creek flows 

on an alluvial fill derived predominantly from the 

White River group; no change in valley character was 

recognized due to change of rock type . 

.Alluvium.-The sediment derived from the erosion 

of the White River Badlands is fine grained (table 2). 

Median grain size of channel-sediment for all sections 

is 0.31 mm, Trask's sorting coefficient is 5.29, and 

Hazen's effective size is 0.0037 mm. Using Burmister's 

tables (1952), it is found that a soil having D10 of 0.0037 

mm is nearly impermeable. Potential capillarity of the 

soil is high, and it is very susceptible to frost heaving. 

This indicates that the alluvium in general is highly 

cohe!3ive, and much energy is required to detach a 

particle from the mass of alluvium. The mean percent 

silt-clay in the channel samples is 64, confirming the 

highly cohesive nature of this material. 

Samples taken of overbank deposits and deposits on 

the sides of the channel contain an even greater percent 

silt-clay and smaller value for D1o (table 2)' indicating 

even higher cohesion. 

Samples were taken from the recently trenched valley 

sides. These contained 89 percent silt-clay and D1o was 

0.0022 mm, indicating that the fill is impermeable. The 

potential capillarity is high, and the banks are suscep· 

tible to frost heaving. 

GENERAL CHANNEL VARIATIONS 

Considering the nature of the alluvium, a general 

discussion will now be presented of the changes in channel characteristics at the 14 sections surveyed. 

On figure 18 the location of the 14 surveyed sections 

and 5 photograph stations are shown. Photographs of 

some sections, shown on figures 20 and 21, pertain to 

the following discussion. Cross sections are shown at 

their respective positions on the longitudinal profile in 

figure 19· which is based on topogr.aphic maps. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 37 

The uppermost section (No. 1) has only a poorly 

defined channel. Past deposition has smoothed the 

valley floor until now flood waters cover a width of 

about 240 feet (fig. 20A). A short distance below 

section 1 is a headcut below which the valley floor is 

trenched. The headcut is not shown on figure 19, for 

its height is much less than the contour interval of the 

topographic maps on which figure 19 is based. 

Bank caving has widened the newly incised channel 

between sections 2 and 3 (figs. 19, 20B), but the cohesiveness of the bank material prevents rapid disintegration of the slump blocks. Some of the la.rger 

fragments on which vegetation still grows are nuclei 

for deposition in the channel (fig. 200). 

At section 3 this process has become more pronounced, 

and locally what seem to be incipient point bars are 

forming about slump blocks. The channel widens and 

deepens rapidly downstream as tributaries join the main 

chn,nnel. At section 4 an inner chn,nnel has formed; 

that is, deposition has built terraces along both banks 

(fig. 20D). The characteristic mode of deposition in 

this area seems to be a plastering of sediment along the 

sides of the channel. The recently deposited, finegrained highly cohesive sediment aids plant growth, 

and all recent sediment deposits are covered with grass 

and weeds, which in turn further increase deposition. 

Midway between photograph station 1 and section 

5 there is a noticeable decrease in stream gradient (fig. 

19), suggesting that deposition probably increases below section 5. Downstream the inner terraces encroach 

on the channel proper and mount higher on the original 

banks of the channel until at section 6 only about 3 feet 

of these banks are exposed, and a short distance downstream only 1 foot (fig. 20£), and finally at section 7 

(fig. 20F), the banks are completely covered. Owing 

to this deposition along the sides of the channel, bank 

caving has stopped. Overbank deposition is important 

at section 7, and the vegetation on the surface adjacent 

to the channel is partly buried. Downstream from 

section 7 the channel becomes shallower and narrower, 

approaching complete filling by building from the sides 

as well as from the bottom of the original channel. 

Thus in about 5 miles the channel of Sage Creek has 

been transformed from a raw trench to one almost completely aggraded. The complete filling of the channel, 

however, is interrupted by renewed trenching below 

section 8. At section 8 the channel is deeper and wider 

than at section 7; however, at section 8 much recently 

deposited alJuvium is found in the channel, suggesting 

that aggradation may be occurring or has occurred 

after channel erosion. 

Figure 21A shows a reach of intense erosion accompanied by channel widening below section 8. At section 9 the channel is at maximum depth but about 1 

mile downstream at photograph station 4 (fig. 21B) 

deposition has begun again, especially along the sides 

of the channel and on slump blocks in the bottom. A 

short distance below this point a remnant of the old 

channel floor is preserved on the west wall of the new 

trench. It lies 6 feet below the prairie surface, suggesting that complete filling had not occurred before 

the renewed trenching. This may explain the absence 

of a headcut above section 8. Between photograph 

station 4 and section 10 (fig. 210), more than 1 mile 

downstream, channel deposition becomes increasingly 

important, until at section 10 the cross section most 

nearly resembles that of section 7 (fig. 19). 

Deposition at section 10 (fig. 210) was heavy during 

the floods before the survey, and at this section the 

manner of deposition is most clearly illustrated. Just 

above section 10 the stream bends sharply to the north. 

With rapid deposition, the point bar deposit on the 

inside of the bend is expected, but of greater interest is 

the deposition on the outside of the bend where bank 

cutting should logically occur. Figure 21D is a view 

of the deposit on the outside bank. The fieldbook is at 

the contact between what. may be deposition by floods in 

the spring of 1957 and earlier floods. The sediment is 

la.id in against the bank, effectively narrowing the 

channel at this section with little or no decrease in 

channel depth. 

A trench dug across the deposit reveals that stratification is not horizontal; rather it curves downward 

from the bank toward the channel. Therefore, these 

lateral deposits are built upward as well as outward 

from the bank. The deposit shown on figure 21D has 

been scoured by reeent. floods, but on the inside of the 

bend (figs. 210, E) the buildinp: of the deposit outward 

into the channel has not been hindered by erosion. 

Complete filling of the channel by a union of the laternJ 

deposits and deposition on the .channel floor will result. 

in a channel-fill deposit containing concave-up stratification (Schumm, 1960a). This type of lateral deposition could only occur in areas of fine-grained cohesive 

sediments containing a high percentage of silt-clay. 

Figure 21£ shows the growth of weeds on the recent. 

deposits. The c,ohesiveness, ability to hold water, and 

the fertility of the fine sediment aids rapid and luxurious vegetative growth. 

Proceeding downstream from section 10, the water 

table apparently approaches the surface, for willow 

a.nd cottonwood saplings appear which further promote deposition. At section 12 the growth of willows 

is dense and at section 13 (fig. 21F) the channel is 

almost completely filled. A dam built in the fall of 

1956 may be the cause of ponded water in the channel 
38 EFFECT OF SEDIMENT CHARAC'I,'ERISTICS 

A D 

B E 

0 F 

FIGURE 20.-Sage Creek, S. Dak., above section 8. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 39 

at section 13, but the heavy deposition alone might 

have caused it. The 8 months since construction of the 

dam probably has been insufficient for that structure 

to cause accumulation of any measurable alluvial 

deposit at section 13. At section 14 just below the new 

dam, overbank deposition ranges in depth ·from 1 to 1ljz 

feet. A second dam was constructed at the mouth of 

Sage Creek in the fall of 19!\6. 

Probably little runoff enters the White River from 

the mouth of Sage Creek. The upper dam diverts 

much of the flow across the flood plain a.nd into the 

river upstream. The remaining runoff is held by the 

lower dam. As a result high flows in the '~ hite River 

enter the lower end of Sage Creek and deposit sediment in that channel, thus plugging the mouth of the 

creek. This type of deposit has been termed a "reverse 

delta" by Leighly (1934). 

QUANTITATIVE CHANNEL VARIATIONS 

As shown in the general discussion of variations, 

there are marked changes in channel character along 

Sage Creek. A comparison now will be made of the 

channel and sediment characteristics at each cross section. The parameters of most importance to this discussion are channel gradient, the shape of the channel 

expressed as a width-depth ratio, median-grain size, 

sorting index of sediment, and the percent silt-clay in 

each channel sample. As noted above, the percent 

silt-clay is taken as that part of the sample smaller 

than the 200-mesh sieve or 0.074 mm. 

To illustrate graphically the changes, the value for 

each of the above indices is plotted against section number on figure 22. 

On figure 22 the width-depth ratio is much lower 

at section 2 than at section 1. This change occurs 

abruptly as one passes the headcut between sections 1 

and 2 because channel degradation commonly causes a 

narrow and deep channel. Where the trench at sections 

3 and 4 has been widened by bank caving, the widthdepth ratio is higher. Gradient is less at sections 3 

and 4 than at sections 1 and 2, and it shows a large 

decrease between sections 4 and 5. Percent silt-clay 

also decreases to 1;t minimum for the sections at section 

5; however, the largest median grain size was found at 

section 5 ( 3 mm). To keep within the limits of the 

diagram, median grain size for section 5 is not plotted 

on figure 22. 

A question arises at this point as to the reason for 

increased median grain size at section 5. Perhaps the 

low graident at section 5 (fig. 19) causes deposition of 

the coarser fraction of the sediment in the channel; 

whereas, the finer fraction continues downstream and is 

deposited where the channel becomes smaller because 

of aggradation (section 7). Deposition becomes important below section 5, and this is accompanied by a 

decrease in width-depth ratio and sediment size and an 

increase in percent silt-clay and gradient. 

To understand the above changes in channel and sediment character as measured at the cross sections, the 

manner of channel filling by aggradation should be 

summarized. In general, the beginning of aggradation 

decreases channel gradient and relatively C!>arse sediment is deposited in this reach of the channel. The 

finer sediments continue to move down the channel 

across the reach of reduced gradient. At some point 

down stream, however, aggradation has almost completely filled the channel, and it is here that overbank 

flooding and deposition become important. With each 

flow of water much fine sediment is deposited in the 

remaining vestiges of the channel and on the flood 

plain. Continued deposition on the flood plain causes 

steepening of the gradient of the valley. The finer 

fraction of the alluvium is therefore found on the 

steeper reaches of the channel and valley floor. The 

above is discussed in more detail later in the report. 

To return to a discussion of the cross sections on Sage 

Creek, the renewed degradation beginning below section 

8 and continuing below section 9 causes a decrease in 

the width-depth ratio at section 9. Percent silt-clay 

decreases and median grain size increases at section 10, 

EXPLANATION OF FIGURE 20 

A, Cross section 1, view upstream. Channel has been filled by aggradation and is now grass covered. Badlands are visible in 

background. 

B, Cross section 2, view upstream. Remnants of blocks of alluvium in channel are the result of bank caving, and indicate active 

widening of channel. 

0, Recent deposition around blocks of bank material which have caved into the channel near cross section 3. The block has not 

rotated, and flood-plain vegetation is flourishing on the surface of the block several feet below its original position. 

D, Station 1, view upstream. The inner terrace formed by lateral deposition along both banks is best displayed in this reach, between sections 4 and 5. 

E, View upstream from station 2 a short distance downstream from section 6. Lateral deposition of fine sediment has almost 

covered banks of gully. 

F, CroRs section 7, view downstream. Banks are completely covered by recent deposits. The small raw channel at the bottom ot 

the gully may be the result of renewed degradation. 
40 EFFECT OF SEDIMENT CHARACTERISTICS 

A D 

B E 

0 F 

FIGURE 21.-Sage Creek, S. Dak., below section 8. 
EROSION AND SEDIMENTATION IN, A SEMIARID ENVIRONMENT 41 

but the general trend of the channel sediment is toward 

increasing fineness below section 7. Gradient also de..:! 

creases slightly as does width-depth ratio below section 

10. The plot of sorting ipdex indicates little except 

that the sediment is less well sorted on the aggrading 

reach between sections 5 to 7. 

The percent silt-clay in samples of the inner terrace 

(table 2) changes little in a downstream direction, suggesting that sediments with more than 70 percent siltclay are very susceptible to the type of deposition in the 

Sage Creek channel. 

SUMMARY OF SAGE CREEK AREA 

In a drainage channel carrying large sediment loads 

of fine-grained highly cohesive sediment, deposition 

occurs on the sides of the channel and deposition can 

occur on the outside of bends as well as on the streambed itself. The result is a reduction in the width-depth 

ratio across the aggrading reach. Median grain size 

increases and percent silt-clay and gradient decreases 

as deposition begins. This is followed by a decrease in 

median grain size and an increase in silt-clay and gradient as deposition increases. The final result will be 

a broad convex stream profile, the downstream part 

covered by the finer sediment. 

Rapid growth of vegetation on recently deposited alluvium seems to aid deposition. Bank caving yields 

only minor amounts of sediment except in the recently 

cut reach, and caved blocks become nuclei for deposition 

along the sides of the channel. 

SAND CREEK, NEBRASKA 

DESCRIPTION 

Location.-Sand Creek (fig. 17) drains an area of 

about 26 square miles in western Dawes County and 

northeastern Sioux County. The southern drainage 

divide is 6 miles north of the town of Crawford, Nebr., 

and the headwaters drain a part of the Pine Ridge 

escarpment, whieh here forms the major drainage divide between the White and Cheyenne River basins. 

Sand Creek flows to the east from the escarpment and 

enters the White River northeast of Ora wford (fig. 23). 

No topographic maps were available for this area. 

Annual preeipitation.-Mean annual precipitation is 

17.19 inches (table 1) based on a 40-year record at Fort 

Robinson, 3 miles southwest of Crawford. Most of 

this precipitation occurred during the months of April 

through October. According to Thornthwaite's (1941) 

climate classification, the Sand Creek area lies near the 

east limit of semiaridity. 

Vegetation and land use.-The short grass prairie 

vegetation, covering all the drainage basin except the 

badland areas near the western divide, affords good 

grazing land. Only a small part of the basin shows 

evidence of former attempts at agriculture; however, 

farming is important to the south and along the White 

River. A ]arge part of the headwater area of the drainage basin is badlands. 

Physiography and geology.-The upper reaches of 

Sand Creek are supplied with large amounts of sediment derived from erosion along the western drainage 

divide. A photograph taken near the divide (fig. 25A) 

shows the removal of the proteetive grass cover from 

slopes by gullying and the formation of badlands. 

Badland development is characteristic of the White 

River group of Oligocene age. 

The westward migration of the 'Vhite River drainage divide leaves a pediment at the base of the Pine 

Ridge escarpment. The area near the scarp resembles 

that near Sage Creek, S. Dak., but farther to the east 

this surface has been dissected to form an area of gently 

EXPLANATION OF FIGURE 21 

A, View downstream from station 3. Recent trenching has exposed cottonwood tree roots on floor of channel. View typical of 

conditions at section 9 except for exposed roots. 

B, View upstream, station 4. This reach is about 1 mile downstream from section 9. Deposition of sediment has begun here 

on floor of channel .and along banks. Depth of gully is 16 feet. 

C, Cross section 10, view Upstream. Progressive deposition in the channel between station 4 and this section has almost filled the 

channel. The maximum depth of the gully was probably only 10 feet in contrast to the upstream sections. 

D, Lateral bank deposit on outside of bend at cross section 10. Fieldbook is at contact of what is assumed to be the deposits from 

the 1956 and 1957 ftoods. A trench dug into these deposits reveals straUfication planes curving downward (convex) toward 

the channel ftoor. 

FJ, Weeds growing on recently deposited alluvium at cross section 10. Older plants (cockleburrs) are partly buried. Establishment of vegetation is rapid on the fine alluvium. 

F, Cross section 13, view downstream. A more advanced stage of aggradation and vegetative growth is shown at this section 

about 3.3 miles downstream from cross section 10. 
42 EFFECT OF SEDIMENT CHARACTERISTICS 

CROSS SECTIONS 

12 3 4 5 6 7 89 10 11 12 1314 

12 1 I I I I I I I I I I 

10 - -

2 

i If- -

:r: • li: 6 f- • -

~ • • • • • b 4 f- - i 

• • • 2 I! • -

0 !I I I I I I J L I I I 

!I I I I I I I I I I I I 0.006 

• 

!""" - 0.005 

• • - - 0.004 ~ 0 

- - 0.003 i 

...J 

"' z 

• • z f- • • • • • - 0.002 ~ 

() 

f- - 0.001 

• 11_ _l I I I I I J I I I I 

100 II I I I I I I I I I 

~ 

5 • • eor- • - I 

!:; • u; 60- • - ~ • w • • ~ 40- • - r 

• 20 II I I I I I I I I I I I 

0.12 en II I T I I I I I I I I I 0: 

1- "' 

"' f- • - 0.10 ~ 

3 

• i 

f- - 0.08 ~ 

IIi 

• N 

u; 

!e • - 0.06 2: 

< 

• 0: 

0 

~ - 0.04 z • • .: 

iS 

IJ J I I I I I ·I I I I I 0.02 "' ~ 

I I I I I· • I I I I I I 

12 .__ • - ~ 0 • ~ 8 - - 0 

z • ~ 4 - • • -

• 0 • ~ T I l I I l_l I I 

12 3 4 s 6 7 89 10 11 12 1314 

CROSS SECTIONS 

FIGURE 22.-Variations in channel and sediment characteristics; Sage Creek, S. Dak. The 

spacing of the cross sections along the abscissa is proportional to the distances between 

sections in the field. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 43 R53 W. R. 52 W. 

~-----~~ Nl 

j ~Q~ \ /§"1~ q "' zz / Badlands ----~ ~ S 

EXPLANATION 

2 

----t--

Location of cross section 

Boundary of drainage basin T. 

33 

N. 

~ ~Ira 2 3 

L-~ Sri:~:: <n;3 1 Badlands 

+ ~ - /v--"'-

...__.....---.....--- ~ 

Base from Soi I Conservat1on Service 

and State Highway Commission maps 

'---__./ 

0 2 MILES 

T 

32 

N. 

FIGURE 23.-lndex map of Sand Creek basin showing locations of cross sections. 

rolling hills tha.t contributes relatively little sediment 

to the channel. 

Again, as in the South Dakota area, almost the entire 

drainage basin of Sand Creek is underlain by formations of the White River group. A negligible part, as 

regards influence on the stream, is underlain by the 

Pierre shale of Cretaceous age. 

Alluvium.-Although derived from erosion of rocks 

of the White River group, the alluvium sampled in the 

Sand Creek valley is somewhat different froin that 

derived from the South Dakota badlands (table 3). 

As suggested by the name of the creek, it is coarser. 

Median grain size for all channel samples is 0.37 mm; 

Trask's sorting index is 2.81 and Hazen's effective size 

0.040 mm. Burmister's tables indicate that a soil with 

D1o of 0.040 is not impermeable, but it is drained 

with difficulty and is near the approximate lower limit 

of effective use of well points for lowering ground 

water ( 0.02 mm). The mean values for all sections 

show that potential capillarity is high and that the soil 

is very susceptible to frost heaving. 

The above data suggest that the sediment would be 

cohesive and difficult to detach from adjacent particles, 

but the individual samples differ from the South 

Dakota area in that the channel samples taken above 

the aggrading reach are proportionally more sandy 

(compare figs. 22 and 27), having as little as 10 percent silt-clay at section 6. 

Samples of the bank material contain 69 percent siltclay, and D10 is 0.0044. Effective size (D10 ) is larger 

than that for the Sage Creek sediment, but it still indicates a nondrainable system with high capillarity 

and high susceptibility to frost heaving. 

GENERAL CHANNEL VARIATIONS 

The high sediment yields from badlands in the headwaters of Sand Creek cause deposition in the downstream reaches of the channel. No topographic maps 

were available so the profile was surveyed from above 

section 6 to the mouth of Little Cottonwood Creek (fig. 

23). At sections 4 and 5 the altitude of the channel 

floor was obtained by altimeter. Above State Highway 2 the gradient was measured at each section. On 

figure 24 are shown the longitudinal profile of the 

stream, the cross sections across the aggrading reach, 

and the profile of the flood plain above the channel. 

· Starting upstream in the area of badlands (fig. 25.A.), 

a continuous channel exists to the area of major aggradation at section 9. The channel is relatively deep 

at sections 1 and 2, but it becomes shallower and wider 

until at sections 3 and 4 (fig. 25B) it is a fairly wide 

sandy channel. The gradient at section 4 is about onehalf of that of section 1, and as the aggrading reach is 

approached, the gradient decreases and the channel 

becomes shallower. Bank caving occurs in many 

places. At section 6 (fig. 250) a recent slump has occurred. The l·arge block, although cohesive (D10 is 

0.005), has broken into many smaller fragments. The 

lack of other blocks in reaches of bank cutting strongly 

suggests that, unlike the Sage Creek area (D10 is 

0.00035) slump blocks will not remain in the channel 

and form nuclei for channel deposition but will be 

swept away. 
44 EFFECT OF SEDIMENT CHARACTERISTICS 

TABLE 3.-Channel and sediment data, Sand Creek, Nebr. 

[Class: A=aggrading; D=degrading; S=stable; U=unclassified] 

Drainage Distance Weighted area between Median Hazen's Trask Silt-clay Channel Channel Width- mean Cross section Class (square cross grain size effective size sorting (Pt>rcent) Gradient width depth depth silt-clay, miles) sections D5o(mm) D10(mm) index (feet) (feet) ratio (F) M 

(miles) (percent) 

Channel sediment 

1 ___________ u 10. 6 -------- 0. 30 0. 045 2. 95 2 ___________ u 16. 1 2. 0 . 70 . 074 2. 17 3 ___________ s 16. 5 .6 . 30 . 020 2. 28 4 ___________ s 17. 9 1. 25 . 72 . 0013 2. 77 5 ___________ s 22. 2 1. 00 .73 . 030 2. 93 6 ___________ s 22. 5 . 75 . 35 . 075 1. 82 7 ___________ A 22. 7 . 30 . 024 . 0013 2. 62 8 ___________ A 23. 1 . 15 . 020 . 0013 2. 25 g ___________ A 23. 9 . 60 . 019 . 0013 2. 48 10 __________ D 24.2 . 50 . 027 . 0010 2. 39 1L _________ D -------- 1. 20 1. 10 . 14 1. 98 12 __________ u -------- . 30 . 49 . 13 1. 53 13 __________ u 25. 7 . 45 . 055 . 0013 8. 35 

Mean_ -------+ -------- -------- 0. 37 0.040 2. 81 

·16 0. 003 20 

7 . 0018 16 

17 . 0035 36 

14 . 0015 75 

15 . 003 65 

10 . 001 36 

87 . 0015 44 

91 . 0005 40 

100 . 003 --------

85 . 0065 15 

5 . 0017 21 

5 . 0017 16 

56 . 001 6 

39 -------- --------

4 

6 

3 

7 

7 

4 

4 

1.5 

-------- 10 

10 

7 

4 

--------

5 

2. 7 

12 

10. 7 

9. 3 

9 

11 

26. 6 

-------- 1.5 

2. 1 

2. 3 

1.5 

--------

4 

2 

2 

2 

2 

8 

8 

2 

3 

3 

2 

0 

7 

9 

--------9 

2 

0 

7 

21 

--------

--------

Bank sediment 

2 ___________ -------- -------- --------

3_---------- -------- -------- -------- 4 ___________ -------- -------- --------

5 ___________ -------- -------- --------

6_------- --- - -- - -- -- - -- -- -- - -- -- - -- - 7 ___________ ------------------------

8_----- -- -- - --- ----- - -- -- --- ---- - -- - 10 __________ -------- -------- -------- 11 __________ -------- -------- -------- 12 __________ -------- -------- --------

Mean_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ______ _ 

0. 040 

. 049 

. 053 

. 050 

. 045 

. 030 

. 043 

. 014 

. 080 

. 095 

0. 050 

0. 011 

. 0014 

. 0016 

. 002 

. 006 

. 002 

. 0013 

. 00065 

. 008 

. 010 

0. 0044 

1. 47 

1. 30 

1. 29 

1. 64 

2. 74 

2. 17 

3. 27 

3. 59 

2. 12 

1. 82 

2. 14 

90 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 70 

70 

60 

65 

88 

63 

93 

48 

39 

69 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Overbank sediment 

2 ___________ -------- -------- --------

g ___________ -------- -------- --------

12 ____ ------ -------- -------- -------- 13 ____ ------ -------- -------- --------

0. 040 

. 019 

. 026 

. 017 

0. 011 

. 0002 

. 001 

. 0011 

Mean_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 0. 026 0. 0033 

Point bars and sandbanks are forming on the major 

bends of the channel, and in many places coarse gravel is 

introduced into the channel at the cut banks. Below 

section 6, pronounced changes occur, which are due to 

deposition. Between sections 6 and 7 the gradient 

decreases. There is a change in the nature of channel 

sediment and some 'teiidency for deposition along the 

banks (fig. 25D). Changes in channel depth are much 

greater than those in channel width. Vegetation grows 

on both sides of the channel, and it in turn aids further deposition. Deposition increases downstream until 

at section 8 (fig. 25E) the channel is almost completely 

filled and can be recognized only by its bare appearance. Vegetation, however, is encroaching on this bare 

channel from both sides. Flood waters at section 8 

cover a width of 190 feet of the valley floor. A short 

1. 47 

4. 06 

2. 32 

2. 93 

2. 69 

90 

92 

100 

93 

94 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

distance below section 8 the vegetation has covered the 

entire channel, and the gradient of the valley floor increases sharply in this area of maximum deposition. 

The deposition, no longer confined to filling the channel, 

has built up the center of the valley until it has a convex 

cross section at sections 9 and 10 (fig. 24). There is 

ponded water on the south side of the valley between 

sections 8 and 9 where deposition in the valley center 

has exceeded that near the south margin forming an 

undrained depression or natural lake. 

The aerial photographs (fig. 26 A, B,) show the 

area of greatest deposition along Sand Creek. The 

light color of the recently deposited alluvium delimits 

the aggrading areas. Downstream from the area of 

major deposition vegetation becomes dense on the flood 

plain and trees are more abundant. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 

At section 9 there is a channel at the south side of 

the valley. Cross section 9 (fig. 24) reveals that the 

edge of this channel is higher than the rest of the valley 

floor. Field investigation shows that this channel 

carries only minor amounts of water. It has not been 

aggraded completely because the flood waters are 

diverted to the low north side of the valley, where 

renewed trenching has occurred at section 10. The 

headcut and longitudinal profile of the floor of the 

recent trench are shown on figure 24. Thus, although 

the locus of points of maximum deposition is migrating 

up channel, following it is a trench, which unless controlled will probably unite with the upper channel to 

form a continuous channel throughout Sand Creek 

valley. Figure 25F shows the new channel dissecting 

the valley fill. Much bank caving is in progress, widening the channel. Downstream at the confluence of Sand 

and Little Cottonwood Creeks the channel has filled 

again and overbank deposition is important. 

QUANTITATiv;E CHANNEL VARIATIONS 

It has been suggested under the general discussion 

that marked changes occur in channel and sediment 

characteristics as the aggrading ar~a is approached and 

crossed and that these changes differ somewhat from 

the changes in the Sage Creek area. The values for 

sediment and channel characteristics are plotted for 

each cross section on figure 27. 

Above section 6 width-depth ratio is a maximum at 

section 3 but shows a general increase from section 1 to 

section 6. Gradient decreases at downstream sections 

in the manner to be expected. Percent silt-clay remains 

nearly constant. Median grain size varies but not in 

relation to any known control. Thus above section 6 

the parameters vary in a downstream direction, probably much as they would in any channel. Between sections 6 and 7 deposition becomes noticeable, and at 

section 7 a pronounced change in channel and sediment 

character occurs. ·width-depth ratio increases, suggesting a widening and shallowing of the channel or a 

greater decrease in depth than in width. This increase 

continues to section 8 beyond which the channel is completely filled. Accompanying the increase in widthdepth ratio is a slight decrease in gradient, a great increase in percent silt-clay, and a decrease in median 

grain size. 

Continuing downstream toward the headcut, at section 10, little change occurs except for an increase in 

gradient and a sharp decrease in width-depth ratio. 

Stream gradient is initially decreased by deposition 

and then steepened as deposition becomes excessive. 

With renewed trenching gradient decreases, but it still 

is steeper than above the aggrading reach. Percent silt-

~I 

~ I 

.,I 

I 

I 

I 

I 

7 

I 

I ~ j 8 

I f J e. g J o

0 

/) : I : 

I 

I 

~I 

I 

" ]' I.&.J 0 0 0 o ~.. N 

I 

I 

..,I 

I 

I 

t 

45 
46 EFFECT OF SEDIMENT CHARACTERISTICS 

A D 

B E 

0 F' 

FIGURE 25.-Sand Creek, Nebr. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 47 

clay decreases and median grain size increases. Downstream from section 11 recent cutting probably has not 

occurred, and the channel and sediment characteristics 

vary accordingly. 

Between sections 8 and 13 the samples of overbank 

material show a slight increase in the percent of siltclay (table 3). The sorting index does not change 

greatly between sections 1 and 12. 

HISTORIC CHANGES 

Conversations with local ranchers reveal that the 

zone of maximum deposition, now located above and 

below section 9 (fig. 24), was about a mile downstream, 

near the county road, in 1917, 40 years ago. The filling 

of the valley has been a type of backfilling, that is, a 

migration upstream of the zone of maximum deposition 

rather than a general raising of the lower part of the 

valley. 

A fence at section 9 has been replaced 5 times in 40 

years as each installation was partly buried by alluvium. 

Total deposition at section 9 was estimated by the owner 

to be about 15 feet. 

The headcut, now near section 10, started about 1950 

between liz to 1 mile below the county road. It has then 

advanced at least 1 mile in 7 years. This information 

suggests that incision did not begin at the confluence 

of Sand and Little Cottonwood Creeks but on the section of steepest gradient below the road. This indicates 

further that the s)1allowing of the channel below the 

road may not be due predominantly to deposition but 

may result from lack of recent trenching in lower 

reaches of the valley. Segments of the old shallow 

channel are preserved near the new trench below the 

road. The old channel was about 5 feet deep and 13 feet 

wide; whereas, the new channel is now 13 feet deep and 

20 feet wide. 

The two aerial photographs in figure 26 show the 

changes that have occurred at and near the reach of 

maximum deposition between 1939 and 1954, a period 

of 15 years. Upon comparing the photographs, the 

channel between sections 7 and 8 seems to have filled. 

Above section 7 the photographs suggest no more than 

minor channel changes. Overbank deposition is present farther to the west (upstream) in 1954 as indicated 

by the upstream and lateral expansion of the lightcolored areas of recent deposition, and the cutoff channel has been filled. The lake between sections 8 and 

9 was formed by 1954, and the darker patches of flood 

plain to the north of the lake are gray probably due 

to flooding and recent deposition on these surfaces. 

Also, the aggradation in the center of the valley has 

forced the channel to the south side of the valley in 

1954. Another noticeable difference between the two 

photographs is the growth of vegetation on the lightcolored surfaces of recent deposition, shown on the 

1939 photograph near the county road, giving them a 

darker appearance on the 1954 photograph. 

In addition, a trenched channel exists below the 

county road in the 1954 photograph in contrast to the 

smaller 1939 channel. Perhaps the darker color of the 

valley below the road in the 1954 photograph is the 

result of this trenching, for the channel now carries 

all the flood water, preventing overbank deposition. 

Marked changes have occurred, suggesting progressive 

aggradation in the upper area and trenching downstream. The upper limit of both deposition and new 

vegetation seem to have moved up chap.nel. 

SUMMARY OF SAND CREEK 4REA 

In a drainage channel in which the sediment is composed of silt-clay and sand in the proportion 2 to 3, 

deposition occurs on the channel floor. Plastering of 

fine sediments on the channel banks occurs only in those 

aggrading reaches in which sand has become a minor 

part of the sediment. The width-depth ratio therefore 

increases along the aggrading reach until the channel 

has been completely filled. 

Vegetation is of little importance except in the reaches 

of channel where fine sediments cover the banks and 

channel floor. Sediment from bank caving is moved 

downstream almost immediately and rarely aids in the 

beginning of channel deposition. A noticeable contrast with Sage Creek is the short distance in which 

deposition appears and channel-filling is completed, bet ween sections 6 and 9 ( 6,000 feet) . 

EXPLANATION OF FIGURE 25 

A, Badlands in headwaters of Sand Creek. Removal of the protective sod cover allows development of badlands in soft rocks of 

the White River group. 

B, Cross section 3, view downstream. Relatively wide sandy channel typical of Sand Creek above aggrading reaches. 

0, Cross section 6. Note recent slump block at base of bank. Fence to left shows signs of recent-partial burial. 

D, Cross section 7, view downstream. Note development of berms along sides of channel. Vegetative growth is promoted by deposition of fine sediments. 

E, Cross section 8, view downstream. Channel is almost completely filled by recent deposition. Vegetation is encroaching on 

the parts of the channel that are bare. 

F, Recently formed gully at county road between cross sections 10 and 11. 
48 EFFECT OF SEDIMENT CHARACTERISTICS 

A. 

B 

FIGURE 26.- Aerial photographs of part of the Sand Creek drainage basin, Nebr. A, taken in 1939; B, taken In 1954. Light·colored area at 

upper left of both photographs is area of maximum deposition. Locations of cross sections 6 to 10 are given on photograph B. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 49 

CROSS SECTIONS 

1 2 3 4 5 6 7 8 9 10 1112 13 

30 .-----~,~~,---~,--~,-~,~rr~~r~r---~~~~ 

• 25 1- -

0 

~ 20 1- - a: 

::r 

~ L&.l 

0 

-.i:. 

b 

~ 

15 1- -

• 10 1- • • -

• • 

5 ~ -

• • • o~----~'--L-I ___ L_I __ L_I_LI~IL_IL_l~L----~~~~ 

.-----~,~~,---~,--~,-~,,~,~,-~,~--~,,-,0.~ 

1-

- • - 0.006 ~ 

0 

< a: 

> 80 

~ () 

I 

~ 60 iii 

1-

- • - • • 

• • 

• 

- 0.004 ~ ...J 

LLI 

z 

z •• - 0.002 :2 

() 

• ~----~~--L-~--~~~--L-1-LILifL-L_IL_I __ ~ILLI~O 

• • • 

~ ~ ~ 

~ 2

: .____ ____ -----.J.'----.Le ----·-'------·L___•j_l -LI j_l _____t___L ____ .LI J.,___J] 

r---------,,r-.--1---r-,-r-,-r-,--. IT~T~I...-----........ ,---, 1.2 

(f) 

a: 

L&.l 

1-

L&.I 

~ 

:J 

...J 

~ 

- - 0.8 ~ 

• • • 

• u.i 

N 

iii - - 0.4 z 

••••• • ~----~IL-_J__I ___ L_I __ L_I_~IIIL_L_lL_l ___ ~II_JO 

i:S 7 0 

i= 

~ 5 ~ a:: 

0 

00 J'-------'---'----'-----'--__L_l_L___L_ ---'--.L__L_j 

2 3 4 5 6 78 9 10 1112 13 

CROSS SECTIONS 

< a: 

~ 

z 

< 

0 

L&.l 

~ 

FIGUm; 27.-Variations in channPl and sediment characteristics, Sand 

C'rt>Pk, Nt>br. 1The spacin~ of the cross sections along the abscissa is 

proportionnl to the distances between cross .SfCtions in the field. 

In summary, the Sand Creek channel had few similarities in the manner of aggradation with Sage Creek. 

These differences are attributed mainly to the smaller 

percentage of silt-clay in the alluvium of Sand Creek . 

ARROYO CALABASAS, NEW MEXICO 

DESCRIPTION 

Looation.-Arroyo Calabasas (fig. 17) drains an area 

of about 48 square miles in central Santa Fe County, 

N. Mex. The drainage basin lies parallel to and about 

1 mile northwest of U.S. Highway 85 between Santa 

F'e and the Santa Fe Airport (fig. 28), and a part of its 

htadwaters lies within the Santa· Fe city limits. Arroyo Calabasas is a tributary to the Santa Fe River . 

The Arroyo Calabasas drainage basin is shown on the 

following Geological Survey topographic maps: Santa 

Fe, Agua Fria, Turquoise Hill, Tetilla Peak, and 

Montoso Peak. 

Annual preoipitation.-A 40-year record of precipitation for Santa Fe (table 1) gives a mean annual 

precipitation of 14.19 inches. Of this total, 10.69 inches 

fell during the months of April through October. At 

the Santa Fe Airport a 15-year record gives a mean 

annual precipitation of only 10.80 inches. A comparison of the same 11 years of record from the 2 stations 

shows an average of 3.1 inches less rainfall at the airport. The Sangre de Cristo Mountains to th~ east 

probably produce a sufficient orographic effect to cause 

this difference in rainfall between mouth and headwaters of Arroyo Calabasas. The drainage basin a pparently lies within a narrow zone of semiarid climate 

between the more humid mountains and the arid zone 

to the west. 

Vegetation and land u.Ye.-Vegetational cover is poor 

except on the untrenched valley bottoms. Pinon and 

juniper grow on the hills. Grass is nearly absent except in the valleys, but slopes have a scant cover of 

Russian-thistle. Cattle and horses gra.ze the undissected valley bottoms. The few dwellings within the 

drainage basin apparently are owned by people who 

work in the city of Santa Fe. No land cultivation has 

been attempted within the drainage basin. 

Physiography and geology.-Unlike the two areas 

described previously, no prominent escarpment forms 

the drainage divide and no pediment development has 

taken place. Instead, a dendritic drainage pattern has 

developed, dissecting the poorly consolidated sediments 

into a network of valleys and divides. Hilltops are 

convex, bordered by val1ey-side slopes of varying steepness depending on the activity of the adjacent ephemeral stream. 

The Arroyo de los Frijoles is a major tributary to 

Arroyo Calabasas. Their drainage areas are about 
50 EFFECT OF SEDIMENT CHARACTERISTICS 

N 

Based on U.S. Geological Survey 

topographic maps 2 

R. 9 E. 

0 2 MILES 

17 

N. 

EXPLANATlON 

Location of cross sections 

Inferred contact between volcanic 

and sedimentary rocks 

Boundary of drainage basin 

FIGURE 28.-lndex map of Arroyo Calabasas, N.Mex., drainage basin showing location of cross sections. 

equal at the junction of the two (fig. 28). In the field, 

Arroyo de los Frijoles seems to be the main stream, for 

at the junction it is a well-defined trench; whereas, 

the Arroyo Calabasas channel is completely aggraded 

and grassed a short distance above the junction. 

The drainage basins are underlain by the unconsolidated or partly consolidated silts and sands (marls) 

of the Santa Fe group of middle ( ? ) Miocene to 

Pleistocene ( n age. The irregular shape of the drainage basin is due to volcanic rocks in the western part 

of the basin. The extent of the volcanic rocks, as inferred from the topographic maps, is indicated on 

figure 28. The streams draining this area enter Arroyo 

Cal·abasas near its mouth, but the resistance and permeability of the volcanic rocks result in little sediment and 

runoff from this part of the basin. 

A.lluviu1n.-The stream channels transporting sediment eroded from the Santa Fe formation seem to be 

filled entirely with sand, and the percent silt-clay in 

the channel of Arroyo Calabasas is only 17 (table 4) . 

Median grain size is 0.59 mm; Trask's sorting index 

is 2.01, and Hazen's effective size, 0.17 mm. Burmister's 

tables suggest that soil with D10 of 0.17 mm is permeable with free flow. Potential capillarity is slight 

and frost heaving is negligible. The above factors suggests that none of the particles in the channels are 

bound together but act independently. The bank material, however, where sampled, has a mean of 20 percent silt-clay and Dto of 0.040 mm. Thus, although 

the material in the channel bottoms is not cohesive 

the material in the banks is, forming stable sides to the 

channel. 

The samples taken in the channel are not truly representative, for in some sections gravel, cobbles, and even 

a boulder in some places cover about 5 percent of the 

streambed. However, this small fraction of larger 

grain sizes did not seem to.influence the general channel 

and sediment character in any noticeable manner, and 

for convenience only the finer material was sampled. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 51 

TABLE 4.-Channel and sediment data, Arroyo Calabasas and Arroyo de los Frijoles, N. Mex. 

[Class: A=aggrading; S=stable; U=unclassifted) 

Drainage Distance Weighted area between Median Hazen's Trask Silt-clay Channel Channel WidthCross section Class (square cross grain size effective size mean sorting (percent) Gradient width depth depth silt-clay, miles) sections Dso(mm) D1o (mm) index (feet) (miles) (feet) ratio (F) M (pereent) 

Channel sediment 

A ___________ s 3. 83 -------- 0. 84 0. 28 1. 68 3. 0. 013 79 3. 26 4. 1 B ___________ A 4. 68 I. I7 . 58 IS 1. 91 4 . OI3 257 2 129 4. 1 c ___________ u 4. 8I . 27 -------- ---------- -------- -------- . OI3 88 2. 5 35 -· ------ n __________ s I2. 85 2. 90 I. 00 . 20 2. IO 5 . OI4 67 4. 5 I5 8. 5 I ___________ s 8. 47 -------- . 85 . 16 2. 34 6 . 010 84 5 17 9.0 2 ___________ s 8. 91 . 30 . 90 . 26 2. 33 2. . 010 63 1 63 2.4 3 ___________ 

4 ___________ A 9.30 . 43 . 51 . 24 1. 53 .5 . 013 -------- -------- -------- -------- A 9. 85 . 61 . 56 . 27 1. 52 2 . 014 -------- -------- -------- -------- 5----------- 6 ___________ A -------- . 76 . 035 . 0012 3. 37 80 . 018 -------- -------- -------- -------- s 24. 2 . 64 . 50 . 18 1. 47 3 . 009 92 4 23 4. 8 7 ___________ s 25. 8 . 49 . 75 . 16 2.01 5 . 011 100 4 25 5. R 8 ___________ A 26. 4 . 34 . 48 . 14 1. 92 5 . 009 90 1. 5 60 5. 1 9 ___________ A 42. 3 1. 89 . 031 . 0015 1. 94 89 -------- -------- -------- -------- -------

Mean_ -------- -------- -------- 0. 59 0. 17 2. 01 I7 -------- -------- -------- -------- --------

Bank sediment 

A ___________ ------------------------ 0. 28 0. 034 

B ___________ -------- -------- -------- . 30 . 060 

D __________ ------------------------ . 13 . 001 

I ___________ ------------------------ . 15 . 005 

2 ___________ -------- -------- -------- . 20 . 045 

6 ___________ -------- -------- -------- . 16 . 040 

7 ___________ ------------------------ . 30 . 035 

8_---------- -------- -------- -------- . 32 . 090 

Mean ________________________ _ 0. 23 0. 040 

Overbank sediment 

3 ___________ -------- -------- -------- 0. 034 0. 0049 2. I6 80 

4 ___________ -------- -------- -------- . 33 . 044 2. 07 16 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 9 ___________ -------- -------- -------- . 033 . 0015 1. 94 89 ---------- ------ -------- -------- -------- 1------1--------1-------1-------1 

Mean_------------------------ 0. 13 0. 003 2. 39 62 

GENERAL CHANNEL VARIATIONS 

Five cross sections were surveyed on Arroyo Calabasas above the confluence with Arroyo de los Frijoles; 

3 were surveyed onArroyo de los Frijoles, and 4 below 

the junction of both streams. 

Above the junction, Arroyo Calabasas has a broadly 

convex longitudinal profile (fig. 29). At section 1 the 

stream channel is 8 feet deep and seems recently cut 

(fig. 29). Downstream the channel is progressively 

filled with sand until at section 3 recent floods have 

covered the entire valley floor (fig. 30A) . The· valley 

floor in turn becomes better covered with vegetation 

until at section 5 there is no indication that a channel 

exists upstream (fig. 30B). This part of the valley is 

used for grazing. 

A short distance below section 5 is the junction of 

Arroyo Calabasas and Arroyo de los Frijoles. Section 

6, located just below the junction, was surveyed across 

the deep chatmel, which is continuous in the Frijoles 

drainage basin (fig. 300) . A headcut has started to 

migrate up the Arroyo Calabasas valley, but the valley 

floor is hanging 8 feet above the trenched channel near 

the junction. 

The longitudinal profile of Arroyo de los Frijoles is 

flatter than that of Arroyo Calabasas near their junction 

(fig. 29), but 3 miles upstream it steepens and at section 

B the channel is 30 feet higher than the corresponding 

point in the Calabasas channel. At section A the. channel is shallow and sandy (fig. 29). Downstrea~ at section B (fig. 29) the channel has widened considerably 

and aggraded. The photograph of the channel below 

section B (fig. 30D) shows that sand has been deposited 

out of the channel and among the trees near the channel. 

Several channels have formed; these cut around the 

partly buried brush and trees forming islands and a 

braided channel. No headcut is present, but the channel 
52 EFFECT OF SEDIMENT CHARACTERISTICS 

~---~--/~---------------------

3 

------------~5 __________ ____ 

8 

9 

Profile measured on Santa Fe, Agua Fria and 

Turquoise Hill topographic maps; based 

on aerial photographs taken in 1951 

10 

FEET 

150 

100 20L 50 

0. 

0 40 80 FEET 0 

o 1000 2000 3000 FEET 

CROSS SECTIONS LONGITUDINAL PROFILES 

4 

FIGURE 29 . .--lliongitudinal profile and cross sections surveyed along Arroyo Calabasas and Arroyo de los Frijoles, N. Mex. Cross sections 

on Arroyo de los Frijoles are given a letter designation. 

is nevertheless being lowered downstream from the 

aggraded cross section B, as indicated by plant roots 

that are exposed in the banks and by the rough appearance of the channel floor (fig. 30E) as contrasted with 

the smooth, flat floor characteristic of reaches of deposition. At section 0 the channel is narrower and bordered by well-defined banks. Continuing downstream 

the channel becomes deeper until at section D it is 5 feet 

deep. This, then, is the first example of channel rejuvenation in which headcutting is not important. A 

definite nick in the longitudinal profile is noted (fig. 

29), but the noncohesive material in the channel prevents the development of a headcut, so typical of 

reju~enation in the finer, more cohesive alluvium (Sage 

Creek between sections 1 a1,1d 2 ; Sand Creek near section 

10). Below the junction of the two streams the channel, 

perhaps due to the decrease in gradient (fig. 29), is 

filled within a short distance. 

At section 7 the chanliel. is only 4 feet deep and at 

section 8 it is only 1.5 feet deep (fig. 30F). The width 

changes little, for the. lack of large amounts of silt-clay 

prevent any deposition along the sides. Even at section 

8 the remnants of the· once high banks seem unchanged. 

Channels are filled solely from the bottom up.· Deposition is complete below section 8 and vegetation covers 

the fill. A small discontinuous gully has formed 

between sections 8 and 9, but as its headcut moves 

upstream the lower end of the channel is filled. Here a 

headcut has formed. Samples of the alluvium taken 

across the valley show that, where the channel has been 

filled, the alluvium contains 66 percent silt-clay, which 

forms a tough, cohesive layer·over the underlying predominantly sandy channel fill. This cohesive layer 

plus the vegetation cover is a protective cap, and headcut erosion again is the means of gully lengthening. 

At section 9 the channel is completely filled, and the 

valley floor resembles that of section 5 (fig. 30B). The 

grass ~over is good and cattle graze the valley floor. 

Below section 9 is another headcut that has worked· 

headward from the junction of Arroyo Calabasas with 

the Santa Fe River (fig. 29). 

Vegetation apparently does not aid the filling of 

channels here. The sandy nature of the alluvium prevents deposition along the banks, and no vegetation was 

noted in the channel bottom. The lack of vegetation in 

the channel is ~ttributed to the ·l~tck of cohesion of the 

channel sediment. Any plant that takes root in the 

channel bottom would be washed out during the first 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 53 

flood as the sandy sediment is set in motion. The 

growth of weeds in a sandy channel downstream from 

a stock-water reservoir demonstrates that the channel 

sediments will support vegetation if undisturbed. The 

reservoir has retained all flow, and the seeds and plants 

were not washed out of the sediment by floods. 

Blocks of bank-caved material were not common in 

the channel, probably because the low cohesion of the 

bank material allows crumbling and removal of fallen 

material by the next flood. 

QUANTITATIVE CHANNEL VARIATIONS 

To simplify this discussion the data obtained at the 

cross sections on Arroyo de los Frijoles have been 

plotted as if they were sections measured upstream from 

section 1 on Arroyo Calabasas (sections A-D, fig 31). 

Moving downstream from section A (fig. 31), three 

areas of deposition are crossed at sections B; 3, 4, 5; and 

8, 9. These sections are characterized by a high widthdepth ratio. Deposition causes filling of the channels 

from bottom to top without changing the width, thereby 

greatly increasing this ratio. Even in the trenched 

areas, this ratio is nearly 20, which greatly exceeds that 

in the areas of finer grained sediment. 

Gradient is steep between sections B and 0 owing to 

recent incision. It is steep also at section 5 and where 

measured below section 9 near the Santa Fe River (X 

on fig. 29). These last two increases in gradient are 

due to the steepening of the valley floor by deposition. 

Each of the steeper reaches of deposition is associated 

with sediments of smaller grain size, although percent 

silt-clay is high only at sections 5 and 9. Perhaps this 

is because aggradation at section B is not so far advanced as at sections 5 and 9. This relation seems 

anomolous- finer sediment on the steepest parts of the 

longitudinal profile-but it.is simply the result of channel and flood-plain deposition as discussed previously. 

The samples at sections 3, 4, and 8 are not so fine as 

those at sections 5 and 9 even though in areas of heavy 

aggradation, because they were taken in the last vestiges of the channels, which wen~ generally little more 

than sand-filled swales. However, the valley floor as 

a whole is characterized by finer sediments. For example, sediments adjacent to these inadequate channels 

contained up to 80 percent silt-clay (table 4). As mentioned above, D50 and percent silt-clay show the greatest 

changes on the aggraded reaches. If the samples taken 

at sections 5 and 9 were not plotted on figure 31, percent silt-clay would show hardly any change; whereas, 

D50 would decrease, but only slightly in a downstream 

direction. 

The sorting index is highest at section 5 in the aggraded Arroyo Calabasas valley above the junction 

with Arroyo de los Frijoles. 

SUMMARY OF ARROYO CALABASAS AREA 

In a drainage area in which silt-clay occurs in small 

amounts the channels are filled from bottom to top. 

No plastering of fine sediments on the banks occurs, 

and vegetation is not an important cause of deposition. 

Vegetation, however, becomes important in stabilizing 

the deposit when flooding occurs over the entire valley 

floor. 

Headcutting occurs only where channel-filling has 

been completed, and the coarser sediments are capped 

by a layer of fine material which supports a heavy grass 

cover. The suggestion here is that a channel in coarse 

sediment, when filled, may acquire the characteristics 

of an area of finer sediment. 

The gradient of the valley increases on the reaches of 

deposition due to piling up of sediment on the valley 

floor. These steep gradient sections are covered with 

the finest sediments. 

BAYOU GULCH, COLORADO 

DESCRIPTION 

Location.-Bayou Gulch (figs. 17, 32) drains an area 

of about 23 square miles in northeastern Doughts and 

northwestern Elbert Counties, Colo. It is a tributary 

of Cherry Creek, entering that stream at a point about 

3 miles north of Franktown and about 5.5 miles south 

of Parker. State Highway 83 crosses Bayou Gulch 

about 0.4 mile above the mouth. The drainage basin 

is shown on the Castle Rock and Elizabeth quadrangles. 

Annual precipitation.-Based on a 23-year record of 

precipitation at Parker (9E) (table 1) the mean annual 

precipitation is 13.42 inches. Of the total, 11.81 irtc;hes 

fell during the months of April through October. According to Thornth waite's climate classification ( 1941) 

the area is semiarid. 

Vegetation and land use.-Vegetational cover is 

heavy. Pine trees grow on the divides and cottonwood 

trees are plentiful along the upper reaches of the creek. 

Alfalfa is raised on a terrace above the present flood 

plain. The rest of the drainage basin is used for grazing except for a few fields of wheat and corn on the 

divide. 

Physiography and geology.- This drainage basin, except for the heavier vegetational cover and a general 

appearance of coarser sediment, resembles that of the 

Santa Fe area. A dendritic drainage pattern has incised the poorly consolidated sediments into a network 
EFFECT OF SEDIMENT CHARACTERISTICS 

D 

B E 

0 F 

FIGURE 30.-Arroyo Calabasas, N. Mex. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 

of valleys and rounded divides in marked contrast to 

the South Dakota area. 

The entire drainage basin is underlain by poorly consolidated sandstones and shales of the Denver formation 

of Late Cretaceous and Paleocene age. 

Alluvium.-The channel sediment contains 5 percent 

silt-clay (table 5). The median grain size is 0.57 nun, 

Trask's sorting index is 1.84, and Hazen's effective size 

is 0.17 mm. The characteristics of the channel sediment 

.here and its behavior are little different than that in the 

Santa Fe area (table 4) for effective size is the same for 

both areas; however, the mechanics of aggradation seem 

to be somewhat different, and, indeed, the width-depth 

ratios are greater in this area. The bank material in 

Bayou Gulch differs from that in the Santa Fe 

area, for silt-clay is only 8 percent where sampled. 

Median size is 0.49 mm and D10 is 0.12, indicating that 

cohesion is low and that the banks would not have any 

great resistance to erosion in contrast to the more· cohesive banks of the Santa Fe area. 

GENJIRAL CliANNBL V ABIATIONS 

Above section 1 (figs. 32, 33) the channel narrows 

as the divide is approached. In the reaches of permanent flow above section 1, vegetation is encroaching 

on the channel but with difficulty because of the mobility 

of the channel material. 

Below section 1 (fig. 34A.) a wide sandy channel is 

characteristic of Bayou Gulch to its mouth. At sections 

4 and 5 (fig. 34B) channel deposition may be occurring, 

for sand has been deposited over the flood plain, and 

the channel has been filled and widened. The observer 

has difficulty in detennining what prQCeSS is operative 

in each section of this channel. Where deposition is 

assumed to occur, widening of the sand-covered areas 

TABLE 5.-Channel and sediment data, Bayou Gulch, Colorado 

[Class: A=aggradlng; s-stable; U-=unelasslfled] 

Drainage Distance area between 

Cross section Class (square cross mOes) sections 

(miles) 

l ___________ s 13.00 2 ___________ -------- s 18.95 0.43 3 ___________ s 19.66 . 24 4 ___________ A 21. 95 . 47 5 ___________ A 22.44 . 16 6 ___________ u 22. 90 . 95 

Mean_ -------- -------- --------

2 ___________ -------- -------- -------- 3 ___________ -------- -------- --------

4_---------- -------- -------·_ -------- 5 ___________ -------- -------- --------

6_ ---------- - ------- ----- - - - - -- -----

Mean_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ______ _ 

Median 

grain siZe D50 (mm) 

0. 74 

.50 

. 58 

. 55 

. 47 

. 55 

0.57 

0. 82 

. 40 

. 48 

. 27 

. 46 

0. 49 

Hazen's Trask 

effective siZe sorting Dto (mm) index 

Channel aedbneat 

0.27 1. 63 . 13 1. 85 . 17 1. 69 . 17 1. 84 . 06 2.50 . 21 1. 51 

0. 17 1. 84 

Balik &ediment 

0.26 

. 055 

. 075 

. 070 

. 15 

0. 12 

2. 13 

1. 60 

Silt-clay 

(percent) 

2 

6 

4 

2 

12 

4 

5 

2 

13 

10 

11 

6 

8.4 

EXPLANATION OF FIGURE 80 

Channel Channel Width- Weighted mean Gradient width depth depth silt-clay, (feet) (feet) ratio (F) M 

(percent) 

0.013 207 2. 5 83 -------- . 009 122 2 61 5.9 . 010 130 3 43 4.4 . 011 330 2 165 2.0 . 010 250 1. 5 167 12.2 . 016 128 1. 5 8~ 4.1 

-------- -------- -------- -------- --------

A, Cross section 2. Channel has been filled and tlood waters cover area between truck and hill in background. Three small 

sandy swales convey low tlows through this reach. 

B, Cross section 5, view upstream. The valley tloor is grass covered and is utilized for grazing. No channel is present at 

this cross section. 

0, Near cross section 6, view upstream toward con.tluence of Arroyo de los Frijoles on right. Headcut on Arroyo Calabasas 

is less than 200 feet upstream from confluence. 

D, Channel of Arroyo de los Frijoles near cross section B. Several channels appear to be degrading between sagebrush and 

juniper trees, thereby forming a braided channel. 

E, Irregular channel surface suggests incipient degradation in Arroyo de los Frijoles downstream from cross section 0. 

F, Near cross section 8, view downstream. Channel is being progressively filled. Compare with photograph of section 6 

(fig. 80 0). 
56 EFFECT OF SEDIMENT CHARACTERISTICS 

CROSS SECTIONS 

A Be D 12 3 4 5 678 9 X 120 

100 

80 

60 • • 

40 

• 

20 • • 

• • 

0 - -- _l__l________ 

• 

L 

I •• ' • • • 

f..-

I •• • ! 

80 

60 

40 

20 

0 

I I "T I I I I I I I I 

• • f- -

• f- • - • • • • I- -

• f- -

I I • I I I 

~t ~: : ~.;7 ~ ~:; :l A BC D 12 3 4 5 678 9 X 

CROSS SECTIONS 

0.020 

0.015 

0.010 

0.005 

VI 

1.0 0:: 

1.1.1 

1-

1.1.1 

:E 0.8 3 

~ 

0.6 ~ 

w 

N 

u; 0.4 z 

:;;: 

0:: 

0.2 <!l 

z 

< 0 0 1.1.1 

:E 

FIGURE 31.-Variations in channel and sediment characteristics, 

Arroyo Calabasas and Arroyo de los Frijoles, N. Mex. frhe spacing 

of the cross sections along the abscissa is proportional to the distances 

betwePn cross sections in the field. 

can be noted, but in contrast to the other areas investigated, there is no veneer of fine sediments over the 

coarser material and no encroachment of vegetation 

into the channel. 

Below section 5, in order to confine the channel and 

thereby to prevent deposition of sand on the adjacent 

field and possible cutting around the bridge abutments, 

a dike has been built. The dike is sand piled in a ridge 

R.66 W. R.65 W. N 

EXPLANATION 

Location of croes section 

Boundary of drainage basin 

1 MILE 

FIGURE 32.-lndex map of Bayou Gulch, Colo., drainage basin showing 

location of cross sections. 

along the channel, and although it looks unstable it 

seems to be effective at least for the low floodflows. 

At the bridge over Bayou Creek, the gradient steepens 

in response to erosion (fig. 33). In contrast, the 1939 

stream profile, obtained from the topographic maps, 

shows no such steepening. This steepening and the 

narrowing of the channel at section 6 (fig. 340) suggest strongly that erosion is occurring or has occurred 

in the past in the lower reaches of Bayou Gulch. Owing 

to the lack of cohesion in the sediment, no headcut is 

present, and as will be shown later, the only indication 

that erosion is occurring other than the profile change 

is the increase in gradient at section 6 and a decrease in 

the width-depth ratio. Above section 6, 2 feet of sand 

has been deposited on the surface adjacent to the channel, indicating recent deposition before incision. 

Very little vegetation grows on what are assumed to 

be reaches of major deposition, sections 4 and 5. Although bank caving has contributed to channel widening, the caved material is removed by the next flood. 

QUANTITATIVE CHANNEL VARIATIONS 

The downstream changes are summarized in figure 35. 

The major change in channel width-depth ratio is at 

the aggrading reach where the channel becomes very 

wide and shallow, sections 4 and 5. Gradient varies 

little but it increases at section 6 where the steepening 

is attributed to incision of the channel. The percent 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 57 

FEET 

80 

40 

O 0:-----'--1.,.._000 __,_____, 2000 FEET 

LONGITUDINAL PROFILE 

FEET 

20L 

10 

0 

0 40 80 FEET 

CROSS SECTIONS 

'~------------------~ 

FIGURE 33.- Longitudinal profile and cross sections surveyed along Bayou Gulch, Colo. 

of silt-clay in the samples is a maximum at section 5 

where deposition is most noticeable. 

Median grain size decreases downstream but increases 

slightly at section 6 due perhaps to channel incision. 

Sorting index varies slightly but increases at section 5, 

suggesting that sorting is poorer on reaches of deposition. 

SUMMARY OF BAYOU GULCH AREA 

The conclusions reached are little different from those 

of the Santa Fe area. The sediment is deposited on 

the channel floor, resulting in an increase in width-depth 

ratio. Vegetation apparently does not aid initial 

deposition. 

In spite of the similarity of the Santa Fe and Bayou 

Gulch areas the width-depth ratio is much greater in 

Bayou Gulch. This is probably due to the decreased 

silt-clay content of bank material (see tables 4, 5). 

MEDANO CREEK, COLORADO 

DESCRIPTION 

To complete the series of silt-clay to sand sediment 

types, Medano Creek was selected because both channel 

and bank material are composed almost entirely of sand. 

Location.-Medano Creek (figs. 17 and 36) drains 

an area of about 29 square miles along the west flank 

of the Sangre de Cristo Mountains in the southeastern 

part of Saguache County and the northeast corner of 

Alamosa County. The part of the stream investigated 

lies within the boundary of the Great Sand Dunes 

National Monument in San Luis Valley. The drainage 

basin is shown on the Huerfano Park and Great Sand 

Dunes National Monument quadrangles. 

Annual precipitation.-Precipitation records at the 

Great Sand Dunes National Monument headquarters 

are of short duration, but an 11-year record at Blanca, 

about 20 miles to the south, shows the mean annual 

precipitation to be 9.20 inches (table 1). However, 

rainfall in the mountains is much higher than in the 

San Luis Valley. A 30-year record at La Veta Pass 

about 16 miles to the east shows mean annual precipitation to be 20.98 inches. Thus the climate ranges from 

subhumid to arid from divide to the lower parts of 

the valley. 

Vegetation and land use.- Vegetation ranges from 

typical alpine types in the high mountains to none on 

the sand dunes adjacent to the part of the creek studied. 

The basin is not used for agriculture. 

Physiography and geology.-The mountains are composed of Permian sedimentary rocks and Precambrian 

metamorphic and igneous rocks. Sediments derived 

from erosion in the mountains are not found in large 

amounts in the channel sediment a short distance below 

the mountain front, for as the stream flows between 

the mountains and the main sand dune mass to the west 

(fig. 34D), windblown sand becomes the predominant 

sediment found in the channel. What might be a perennial flow from the mountains is, within a few miles, 

completely absorbed by the sand. In 1957 the channel 
58 EFFECT OF SEDIMENT C:J;IARACTERISTICS 

D 

B E 

0 F 

FIGURE 34.-Bayou Gulch nnd Medano Creek, Colo. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 59 

TABLE 6.-Channel and sediment data 

[ Class S =stable ] 

Drainage Distance Weighted area between Median Hazen's Trask Silt-clay Channel Channel Width- mean Cross section Class (squar.e cross grain size effective size sorting (percent) Gradient width depth depth silt-elay, miles) sections Dso (rom) Dro(mm) index (feet) (feet) ratio (F) M 

(miles) ~ercent) 

Channel sediment 

L _ _ _ _ _ _ _ _ _ _ S 25. 85 _______ _ 

2___________ s 26. 10 0. 36 

3--- - - - - - - - - s 28. 84 . 65 

Mean _________ ----------------

0. 24 

. 24 

. 24 

. 24 

0. 15 

. 15 

. 15 

. 15 

1. 23 

1. 23 

1. 23 

1. 23 

1 

1 

.5 

.8 

0. 017 

. 019 

. 016 

340 

800 

820 

2 

3 

2. 5 

170 

267 

328 

1 

1 

. 5 

Bank sediment 

} ___________ -------- -------- -------- 3 ___________ -------- -------- ------ Mean ________________________ _ 

0. 24 

. 31 

. 28 

0. 014 

. 019 

. 017 

contained migrating sand dunes 2 miles below the campgrounds of the national monument. 

The eolian sand aparently accumulates here due to 

a re-entrant in the mountain front. Sand blown along 

the mountain front is trapped in this pocket, and over 

the years has accumulated into a tremendous mass of 

sand in places more than 500 feet high (fig. 34F). 

Alluvium.-Both the banks and channel of Medano 

Creek, in the reaches studied, are composed of sand 

derived from the main dune mass to the west (table 

6). In the channel at the 3 cross sections surveyed, 

median grain size is identical, 0.24 mm, as is D10, 0.15 

mm. Mean percent silt-clay is 0.8. Sorting index is 

1.23. The bank material has even less silt-clay, 0.5 

percent, but median size is 0.28 mm and D1o is 0.17 mm. 

The almost complete absence of silt-clay indicates 

(Burmiste~, 1952) that the material in banks and channel is freely drained and has no cohesion. The banks, 

in fact, are at the angle of repose of dune sand, about 

26°. 

Although by inspection this area would seem to have 

the coarsest sediment, the average median size for all 

cross sections is finer than for any of the other areas. 

1. 23 

1. 19 

1. 21 

0. 5 

. 5 

.5 

This is due to the excellent sorting of the windblown 

sand; no appreciable sediment larger than 0.5 mm or 

smaller than 0.15 mm was found in the samples. 

QUANTITATIVE CHANNEL VARIATIONS 

Neglecting changes in channel character near and in 

the mountains, the several miles of channel readily 

accessible showed only slight changes. At section 1 

the channel is confined between the dunes to the west 

and sand piled against the mountain front (fig. 34E). 

Downstream, however, the channel widens and widthdepth ratio increases (fig. 37; table 6). Gradient decreases but the character of the alluvium remains almost 

constant. 

Water was flowing in the channel during the study, 

and discharge was estimated as 10 cubic feet per second. 

A remarkable feature is that the water was flowing 

on a part of the channel well above the lowest point on 

the cross section. The water covered only a small part 

of the total channel width, about 80 to 100 feet, and 

ranged in depth from a thin film to 0.3 foot. During 

a flood, however, water would cover 800 feet of channel 

at the campground (fig. 34F). 

EXPLANATION OF FIGURE 34 

A, Bayou Gulch, view upstream between cross sections 2 and 3. Channel is relatively wide and sandy. Bank cutting is 

common in this reach. 

B, Bayou Gulch, view downstream toward cross section 5. Small levee on right side of channel prevents flooding of field to right 

of photograph. 

0, Bayou Gulch, view downstream from highway bridge toward cross section 6. Tree line indicates location of Cherry Creek. 

D, View from crest of sand dunes in Great Sand Dunes National Monument across Medano Creek toward the Sangre de Cristo 

Mountains. 

E, Medano Creek, cross section 1, view upstream. Note that water is flowing on only a small part of the cross section. The 

cross section plotted on figure 37 shows that the water is not flowing on the lowest part of the channel floor. 

F, Medano Creek, cross section 3, view to west toward main sand-dune mass. Point in channel from which photograph was taken 

is about 2 feet lower than part of channel on which water is flowing. 
60 EFFECT OF SEDIMENT CHARACTERISTICS 

Possibly between floods the water shifts laterally 

across the channel, building up by deposition that part 

of the channel on which it flows. This would and does 

result in the peculiar situation of water flowing at an 

altitude higher than much of the channel bottom. The 

channel surface on which water flows is braided. Pos0 

~ c( 

0:::: 

I 

1-

0.. 

UJ 

0 

± 

1-

0 

~ 

>- c( 

...J 

(.) 

I 

1-

...J 

c;; 

1-

z 

UJ 

(.) 

0:::: 

IJJ 

0.. 

X 

IJJ 

0 

z 

C!' 

z 

~ 0:::: 

0 

fJ) 

200 

160 

120 

80 

-

3 

2 >--

1 

CROSS SECTIONS 

2 3 4 5 

• • 

• 

I I I I 

• • • 

I I • I I 

I 

• 

I 

2 

I 

• 

I I 

• 

• 

I I I 

3 4 5 

CROSS SECTIONS 

6 

0.8 

0.4 

-

0 

6 

LJ' 

~(/) 

(/)0:::: IJJ ZI- -UJ c(:::E a::_ 

e,...J ...J 

z-

<:::E 

az UJ-

:::E 

FIGURE 35.-Variations in channel and sediment characteristics, 

Bayou Gulch, Colo. The spacing of the cross sections along 

the abscissa is proportional to the distances between cross 

sections in field. 

sibly, this is because there are no banks confining the 

small flows and also because water loss into the sand 

is high. 

Although only a shallow layer of water was moving 

downstream, large a1nounts of sand were in movement. 

Sand grains and even a few half-submerged pebbles 

were being rolled along the channel bottom. The water 

was clear, owing to the low percent silt-clay, and this enabled the author to see antidunes form on the channel 

bottom and move upstream. The upstream movement 

of the antidune itself could be observed rather than 

just its effect on the water surface. 

EXPLANATION 

3 

- I 

Location of cross section 

Boundary of drainage 

basin 

N 

Headquarters 

National Monument 

Based on U.S. Geological Survey 

topographic rna ps 

2 0 2 MILES 

FIGURE ;{~-Index map of Medano Creek, Colo., showing location of 

cross sections. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 61 

3 

f+-. Flowing water~ " _____________________________ J ____ __ i_ 

FIGURE 37.--cross sections surveyed along Medano Creek, Colo. 

Water flowing at and above the campground was 

completely absorbed by the sandy channel within onehalf mile downstream. It was not possible to study the 

aggrading reach below the campground because sand 

dunes had migrated into that part of the channel. In 

addition, a veneer of windblown sand covered the channel making the distinction between aeolian and fluvial 

deposition difficult. 

SUMMARY OF MEDANO CREEK AREA 

This channel, composed of dune sand with banks of 

the same material, is the widest and shallowest of the 

channels studied. The channel anti sediment character 

changes little downstream. Even though average median sediment size is the smallest sampled, this channel 

is typical of those carrying the less cohesive sediments. 

COMPARISON OF SEDIMENT AND CHANNEL CHARACTERISTICS BETWEEN AREAS 

In the preceding section the changes in sediment type 

and channel character along a stream are discussed for 

five areas differing in alluvial characteristics. In this 

part of the report, the differences and comparisons 

among the five areas will be reviewed, additional data 

will be presented, and practical applications of the conclusions will be discussed. 

During this investigation, which was aimed mainly at 

revealing the mechanics of erosion and deposition in 

small ephemeral-stream ehannels,·data were eollected at 

some eross sections that were ultimately determined to 

be stable rather than aggrading or degrading. This was 

unavoidable, for during the early stages of the study, the 

stable eross seetions could not be readily identified. 

However, the availability of the stable cross section 

data allows comparison of the stable channel characteristics among the five study areas. In addition, if the 

stable sections are discussed first, then the characteristics 

and variability of the unstable sections can be compared 

with the characteristics of stable cross sections. 

RELATION BETWEEN SEDIMENT AND CHANNEL 

CHARACTERISTICS 

The stable channel cross sections were selected from 

all those studied. The separation was made partly for 

comparative purposes, for it is assumed that those sections approaching or having reached stability will afford 

a better basis for comparison of channel and sediment 

eharacteristics among the five areas than a mean value 

for all sections studied in each area. 

The mean values for channel and sediment characteristics for the stable cross sections as well as similar 

data for those sections which are being actively aggraded are shown in table 7. It was more difficult to 

determine if a particular section \Yas being actively degraded although three cross sections are so classed 

(tables 2, 3). Those sections not listed in table 7 

were either doubtfully stable or not being aggraded 

and are the unclassified channels of tables 2-5. All cross 

sections classed as aggrading, however, are not listed on 

table 7, for at many such sections the cha:nnel was completely filled. 

The study areas are listed on this table in the order 

in which they were discussed in the preceding sections, 

namely, in the order of apparent coarsening of the 

channel alluvium and increasing width-depth ratio. 
62 EFFECT OF SEDIMENT CHARACTERISTICS 

TABLE 7.-·Mean values of channel and sediment data 

Median 

Study area Type of cross Cross section grain size 

section nso (mm) 

Sage Creek ___________ Stable ________ 2, 3, 4_----- 0.080 

Aggrading ____ 6, 7, 10, 11 . 067 

12. I>o ______________ Stable ________ 3, 4, 5, 6- - - - . 53 

Aggrading ____ 7, 8 ________ . 022 

Arroyo Calabasas _____ Stable ________ A, I>, 1, 2, . 81 

6, 7. 

Aggrading ____ B, 3, 8 _____ . 52 

Bayou Gulch _________ Stable ________ 1, 2, 3 ______ . 61 

Aggrading ____ 4, 5 ________ . 51 ~fedano Creek ________ Stable ________ 1, 2, 3 ______ . 24 

Aggrading ____ ------------ --------

1 Based on sections Band 8 only. 

A glance at table 7, however, i'mmediately suggests the 

order to be incorrect, for there is no progressive increase 

in median grain si=?e ( Dso) or D10 from area 1 to 5 in 

spite of appearances in the field. The percent silt-clay 

in the channel samples, however, progressively decreases 

from a high value for the Sage Creek area to a negligible amount for the Medano, Creek area, although the 

values for Arroyo Calabasas and Bayou Gulch are the 

same. 

The character of the sediment forming the perimeter 

of the channel can be expressed as a weighted mean 

percent silt-clay, designated M (Schumm, 1960b}. ~/ 

is calculated as follows 

ScXW+SbX2D 

M= W+2D 

in which Sc=percent silt-clay in channel alluvium, 

Sb=percent silt-clay in bank alluvium, 

D=channel depth, 

W =cha:d.nel width. 

The weighted mean percent silt-clay, hereafter referred 

to as M in the text, is given for all cross sections at 

which bank and· channel samples were collected (see 

tables 2-6}, and the mean values for st·able and aggrading sections are presented in table 7. M for the stable 

cross sections decreases in a manner similar to that for 

channel silt-clay. 

It is interesting to note that the mean sorting index decreases progressively from area 1 to 5; the 

samples become better sorted as the percent silt-clay 

decreases. This may be explained by the fact that none 

of the streams contained appreciable amounts of gravel, 

so as the finer components of the sediment were eliminated the sorting naturally imprqved. 

The mean width and depth of the channels bear an 

inverse relation to each other, but both show a progressive change as percent silt-clay decreases. The 

mean width-depth ratio shows a progressive increase 

as channel percent silt-clay and A/ decreases. In figure 

Hazen's Trask Channel Channel Width- Weighted effective sorting Silt-clay Gradient width depth depth mean siltsize DIO index (percent) (feet) (feet) ratio clay, M 

(mm) (F) (percent) 

0.0035 5. 74 54 0. 0048 22 6. 3 3. 8 69 . 0049 4~ 96 65 . 002 20 4.0 5. 1 70 

. 032 2. 45 14 . 0023 53 5. 3 10.3 22 . 0013 2. 43 89 . 001 42 2. 7 18.8 88 . 21 1. 99 4 . 011 81 3. 6 28. 1 5. 7 

. 19 1. 79 3. 2 . 012 1 173 11.8 1 94.5 1 4. 6 

. 19 1. 72 4 . 011 153 2. 5 62 5. 1 . 12 2. 17 7 . 010 290 1.8 166 7. 1 . 15 1. 23 0. 8 . 017 653 2. 5 255 . 8 

-------- ------ -------- -------- ------ ------ ------ --------

38 the values of width-depth ratio for 18 stable cross 

sections are plotted against weighted mean percent siltclay· ( M) of the channel and bank samples taken at each 

of the cross sections. 

The relation is such that a narrow and deep channel 

is associated with sediments high in silt-clay; whereas, 

those channels containing little silt-clay are wide and 

shallow. The regression line for figure 38, although 

not the best fit for the data shown, is based on data for 

69 cross sections including those plotted on figure 38 

(Schumm, 1960b). 

The gradients of the streams show a general steepening with decreasing silt-clay, but this is not progressive, 

for the Sand Creek area has a somewhat gentler gradient 

than Sage Creek. A plot of M against gradient (fig. 39) 

reveals that stream gradient increases as M decreases. 

Thus for those small ephemeral streams where the 

annual rainfall is in the range 10 to 20 inches both 

channel shape and gradient are related toM. It is probable that a different relation exists between gradient 

and M for larger streams and those in more humid 

regions. The relation between M and width-depth ratio 

is valid for a large range of streams in different climatic 

regions (Schumm, 1960b). 

SUMMARY OF COMPARISONS 

Differences in sediment type among the study areas 

are related to variations in channel characteristics. As 

progressively wider and shallower sections of the stable 

channels were considered an accompanying decrease in 

the percent silt-clay composing the perimeter of the 

channel was noted. In addition, a decrease in channel 

gradient occurred as M increased. 

It is assumed that these relations may be only the 

most obvious. Further work may reveal that other 

aspects of fluvial hydraulics and morphology are related 

to a parameter, .such as lJf, expressive of the physical 

properties of the alluvium forming stream channels. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 63 

1000 

• '\1\.. 

r-.~ 

•• ~ I 

"" -" 100 

"\. ~ 

• "' •• ~ 

•'" ' 

~~~ ";ss • • r\ ~' • f\ ~a 

-~~ • 

10 ~ • 

"\. 

"' ' • ~ 

r"\. I 

I '\ 

I'\ ~ 

~ ~ 

1\ 

l.O 0.1 1.0 10 100 

WEIGHTED MEAN PERCENT SILT -CLAY (M) 

FIGURE 38.-Relation between width-depth ratio (F) and weighted mean perc~nt silt-clay (M) In bank and channel sediments for stable cross 

sections. 

DEPOSITION IN EPHEMERAL STREAMS 

CROSS SECTIONS 

During the discussion of the study areas, some comparisons between the manner of aggradation in each 

area. were made. The changes in width -depth ratio as 

an aggrading reach was approached and crossed indicates that as deposition increased the channels containing highly cohesive sediments with a high percent 

silt-clay became progressively narrower; whereas, channels containing low-cohesion sediments became shallower with little change in width. The modification 

of the shape of a stream channel by aggradation seems 

to be determined by the mechanics of sediment deposition in the channel. 

Blench (1957, p. 13) describes lateral deposition of 

silt along the sides of Indian regime canals and explains 

how this phenomenon is used in the repair of breached 

canals and in design. According to Blench, the typical 

Indian canal has a sand bed and berms of silty-clay 
64 EF-FECT OF SEDIMENT CHARACTERISTICS 

100 

~ '-'- 10 >- :s 

0 

I 

s 

(j) 

1--

z 

Ul 

0 

0: 

w 

a... 

z 

< 1J..I 

:2: 

0 

l&J 

1--

ili 1.0 w 

~ 

'" "' ' . . \. 

' • 

'\. 

"\ ..... 

• •• -" \ " _'\. - • ' \. .. ,- • •"\. . "I\. 

• ' \ ' " "\. 

"' \. 

' '\. 

It has been suggested that the correlation between 

width-depth ratio and M might be used as a criterion 

of aggradation or degradation (Schumm, 1960b). 

That is, since stable channels fall about the line described by the equation F=255 M-1 •

08, then points that 

plot well above this line might be assumed to be 

aggrading; whereas, those plotting below the line would 

be degrading. To test this hypothesis the data for 

stable degrading, and aggrading cross sections were 

plotted on figure 40 about this regression line. 

The 11 aggrading sections plotted all fall above the 

regression line; however, only 7 would be recognized 

as aggrading from their position on the graph. The 

unstable condition of the remainder would be obscured 

by the natural scatter about the regression line. However, since the Sage Creek cross sections have a decrease in width-depth ratio with aggradation, these 

points would not be expected to fall above the regression line. In any event, it seems that those cross sections which plot well above the regression line may 

be considered aggrading. 

Of 3 cross sections originally classed as degrading, 

there are adequate data for plotting of 2 (Sand Creek 

sections 10 and 11; table 3). These two sections plot 

below the regression line of figure 40. Perhaps on the 

basis of the location on figure 40 of sections known to 

be aggrading or degrading, it may be possible to classify 0.1 0 .004 .008 .012 

CHANNEL GRADIENT 

.016 .o2o the remaining cross sections, that is, those listed as 

unclassified on tables 2 through 6. 

FIGURE 39,-Relation between stream gradient and weighted mean 

percent silt-clay (M) for stable cross sectiom'l. 

loam. The berms are deposited from suspended load. 

Two study areas of this investigation, Sage Creek and 

even Sand Creek at section 7, show berm development. 

The channels with low percent silt-clay in banks and 

channel, Bayou Gulch, Arroyo Calabasas, and Medano 

Creek, show only deposition on the channel bottom. 

Latm~J deposition is due both to the cohesiveness of 

the finer sediment and to its . high concentration 

throughout the mass of flowing water. Hjulstrom's 

( 1935, p. 27 4) calculations demonstrate that a mixture 

of silt and clay with an average grain size of 0.001 mm 

will have a concentration 5 meters above the channel 

floor only slightly less than that measured just above 

the channel floor; whereas, the concentration of sediment with an average size of 0.10 mm rapidly decreases 

to 10 percent of that just above the channel at a height 

of only 25 em. Thus, under similar conditions of turbulence the finest sediment is available for deposition 

along the banks; whereas, the coarse material will only 

be deposited on the channel floor. 

Twelve of the 49 cross sections studied are unclassified. Insufficient data are available to allow computation of width-depth ratio or M for six of these. The 

remaining 6 cro86 sections (Sand Creek 2 and 12; Sage 

Creek 5, 8, and 14; Bayou Gulch 6) are plotted on 

figure 40. Four of these are located close to the regression line (Sage Creek 2 and 8, Sand Creek 5, Bayou 

Gulch 6), and these sections may be stable. 

If the field information on the nature of these cross 

sections is considered, each is located on a reach of 

channel either affected in the past by channel changes 

or to be affected by channel changes in the future. For 

example, the channels of Sage Creek at section 2 and 

Bayou Gulch at section 6 have been degraded in the 

past but have apparently reverted to a stable form. 

Sand Creek section 5, on the other hand, has not yet 

been affected by the aggradation occurring downstream 

at section 6. Sage Creek section 8, although believed 

to have been eroded and then subjected to recent deposition, also has a form characteristic of a stable channel. 

The erosion occurring a short distance downstream, 

however, will undoubtedly cause marked changes at 

this section in the near future. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 65 

1000 

100 

10 

1.0 

0.1 

-

• i\~ 

~-" • ""\ 

EXPLANATION, 

• Stable cross section 

+ Aggrading cross section 

• Degrading cross section 

• 2unclassified cross section 

1.0 

~ 

~ .. ,-

" ~ 

\. 

• " \. 

."J ' "'\ 

• • • 

+ 

A' 

~ 

"\~IS'~ 

\ ~, i 

. ~\ • H ~ •-

" ~ _a 

" • "\. • s ... 

1'\. IT 1~ 

'~ .2 r\ ·.a 

.12 \, • ' II 

10 100 

WEIGHTED MEAN PERCENT SILT-CLAY (M) 

FIGURE 40.-Relation between width-depth ratio and weighted mean percent silt-clay for stable and unstable cross sections. Numbers refer to 

cross sections, tables 2, 3., and 5. 

The remaining two sections plot far from the regression line. Sage Creek section 14 plots well above the 

line, and it was noted in the field that water diverted 

onto the flood plain by a small dam enters the channel 

and causes deposition at this section. Sand Creek section 12 plots well below the line indicating channel 

erosion. Section 12 is located only 0.3 mile below section 11 which was previously classified as degrading. 

Either degradation is still occurring at section 12 or 

the section has not been able to adjust its form as yet 

to stable conditions. The above suggests that the location of the unclassified sections with regard to the 

regression line of figure 40 makes it possible to classify 

the sections as to their stability or lack of it. 

SLOPE 

In addition to changes in channel shape accompanying aggradation, there are concomitant changes in the 
66 EFFECT OF SEDIMENT CHARACTERISTICS 

longitudinal profile of the stream. As is well known 

and was noted in each of the study areas, stream gradient is decreased on the upstream part of the aggrading 

reach. The slopes n1easured at the cross sections in the 

field do not show this in all cases (figs. 22, 27, 31, 35) 

but the longitudinal profiles do (figs. 19, 24, 29, 33). 

All the graphs and longitudinal profiles show an increase in gradient downstream on the aggarding reach. 

In all aggrading channels this steepening enables the 

stream to increase its capacity for sediment load. Generally, incision occurs on the steepened reach (Schumm 

and Hadley, 1957). Where the channel is completely 

filled by deposition on the steeper reach, the anom:;tlous 

situation arises that the finer alluvium is found on the 

steeper aggrading reaches. See section 10 on figure 27 

and sections B, 5, and X on figure 31. 

The data collected at cross sections located on the 

aggrading reaches of the study areas give a fairly complete picture of what might occur at one cross section 

during the complete filling of the channel. During the 

discussion of the historic changes on Sand Creek it was 

concluded that the upstream limit of aggradation 

shifted upchannel as aggradation progressed. Channel aggradation, therefore, may be somewhat analagous 

to backfilling of the channel, rather than to a general 

filling of the channel simultaneously over a long 

distance·. 

Assuming that aggradation begins at one particular 

point in the channel and then progresses upstream, it is 

possible to outline the steps in the complete cycle of 

aggradation and retrenching of that channel. Gradient 

is decreased by the initial channel deposit, and if it is 

not swept away by the next flood, this alluvial deposit 

induces further deposition in the channel. As the channel fills, the zone where major deposition of the coarser 

fraction of the alluvium occurs, migrates upchannel, 

and progressively finer sediment is deposited over the 

originally coarser grained sediment at the site of initial 

aggradation. 

In several reaches of the Sand Creek and Arroyo 

Calabasas channels the sediments in the banks, where 

exposed by trenching, show a progressive coarsening 

of the alluvium from top to bottom of the deposit. 

Thus, at any one point in an aggrading channel, sediment size generally decreases as deposition progresses, 

and accompanying this is a decrease in channel size. 

When the channel is almost filled overbank deposition 

becomes very important. Continued overbank deposition of fine sediment causes a steepening of the valley 

floor. In a short time the channel is completely filled, 

and floodwaters cover almost the entire valley floor. 

Nevertheless, a short distance upstream the channel still 

exists, but it is being filled. The progressive shallowing 

of the channel downstream is best shown between cross 

sections 6 and 9 on figure 24 as is the decrease in gradient as deposition in the channel begins (fig. 24, sections 

6-8). The steepening of the gradient after complete 

filling of the channel (fig. 24, sections 9, 10) is also 

clearly shown (see also Schumm and Hadley, 1957, figs. 

3B, 6A). Accompanying the change in gradient, percent silt-clay increases from section 6 to a maximum for 

all sections at 9, and median .grain size decreases from 

cross section 6 to 9. 

In the trench a hove the aggrading reach on Sand 

Creek the sediments exposed in the bank between sections 5 and 6 from bottom to top are: 2 feet coarse 

g:.;a vel and cobbles; 6 feet coarse-to-fine sand and silt ; 

6 feet of silt and clay with some fine sand. In the 

trench below the aggrading reach the same type of 

sequence occurs. From auger holes bored in the recent 

sediments, the gravel layer was found at the level of 

the channel floor between sections 5 and 6, at a depth 

of 2.5 feet at section 6; at 4.0 feet at section 7; and at 

9.3 feet at section 8. The gravel reappears in the channel floor just above the country road, a,p.d its inferred 

relation to the longitudinal profile is shown by the 

dashed line below the profile on figure 24. This gravel 

layer may be the level of the channel floor before aggradation. If this is so, the convexity on the longitudinal 

profile shown between sections 6 and 10 above the gravel 

layer (fig. 24) is probably the recent channel and valley 

fill. Renewed trenching has occurred on the lower end 

of the deposit between sections 10 and 11. This cycle 

of channel filling, aggradation and steepening of the 

valley floor, and retrenching of the alluvium has been 

previously discussed as the common pattern in semiarid 

valleys (Schumm and Hadley, 1957). 

To summarize, the decrease in grain size and increase 

in silt-clay across an aggrading reach, as shown on 

figures 22, 27, 31, and 35, indicates that as one moves 

downstream along an aggrading ephemeral stream relatively coarse sediment will be found upstream and the 

finer sediments will be found on the downstream edge 

of the deposit. In addition, there should also be an 

upward decrease in grain size from bottom to top of the 

deposit at any one location. The change in gradient is 

from a decrease in the filling channel to an increase 

where the channel has been filled and flood-plain deposition is dominant. 

In the studying of aggrading channels, it is important 

to recognize and to anticipate marked variations in 

channel character and sediment type downstream due 

to aggradation. In other words, the orderly relation 

shown in many streams will be disrupted by aggradation, and anoma~ies are to be expected. This is especially true of channels which do not have permanent 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 67 

flow between floods. The generally assumed ability of 

a stream to adjust to changed conditions does not apply 

to ephemeral streams in channels that are being rapidly 

aggraded. The ephemeral streams, when attempts to 

adjust fail, follow a pattern of alternate deposition and 

erosion (Schumm and Hadley, 1957). 

VEGETATION 

Observations have shown that the influence of vegetation differed between the silty and sandy study areas. 

In silty sediments, typified by the Sage Creek area, 

growth of vegetation was rapid on the modern sediments. The photographs of the Sage Creek channel 

(figs. 20 0-F; 210, E, F) show vegetation on the banks, 

berm, and channel bottom. Figure 21E shows that vegetation quickly establishes itself after the spring floods. 

In a channel the vegetation grows only on areas where 

deposition has occurred since channel incision. V egetation, therefore, may aid deposition once it has begun, 

but fresh alluvium in the channel seems necessary for 

growth of vegetation. 

The establishment of vegetation on the noncohesive, 

highly mobile sediments is much more difficult. These 

channels were remarkably free of vegetation even when 

aggrading (figs. 25B, 0; 300, E, F), but as soon as 

a veneer of finer sediment was deposited on the sand, 

vegetation began to encroach on the channel and (figs. 

25D, E; 30A, B) eventually completely covered it. 

As discussed in the description of Arroyo Calabasas, 

vegetation does grow in the sandy channels when undisturbed-such as below dams that have not spilled 

recently-but flow in the streams of high bed movement 

will cause washing out of seeds and plants and prevent 

the development of permanent vegetation. Vegetation, 

therefore, will accelerate deposition but in the areas 

studied is not the cause of it. 

CAUSES OF DEPOSITION 

One of the primary purposes of this investigation is 

to determine, if possible, the reason for aggradation in 

the study areas. The major part of the sediment load 

is derived from different sources in the different areas. 

In the Bayou Gulch and Arroyo Calabasas drainage 

basins the sediment is produced by slope erosion and 

by channel erosion along the dendritic patterned tributaries and main channel. The sediment is not derived 

from any restricted zone within the basin, but each 

tributary conveys sediment to the main channel. In 

the Sage and Sand Creek basins, most of the sediment 

is derived from an escarpment forming the headwater 

divide. 

In any event, each of the study areas is one of high 

sediment production. No matter what its source, high 

sediment yields are probably the cause of aggradation 

in these stream channels. The increase in sediment 

yields and retrenching of the Sage, Sand, and Arroyo 

Calabasas channels cannot be attributed to any special 

cause, such as climate change or overgrazing. In the 

absence of such evidence the channel cutting can be related only to the presence of reaches of steeper gradient 

in each valley, which have been built up by deposition 

to be inevitably trenched. This sequence of events has 

been proposed as the normal cycle of development of 

ephemeral streams (Schumm and Hadley, 1957). 

Although causes for aggradation may not be 

completely understood, the reason for its occurrence in 

a particular segment of a stream channel should be 

considered. It seems that aggradation in these channels 

is associated with segments of the stream that receive 

only small contributions from tributaries. The loss of 

water into the channels of ephemeral streams (Babcock 

and Cushing, 1941) and its function in promoting 

aggradation (Schumm and Hadley, 1957) have been 

studied and found important. It is logical to assume 

that in those reaches of the channel where contributions 

of runoff from tributaries is minor, the concentration 

of sediment will increase and aggradation might occur. 

Other factors would undoubtedly complicate this relation, and it is know that the entrance of a steep-gradient 

tributary often results in deposition at and below its 

junction with t11e main channel. 

Listed in the tables giving the basic data for each 

study area are the drainage area above each section and 

the stream channel length between each section (tables 

2-5). The increase in drainage area per mile of channel 

length above and on the aggrading reaches are presented 

in table 8. A comparison of the ratios shows a marked 

differenee. In each area, except that of the Arroyo de 

los Frijoles and Arroyo Calabasas, deposition is 

occurring in reaches where the increase of drainage area 

is much less per unit length of channel than on the stable 

reaches. As previously noted, most of the Arroyo Calabasas drainage basin to the west of the channel, below 

the junction with Arroyo de los Frijoles, is composed 

of volcanic rocks from which little runoff and sediment 

reaches the stream. If most of this area is omitted 

from the calculation, then the ratio on the aggrading 

reach drops to about one-half of its value upstream, 

and the ratios are comparable to those for the other 

study areas. This relation suggests that aggradation 

oceurs in these small ephemeral streams on reaches of 

small tributary contribution, a result, perhaps, of 

increased sediment concentration due to water loss into 

the alluvium of the valley floor. 

An additional important point must be the differenee 

in general appearance between the same valley where 
68 EFFECT OF SEDIMENT CHARACTERISTICS 

TABLE 8.-Ratio of drainage area to channel length 

Non. Drainage Channel AreaStudy area aggrading Aggrading area length length 

reach reach (square 

miles) 

(miles) ratio 

Sage Creek ____________ 1-6 12.7 5. 7 2.2 -------~7-- .26 1. 1 .2 

Sand Creek __ --------- 2-6 ------------ 6.4 3. 6 1.8 

6-10 1.7 1.6 1.1 

Arroyo de los Frijoles C-6 ------------ 19.4 3. 2 6.0 

and Arroyo Cala- 6-9 18.1 2. 7 6. 7 basas. 16-9 17.2 2. 7 2. Bayou Gulch __________ 1-4 ------------ 9.0 1.1 8.1 

4-5 .5 . 2 2 . 

1 Adjusted for noncontributing area. 

the channel is completely filled and grassed and where 

trenched. In the flat-floored valley the alluvium may 

contain much water, thereby supporting heavy vegetation. In addition a veneer of fine sediment generally 

overlies the coarser sediment. In areas of predominant 

sand and gravel, therefore, the valley floor will be an 

area of much finer sediment. Upon renewed trenching, 

however, the c~arser sediment will be exposed and a 

wide shallow channel will form, which will appear 

inconsistent with the fine surface sediment. Thus, the 

sampled surface of an untrenched valley floor may 

give an erroneous picture of the composition of the 

major part of the valley alluvium and its behavior 

during erosion. 

EROSION IN EPHEMERAL STREAMS 

The phase of the semiarid cycle of valley erosion that 

has received the most attention is channel incision or 

arroyo cutting. In this section, the differences between 

FEET 

40 

20 

erosion and channel development in the different study 

areas will be discussed. 

CHANNEL INCISION 

Comparisons of channel erosion in the study areas 

and elsewhere show that after initial cutting of a 

grassed valley floor, the channel extends head ward by 

headcut migration. The perpetuation of a headcut 

seems to require some resistant material capping the 

alluvium. This resistant cap may be formed by the 

binding action of plant roots, or a veneer of silt and 

clay overlying the coarser materials. If this resistant 

zone were lacking, the headcut would be rounded and 

lowered as it retreated upchannel, and it would lose its 

identity in a short distance. Where trenching was 

renewed and a headcut was not formed, Arroyo Calabasas, Bayou Gulch, and Sage Creek below the aggrading reach, a well-defined channel was already present, 

typified by a generally uniform resistance in vertical 

section. In the broad sandy channels without fine 

sediments and vegetation no headcut will form. 

An interesting example of the removal of a nick or 

break in the profile of a sandy channel was found in 

New lin Creek, a tributary to Cherry Creek located to 

the west Bayou Gulch in Douglas County, Colo. The 

profile of Newlin Creek, when plotted from a topographic map (Parker, Colo.) prepared in 1939, showed 

a prominent nick in the channel (fig. 41). The part 

of the Newlin Creek profile showing the nick, between 

the two secondary roads, in sections 17 and 20, was 

resurveyed in 1957. The altitude of the channel at the 

1939 profile from Parker topographic map 

FIGURE 41.-Comparison of the longitudinal profile of Newlin Creek, Colo., in 1939 and 1957. 
EROSION AND SEDIMENTATION IN A SEMIARID ENVIRONMENT 69 

lower road had not changed measurably ~uring the 

18-year period, but the profile between the roads had 

been smoothed by removal of the nick. The small 

break in the 1957 longitudinal profile at its upper end 

is due to the road crossing at that point, which 1nay 

have retarded further degradation, for the road acts 

as a small clam. 

The changes occurring on Newlin Creek agree with 

those observed by Brush (1957) during flume experiments and suggest that in poorly cohesive material, not 

capped by a more resistant layer, the headward migration of the nick will be accompanied by a decrease in 

its height. 

The change in channel characteristics along New lin 

Creek from above the upper road to below the original 

position of the nick in 1939 illustrates the changes 

occurring during degradation of a sandy channel. 

Above the upper road, the channel is 130 feet wide, 

and sand is being deposited over another 100 feet of 

the valley floor. Width-depth ratio approaches 90. 

Below the road degradation begins; the channel narrows to 40 or 50 feet, and width-depth ratio decreases 

to 17. The degradation causes erosion along many. 

interconnected water courses, forming vegetationcovered islands separated by degrading sandy channels. 

One channel probably will become dominant after eontinned erosion. 

Downstream, evidence of greater erosion appears; 

roots of trees are exposed and cobbles and boulders appear in the channel bottom. At the site of the former 

nick; the gully is 10 feet deep and is 20 to 40 feet wide; 

width-depth ratio is about 3. Below the lower road 

the channel seems aggraded, shallowing and widening 

to its junction with Cherry Creek. Sediment samples 

taken in the eroding channel show only minor variation 

in grain size (D5o=0.60) and percent silt-elay (2.0). 

BANK CAVING 

Another important phenomenon that occurs during 

the development of a stable channel is bank caving. 

The relative amount of bank caving along the streams 

studied and its effect is markedly different in areas of 

cohesive and noncohesive sediment. 

In poorly cohesive alluvium, or that with a small 

percent silt-clay, the channel widens rapidly by bank 

caving after initial dissection; whereas, in cohesive 

sediment, predominantly silt-clay, the blocks of sediment that fall into the channel are resistant and do not 

move· or disintegrate readily, although this will depend 

on velocity of water movement. They may even form 

the nucleus for deposition along the banks and prevent 

further channel widening (fig. 200). Vegetation, 

growing on banks before the caving, often continues to 

grow on the fallen bloek if rotation of the block was not 

great. The vegetation in turn promotes further aggradation in the channel. 

Blocks of poorly cohesive alluvium, on the other 

hand, disintegrate upon impact or later under the 

eroding action of flood waters, and the sediment adds 

to the bed and suspended load of the stream. Slumping or caving .of bank material into the stream channel 

and the function of slump blocks in sediment supply 

and aggradation, therefore, differ with type of bank 

material. 

PRACTICAL CONSIDERATIONS 

Because of a critical need for effective control of 

erosion and deposition in ephemeral streams, an attempt will be made to suggest how some of the information and conclusions stated previously might have a 

practical application. The differences in erosion and 

deposition in the study areas suggest that different 

types of conservation techniques may be necessary for 

different types of alluvium filling the valley. 

Generally, in valleys where the streams are ephemeral 

conservation measures are not begun until improvement 

will require large expenditures of time and money. 

For example, once a gully has formed the only solution 

is to build either a dam or diversion structure above 

the headcut, to prevent its headward migration, or to 

plug the gully and fill it by inducing deposition in 

the trench (Peterson, 1950). However, if it were possible to determine at what point in the valley 

trenching would begin, then it might be more economical and certainly more practical to prevent the initial 

eroswn. 

Some studies in semiarid valleys suggest that a 

discontinuous gully will for1n on the steeper parts of 

an alluvial valley (Schumm and Hadley, 1957). If 

good topographic maps are available for the areas concerned, it would be possible to plot the longitudinal 

profile of the stream and discover the steeper reaches 

where trenching· might begin. If maps are not available, the valley profile could be surveyed, or perhaps, 

the critical gradient changes could be revealed by 

photogrammetric methods. Once the steeper reaches 

have been discovered, the valley floor could be protected 

by restricting grazing. However, since in many areas 

the valley floor affords the best grazing and because the 

critical reach would need to be fenced, this solution may 

not be acceptable to the local rancher. Another possibility would be the construction of small earth diversion dikes to slow a11d spread the flow of -water- across 

the steeper reach. This, however, might cause deposition and further steepening on this already critical 

reach. This discussion of preventive conservation 

measures on the steeper parts of the alluvial fill empha-
70 EFFECT OF SEDIMENT CHARACTERISTICS 

sizes that only certain parts of a valley need be protected to conserve the entire valley. 

In three of the study areas selected for this investigation, problems of erosion and deposition are acute. 

Conservation measures should be directed at not maintaining but restoring the valley floor to its ungullied 

state. This may be possible only by inducing deposition in the gully proper, thereby filling and healing the 

trench. Such a project can succeed best where the 

method used is designed to take advantage of a natural 

tendency to deposition. In each valley studied deposition was shown to occur naturally where tributary contribution is small, where gain of drainage area per mile 

of channel is less than normal (table 8). Therefore, 

if these limited, observations are confirmed elsewhere, 

a structure placed in a valley to promote deposition 

should be located where the ratio of increase of drainage area to channel length is small. 

Deposition also may be induced by using reverse 

delta deposition near the confluence of the channel with 

a trunk stream. If a dam is built on the tributary a 

short distance above its 1nouth, the retention of water 

will allow flood waters to enter the channel from the 

main stream, causing deposition below the dam and 

plugging of the valley. Deposition in the reservoir 

above the dan1 will also be important. The lower e:pd 

of Sage Creek has been filled in this manner. Once 

deposition has started, care should be exercised that 

renewed trenching on the toe of the alluvial deposit 

does not occur. 

If incipent aggradation could be recognized in a 

channel, it might be accelerated by conservation Ineasures. As indicated by a reexamination of changes in 

sediment and channel character across the aggrading 

reaches (figs. 22, 27, 31, 35) deposition may be detected, where it is not obvious, by a decrease in gradient 

and an increase in percent silt-clay in the channel sediment. Perhaps width-depth ratio can be used, for 

width-depth ratio is increased by aggradation except 

for those areas of high silt-clay where initial deposition 

is along the channel banks for example, Sage Creek. 

Perhaps in similar channels if the width-depth ratio 

in any reach is much higher than that upstream or 

downstream, deposition may be expected (fig. 40). 

Vegetation aids deposition only after deposition has 

begun. Vegetation is readily established on the fine 

alluvium but not in predominantly sandy material. 

It appears therefore if vegetation does not occur naturally in a channel, it probably cannot be induced to 

grow there without great expense. Planting grass and 

trees in a semiarid valley would be practical only as a 

means of stabilizing an alluvial deposit. But care 

should be exercised that the conservation measure may 

not have an eventual adverse effect on valley stability. More work is required on this and other problems related to deposition, but it becomes increasingly 

apparent that a careful study of each valley should be 

made before conservation measures of any type are 

initiated. 

REFERENCES 

Babcock, H. M. and Cushing, E. M., 1941, Recharge to ground 

water from floods in a typical desert wash, Pinal County, 

Arizona: Am. Geophys. Union Trans., v. 22, pt. 1, p. 49-56. 

Blench, T., 1957, Regime behavior of canals and rivers: London, Butterworth's Publications, 138 p. 

Brush, L. M., Jr., 1957, Laboratory study of knickpoint behavior in noncohesive materials [abs.] : Geol. Soc. American Bull, v. 68, p. 1704. 

Burmister, D. M., 1952, Soil mechanics: New York, Columbia 

Univ. Press, 155 p. 

Dunn, I. S., 1959, Tractive resistance of cohesive channels : 

Jour. Soil Mechanics and Foundations Div., Am. Soc. Civil 

Engineers Proc., SM 3, p. 1-24. 

. Hjulstrom, Filip, 1935, Studies of the morphological activity of 

rivers as illustrated by the River Fyris: Univ. Upsala 

Geol. Inst. Bull., v. 25, p. 221-527. 

Leighly, J. B., 1934, Turbulence and the transportation of rock 

debris by streams: Geog. Rev., v. 24, p. 543-464. 

Leopold, L. B. and Maddock, Thomas, Jr., 1953, The hydraulic 

geometry of stream channels and some physiographic implications: U.S. Geol. Survey Prof. Paper 252, 57 p. 

Loopold, L. B. and Miller, J.P., 1956, Ephemeral streams-hydraulic factors and their relation to the drainage net: U.S. 

Geol. Survey Prof. Paper 282-A, p. 1-37. 

Peterson, H. V., 1950, The problem of gullying in western valleys; in Trask, P. D., Applied sedimentation: New York, 

John 'Viley & Sons, 707 p. 

Schumm, S. A., 1956, The role of creep and rainwash on the 

retreat of badland slopes: Am. Jour. Sci., v. 254, p. 693-

706. . 

--1960a, The effect of sediment type on the shape and 

stratification of some modern fluvial deposits: Am. Jour. 

Sci., v. 258, p. 177-184. 

--1960b, The shape of alluvial channels in relation to sediment type: U.S. Geol. Survey Prof. Paper 352-B, p. 17-31. 

Schumm, S. A. and Hadley, R. F., 1957, Arroys and the semiarid cycle of erosion: Am. Jour. Sci., v. 255, p. 161-174. 

Thornthwaite, C. W., 1941, Atlas of climatic types in the United 

States, 1900-1939: U.S. Dept. Agr. Misc. Pub. 421, 7 p. 

Ward, Freeman, 1922, Geology of a portion of the badlands: 

South Dakota Geol. Survey Bull. 11, p.1-59. 

Wolman, M.G., 1955, The natural channel of Brandywine Creek, 

Pennsylvania: U.S. Geol. Survey Prof. Paper 271, 56 p. 

'Volman, M. G. and Leopold, L. B., 1957, River flood plainsSome observations on their formation: U.S. Geol. Survey 

Prof. Paper 282-C, p. 87-109. 

*U.S. GOVERNMENT PRINTING OFFICE: 196Z 0 -6Zl994