A Tentative Classification of Alluvial River Channels
A Tentative Classification of Alluvial River Channels By S. A. Schumm
An examination of similarities and differences among some Great Plains rivers
GEOLOGICAL SURVEY CIRCULAR 477
Washington J 963 United States Department of the Interior STEW ART L. UDALL, SECRETARY
Geological Survey THOMAS B. NOLAN, DIRECTOR
Free on application to the U.S. Geological Survey, Washington 25, D. C. CONTENTS
Page Page ~Abstract------1 The independent variables--Continued Introduction______1 The importance of sediment type____ 3 The independent variables ______1 A relation of percent silt -clay to Restrictions ______1 bedload percent------6 The classification ______7 Discharge ------2 Total sediment load______2 Conclusions______7 Channel stability ______2 References cited ______- _ _ _ _ _ 9 Mode of sediment transport______2
ILLUSTRATIONS
Page Figure 1. Relation of width -depth ratio to silt -clay______4 2. Relation of sinuosity to width-depth ratio------5 3. Relation of sinuosity to silt-claY------5 4. Hypothetical relation of bedload to silt -clay______6
TABLE
Page Table 1. Classification of alluvial channels______8
III
A Tentative Classification of Alluvial River Channels
By S. A. Schumm
ABSTRACT development in the cycle of erosion, such as .youthful, mature, and old (Davis, 1889), and A classification of river channels might be based on the independent variables, discharge and sediment load. Dis also according to their origin on a land sur charge, however, because it determines mainly the size of a face, such as antecedent, superposed, and channel, is not a critical factor for purposes of this study. consequent (Davis, 1890). These classifica Sediment load, because of its influence in determining chan nel stability, shape, and sinuosity, is used as the basis of tions relate the rivers to the geology of an this classification. Three classes of channels, stable, erod area and are used in the geologic interpreta ing, and depositing, are established with regard to channel tion of aerial photographs (Miller, 1962, p. 90- stability or the relation of sediment load to transportability. 97). The numerical classification of Horton A negative exponential correlation between silt-clay in (1945), whereby order numbers are assigned the stream channel and the percent of total sediment load transported as bedload is proposed. From this relation the· to the components of a drainage network, is limits of three additional classes of channels, bedload extremely useful in the morphometric analy mixed-load, and suspended-load, are established, on th~ sis of drainage systems. None of the above basis of the predominant mode of sediment transport or the proportions of suspended load and bedload transported ·classifications, however, gives an indication through a channel. "In all, nine subclasses of river channels of the character of the river channel with re are defined on the basis of channel stability and the dominant spect to the two independent variables, dis mode of sediment transport. charge and sediment load, upon which the INTRODUCTION n1orphology of alluvial river channels de pends. Classifications of natural phenomena are of value because they focus attention on the Melton (1936) related streams to the man key factors which distinguish the individual ner in which they build their flood plains, but phenomena. Conversely, they may tend to discharge and sediment load of the streams restrict thoughtful consideration of a prob did not determine Melton's classification lem by categorizing things which are really rather they were reflected in it. Owing to the members of a continuous series. When this complexity of the relationships and the lack disadvantage is recognized, they may become of data, it is not surprising that a classifica useful guides to further research. tion of river channels has not been attempted previously. This report classifies some relationships between river morphology and sediment type. THE INDEPENDENT VARIABLES The classification was prepared as a means of organizing these relationships as a guide RESTRICTIONS and stimulus to further research on the influ ence of sediment load on fluvial morphology. The proposals must be restricted in appli cation, first, because they are based on data Drainage patterns have been defined by obtained ·and observations made in the west geologists with regard to the respons~ of ern United States, mainly on the Great Plains, streams to geologic structure and lithology though the writer is hopeful that the relation of an area-for example, dendritic, rectangu ships may prove to be of a general nature; sec lar, and radial (Zernitz, 1932). Rivers have ond, the rivers studies are alluvial, generally been classified according to their stage of
1 2 A TENTATIVE CLASSIFICATION OF ALLUVIAL RIVER. CHANNELS having a well formed flood plain and lacking TOTAL SEDIMENT LOAD regulation of course or gradient by rock out crops except over short distances; third, the CHANNEL STABILITY channels contain less than 20 percent coarse gravel and the sediment load is, therefore, A major division of alluvial channels can relatively fine -grained. be made on the basis of stream stability or lack of it. These differences can be thought The classification can only be applied lo of with respect to the total sediment load de cally or to segments of a river system, for livered to the channel. An excess of total the characteristics of a stream channel can load causes deposition, a deficiency causes change significantly within a short distance erosion, and between the extremes lies the if it becomes unstable (Schumm, 1961) or re stable channel. Although often in the field ceives sediment of a different type from a a given river channel cannot be clearly iden tributary (Schumm, 1960a). Therefore, it is a tified as stable, eroding, or depositing, it is classification of channels rather than rivers. possible to think of all rivers as falling into one or another of these three classes. The classification, as indicated in the title, is tentative, for the basic data necessary to The stable channel is one that shows no prove its validity are lacking. Nevertheless, progressive change in gradient, dimensions, it is presented as a stimulus to the collection or shape. Temporary changes occur during of the necessary data and perhaps as ::~ new floods, but the stable channel, if the classifi approach to the consideration of alluvial cation were not restricted to short segments rivers. of the river, would be identical to the graded stream as defined by Mackin (1948) in which, DISCHARGE "over a period of years, slope is delicately adjusted to provide, with available discharge Discharge is an independent variable that and with prevailing channel characteristics, largely determin~s the size of stream chan just the velocity required for the transporta nels (Leopold and Maddock, 1953) and the am tion of the load supplied from the drainage plitude and wave length of meanders (Leopold basin." and Wolman, 195 ... ). Width and depth of the channels to be Jiscussed herein are largely a The eroding channel is one that is being function of dis..;harge, although the sediment progressively degraded or widened by bank forming the channel may introduce important erosion or both. Conversely, the depositing modifications (Schumm, 1962). channel is one that is being aggraded or is having sediment deposited on its banks or Discharge, however, is not a valid basis for both. Classification of river channels on the a classification of ~tream channels unless basis of stability as eroding, stable, or de size is considered most important, though a positing, emphasizes the diversity among qualitative distinction among stream channels rivers and stream channels; each of the three can be made on the basis of discharge char classes can be considered to be distinct from acteristics, that is, ephemeral and perennial the others. streams. This distinction is obvious but not fundamental. Dowr.stream differences be MODE OF SEDIMENT TRANSPORT tween ephemeral ctnd perennial streams have been d(:~monstrated by Leopold and Miller The channels are further subdivided with ( 19 56), but the ,. uthor has shown that some reference to the predominant mode of sedi elements of the pattern and shape of both ment transport through the channel. Mode of ephem,eral and perennial stream channels are transport is simplified here to mean the related to the type of sediment forming the transportation of sediment as either sus perimeter of the channel (Schumm, 1960a, pended load or bedload. 1963) rather than to the amount or character of discharge. THE INDEPENDENT VARIABLES 3
It is necessary to clarify the definition of c:omprises the bedload when the river is in the terms bedload and suspended load as used flood. The 0.351-mm size, because it re herein. According to Einstein, Anderson and flects flood conditions, seems too high, for Johnson (1940, p. 632) "the bed-load consists at low flows sediment particles 0.3 mm in of material moving as surface -creep and diameter will certainly move on the stream material moving in suspension, both of which bed. Sundborg (1956, p. 219) suggested that can be expressed as a rate related to the the finest material transported as bedload stream discharge. ****Primarily the con ranges from 0.15 to 0.20 mm, and Hjulstrom's ception is that bed-load and suspended load curves (1935, p. 274) suggest strongly that do not supplement each other, as a certain sediment larger than the 200-mesh sieve grain may easily be in suspension and still (0.074 mm) will be transported near the be considered bed -load." The term bedload streambed. as used above is synonomous with the term bed -material load, which is defined as that In brief, the proposed distinction among part of the sediment load of a stream which river channels is based on the predominant consists of particle sizes represented signif mode of sediment transport or the propor icantly in the bed of the stream. The empha tions of bedload and_ suspended load trans sis on significant representation excludes ported through a stream channel. Although the small percentage of silt and clay usually a single grain size, which would form the found in most bed material (Einstein, 1950, boundary between suspended load and bed p. 6, 7). load, cannot be selected from available sedi ment -transport data, it is suggested that, in The term suspended lead as used in this general, silt and clay are transported in· sus publication is synonomous with wash load, pension and that sand and coars.er sediment that part of the sediment load not significantly are transported on or near the streambed. represented in the bed of the stream (Einstein, 1950, p. 7). Wash load moves almost entirely THE IMPORTANCE OF SEDIMENT TYPE in suspension and bears no relation to dis charge (Einstein et al., 1940, p. 632). As dis Basic data on the proportions of bed and charge increases in a stream channel the suspended load are available for only a few amount of bed -material transported by sus rivers. Therefore, it is necessary to retrace pension increases. Nevertheless, bed-mate the steps whereby the writer has tentatively rial load differs from wash load because at concluded that the proportions of suspended low flows bed -material load is stationary or load and bedload in a stream may be the moves on the bed; wash load, however, is al critical factor upon which many stream char ways in suspension as it is washed through acteristics depend. the channel. In the channels of the Great Plains rivers bed -material load is usually During a study of channel characteristics composed of sand, whereas wash load is of some western streams, it was discovered composed of silt and clay. that the shape of the channel of stable rivers expressed as a width-depth ratio (F) is re In this report the terms bedload and sus lated to the percentage of sediment finer than pended load will be used, for although for any 0.074mm in the perimeter of the channel (M) short period of sediment discharge the terms (fig. 1). bed-material load and wash load are mean 1 (1) F=255 M- • 08 ingful, there is probably an average grain size which, over a period of years, forms the Further work revealed that the mechanics of boundary between the sediment moved pre deposition and erosion in ephemeral stream dominantly on the bed (bedload) and the sedi channels is related to M or silt -clay in the ment moved predominantly in suspension channel perimeter (Schumm, 1962). The sin (suspended load). uosity of Great Plains streams, expressed as a ratio of channel length to valley length (P), The average grain size separating sus is related to M and width -depth ratio (IF) as pended load from bedload probably varies follows (Schumm, 196 3) : between rivers. Einstein, Anderson, and f'=3.5F-· 27 (fig.2) (2) Johnson (1940) concluded from studies along the Enoree River in South Carolina that mate P =0.94 M •25 (fig. 3) (3) rial larger than the 42-mesh sieve (0.351 mm) 4 A TENTATIVE CLASSIFICATION OF ALLUVIAL RIVER CHANNELS
0 ..... <( a: .. ::r: ..... • ()_ ... w 0 I ::r: ..... 0 10.0 ~
LCO.I 1.0 10 100 PERCENT SILT-CLAY (M)
Figure 1. -Relation of width-depth ratio (F) to weighted mean percent silt-clay (M) (from Schumm, 1960).
Initially these relationships were explained of the cohesiveness of the sediment, M may by the higher cohesion of the bed and bank be related to the average percent of total load sediments, which contain large amounts of transported in suspension. Conversely, the silt and clay. This assumption was reason percent of sediment coarser than 0.074 mm able for bank material, but in most channels exposed in the channel may be related to the the bed material doesn't contain enough silt average percent of total sediment load trans and clay to be cohesive. Yet, it was not the ported as bedload. The data presented by percent silt -clay in the banks alone that Hjulstrom (1935) and Sundborg (1956) on the showed the- best correlation with channel size of sediment transported on or near the shape and sinuosity but rather the percent streambed lend support to this hypothesis. silt -clay in the entire perimeter of the chan nel. After many attempts to explain the sig Supporting evidence for the suggestion that, nific__~ce of M in terms of the physical prop as the percentage of sand and coarser sedi erties of the sediments, it occurred to the ment in the perimeter of a channel increases. writer that, in addition to being an indication the proportion of total load transported as THE INDEPENDENT VARIABLES 5
5.0
_3.0 ct - ~ r---....:. • ~ - • ~ .P•3 r--...... 1- • . 5~~'1 ~ 2.0 ~ r-- • • to;- • U) • 0 ~ • ~ ::::> ~ - z ~ • • ' ~ 4 .. • C/) • • r--- ...... r;.... 1.0 - r--. "'---~---
10 100 WIDTH-DEPTH RATIO {F)
Figure 2. -Relation of sinuosity (P) to width-depth ratio (F) (from Schumm, 1963).
- 3.0~------~----~----~--~-+--~+-+-+------+------+----+---+-~--r-~~ Cl_ ... e I~~~- - ~·~ -~~ . p : 0 .9~ • __....,..... __,::- • ~2.0 . ~~· . Cl) . ~...... 0 . ~~ -----!· ::::> . I ----~ • • z • ~...,_...- • elM Cl) ._,:.-....- .....--- . . . I.0~--~~---+------~--4---+--+~~~~~------r----~----+---r--r-+~~-1
10 100 PERCENT SILT-CLAY (M)
Figure 3. -Relation of sinuosity (p) to silt-clay (M) in perimeter of channel {from Schumm, 1963).
bedload increases is given by the relation of (1927, p. 246), and Mackin (1948, p. 484) to percent silt -clay in the channel perimeter td support the view that to be stable an alluvial width-depth ratio (equation 1), for others channel transporting a large proportion of have noted that wide shallow rivers are char bedload must have a relatively wide and shal acterized by high-bedload transport. Leopold low cross section. In addition, high sinuosity and Maddock (1953, p. 29) state, "At constant is not conducive to efficient bedload trans velocity and discharge, an increase in width port, for Shulits (1959) observed that flume is associated with a decrease of suspended experiments indicate that bedload is one load and an increase in bedload transport." third less in a 180° bend than in a straight They quote from Lane (1937, p. 138L Griffith channel and that in a meandering channel the 6 A -TENTATIVE CLASSIFICATION OF ALLUVIAL RIVER CHANNFLS bedload per unit width is 20 percent less than and Hembree, 1955, p. 111). The percentage in a straight channel. This observation is in of material coarser than silt-clay in the per agreement with that of Dryden and others imeter of the Niobrara River, upstream from (1956, p. 483) that the frictional resistance to the points of sediment measurement, is about flow increases as sinuosity increases. When 98 percent. It can be expected that the pro the relation of sinuosity to width-depth ratio portion of bedload on the average would he and to M (equations 2 and -3-)---are considered less than the percent of sand in the perimeter with regard to the above statements. the case of the channel, for during floods large amounts for the significance of the proportions of sus of fine sediment would be carried through the pended load and bedload in determining chan channel in suspension. In any event, it seems_ nel characteristics is strengthened. Sundborg reasonable to suppose that as the proportion (1956, p. 203-204) suggested that as bedload of bedload increases the percent silt -clay decreases the channel becomes narrow and forming the perimeter of the channel de deeper and tends to meander; the writer con creases. curs in this opinion. It may be possible to suggest the type of An opportunity to examine the effect of relationship that exists between the percent changes in the proportions of suspended load silt -clay in the channel perimeter and the and . bedload in a channel is afforded by the average percent of total load transported as Smoky Hill-Kansas River system. In western bedload. For two localities it is known that Kansas the sinuosity of the Smoky Hill River about 50 percent of total load is transported is about 1.2. the percent silt-clay ( M) is about on the bed when percent silt -clay ( M) is about 5 percent, and width-depth ratio is about 85. 2. Bedload probably is never greater than Near the junctions of the Saline and Solomon about 60 to 70 percent of total load because Rivers with the Smoky Hill, the sinuosity in fine sediments are present in every stream crease to about 2. 5. percent silt -clay ( M) is and generally are transported at a maximum 20, and width-depth ratio decreases to about rate when bedload transport is high. There 10. The Saline and Solomon Rivers appear to fore, a curve relating percent silt-clay l M) introduce a large amount of suspended load to percent bedload might intersect the bed into the Smoky Hill channel. Below the june load axis at about 60 to 70 percent (fig. 4). tion of the Republican River with the Smoky Hill River. sinuosity decreases progressively On the other hand, percent silt~ clay ( M) will to 1.1 at Topeka, percent silt -clay ( M) de approach 100 percent when bedload transport creases to about 3, and width-depth ratio in is zero. Probably bedload transport is low creases to about 45. The Republican River when percent silt -clay ( M) is about 30 per has only about 4 percent silt -clay ( M) in its cent because above this value sinuosity is channel above the junction, and it undoubtedly introduces a large amount of bedload into the 0 100 Kansas River. If, as suggested, the major
Proportion of total Channel stability Mode of Channel sediment load sediment sediment transport (M) percent Suspended Bedload Stable Depositing Eroding load percent percent (graded stream) (excess load) (deficiency of load)
Stable suspended-load Depositing suspended load Eroding suspended-load channel. Width-depth channel. Major deposi- Suspended ratio less than 7; tion on banks cause nar- channel. Streambed 30-100 85-100 0-15 erosion predominant; > load sinuosity greater than rowing of channel; channel widening ~ 2.1; gradient rela- streambed deposition minor. tively gentle. minor. ~
Stable mixed-load chan- ~ nel. Width-depth Depositing mixed-load Eroding mixed-load ~ (I) (I) Mixed ratio greater than 7 channel. Initial major channel. Initial load 8-30 65-85 15-35 less than 25; sinuos- deposition on banks fol- streambed erosion ity~ less than 2.l lowed by streambed followed by channel ~ greater than 1. 5; deposition. widening. ! gradient moderate. ~ Stable bedload channel. § Eroding bedload channel. Width-depth ratio Depositing bedload channel. Little streambed ero- Bedload 0-8 30-65 35-70 greater than 25; sinu- Streambed deposition and ~ osity~ less than 1. 5; sion; channel widening !XI island formation. ~ gradient relatively predominant. !XI steep. () ~ ~ REFERENCES CITID 9 been classified on the basis of one of the two Einstein~ H. A., 1950, The bed-load function independent variables influencing stream for sediment transportation in open chan morphology. Discharge is not used as a basis nel flows: U.S. Dept. Agriculture Technical for the classification because it controls Bull. 1026, 71 p. mainly the size of the channels. The dimen Einstein, H. A., Anderson, A. G., and Johnson, sionless properties of the channels depend J. W., 1940, A distinction between bedload mainly on the sediment load moved into the and suspended load in natural streams: channel. Channel stability depends on the Am. Geophys. Union Trans., v. 21, p. 628- balance~ or lack of it~ between sediment load 633. and transportability~ and three classes of Griffith, W. M., 192 7, A theory of silt and channels result: stable~ eroding~ and depos scour: Inst. Civil Eng. Proc., v. 22 3, p. 243- iting. The type of material transported or 314. the mode of its transport as bedload or sus Hjulstrom, Filip, 1935, Studies of the mor pended load appears to be a major factor de phological activity of rivers as illustrated termining the character of a stream channel~ by the River Fyris: Univ. Upsala Geol. Inst. and a final subdivision of channels is based Bull., v. 25, p. 221-527. on this relation. Horton, R.E.,1945~ Erosional development of streams and their drainage basins; Hydro This classification of stream channels can physical approach to quantitative morphol only afford a basis for discussion until more ogy: Geol. Soc. America Bull., v. 56, p. data become available on the components of 275-370. total sediment load in a stream channel. Hubbell, D. W., and Matejka, D. Q., 1959, In Perhaps the quickest way in which the sug vestigations of sediment transportation, gestions advanced here can be evaluated is Middle Loup River at Dunning, Nebraska: through model studies of stream channels. U.s. Geol. Survey Water-Supply Paper 1476, The experiments would have to be performed 123 p. in flumes wide enough to permit formation of Lane, E. W., 1937, Stable channels in erodible miniature channels. The sinuosity and shape materials: Am. Soc. Civil Eng. Trans., v. of these channels could then be related to 102, p. 12 3-194. varying mixtures of silt -clay and sand intro Leopold, L. B., and Maddock, T., Jr., 1953, duced into the channel. If slope and discharge The hydraulic geometry of stream channels were constant, the variations in the propor and some physiographic implications: U.S. tions of bed and suspended sediment intro Geol. Survey Prof. Paper 252, 57 p. duced into the miniature channels should de Leopold,L.B.,and Miller~J.P., 1956,Ephem termine the type of channel produced. eral streams -hydraulic factors and their relation to the drainage net: U.S. Geol. Sur REFERENCES CITED vey Prof. Paper 282-A, p. 1-37. Leopold,L.B.,and Wolman, M.G.,1957,River Blench~ T ., 1957~ Regime behavior of canals channel patterns: braided, meandering and and rivers: London, Butterworths Publica straight: U.S. Geol. Survey Prof. Paper 2 82- tions, 138 p. B, p. 29-84. Colby, B. R., and Hembree, C. H., 1955, Com Mackin, J. H., 1948, Concept of the graded putations of total sediment discharge, Nio river: Geol. Soc. America Bull., v. 59, p. brara River near Cody, Nebraska: U.s. 463-512. Geol. Survey Water-Supply Paper 1357, Melton, F. A., 1936, An empirical classifica 187 p. tion of flood -plain streams: Geogr. Rev., Davis, W. M., 1889, The rivers and valleys of v. 26 p. 593-609. Pennsylvania: Nat. Geogr. Mag., v. 1, p. Miller, V.C., 1962, Photogeology: New York~ 183-253. McGraw-Hill, 248 p. ----1890, The rivers of northern New Schumm, S. A., 1960a, The shape of alluvial Jersey with notes on the classification of channels in relation to sediment type: U.S. rivers in general: Nat. Geogr. Mag., v. 2, Geol. Survey Prof. Paper 352-B, p. 17-30. p. 81-110. ----1960b, The effects of sediment type Dryden, H. L., Murnaghan, F. D., and Bateman, on the shape and stratification of some H., 1956, Hydrodynamics: New York, Dover, modern fluvial deposits: Am. Jour. Sci., v. 634 p. 2 58, p. 177-184. 10 A TENTATIVE CLASSIFICATION OF ALLUVIAL RIVER CHANNELS
Schumm,S.A.,1961,Effect of sediment char along Cimarron River, southwestern Kan acteristics on erosion and deposition in sas: U.S. Geol. Survey Prof. Paper 352-E. ephemeral-stream channels: U.S. Geol. Sur Shulits, S., 1959, Stability of talweg in natural vey Prof. Paper 3 52-C, p. 31-70. channels (Abs. ]: Geol. Soc. Am. Bull., v. 70 ----1962, Dimensions of some stable al p. 16 75. luvial channels: U.S. Geol. Survey Prof. Sundborg, A., 1956, The River Klaralven, A Paper 424-B, p. B2 6-B2 7. study of fluvial processes: Geografiska ----1963, The sinuosity of alluvial rivers Annaler, v. 38, p. 127-316. on the Great Plains: Geol. Soc. America Zernitz, E. R., 1932, Drainage patterns and Bull., v. 74., p. their significance: Jour. Geol., v. 40, p. Schumm,S.A.,and Lichty, R. W., 1963, Chan 498-521. nel widening and flood -plain construction
IN'l'.DUP.,D.C.63- 31506