S. A. SCHUMM U. S. Geological Survey, Denver, Colo.
Sinuosity of Alluvial Rivers on the Great Plains
Abstract: Data on the morphologic and sediment does not appear to affect the sinuosity of streams. characteristics of stable alluvial rivers of the Great Another possible distinction between straight Plains were collected at 50 cross sections. The and sinuous streams is in the proportions of the channel patterns of these rivers were classified into components of total sediment load. In a wide, five types: tortuous, irregular, regular, transitional, shallow channel much of the sediment transported and straight. Because no clear demarcation existed is bed-material load. In a narrow, deep channel most between each of the types, the pattern of the rivers of the sediment transported is wash load. was described by sinuosity, a ratio of channel length On the Great Plains both straight and sinuous to valley length. The sinuosity (P) of these rivers streams may flow on the surface of alluvial valley is related to the shape of the channels expressed as fills at about the same valley slope. The departure a width-depth ratio (F) and to the percentage of of a stream from a straight course down the alluvial silt and clay in the perimeter of the channel (M) valley results from changes in both the caliber of as follows: the sediment load and in the relative proportions of P = 3.5F-27 bed-material load and wash load during the post- 25 Pleistocene alluviation of these valleys. When P = 0.94 M- . during this alluviation the proportion of wash load Sinuous streams are characterized by a low width- increased, most probably by a decrease in bed- depth ratio (F), a high percentage of silt-clay in the material load, the stream adjusted itself by decreas- perimeter of the channel (M), a high percentage of ing its gradient through the development of a silt-clay in the banks (although the banks of straight sinuous course. Recent changes in stream sinuosity channels may also contain large amounts of silt- in response to changes in the proportions of bed clay), and a lower gradient than straight channels load and suspended load support this hypothesis. having the same mean discharge. Discharge itself
CONTENTS Introduction 1089 3. Relationship between sinuosity and silt-clay in Acknowledgments 1090 stream banks 1093 Description of streams 1090 4. Relationship between sinuosity and silt-clay in Methods of investigation 1090 perimeter of channel 1093 Channel patterns 1090 5. Relationship between stream gradient and Properties of sinuous and straight streams . . . 1091 mean annual discharge 1094 Influences of sediment load on sinuosity .... 1094 6. Hypothetical cross section of a valley showing Effect of valley history on sinuosity 1096 change of shape of stream channels toward Conclusions 1098 top of alluvium as sediment becomes pro- References cited 1098 gressively finer 1097 Figure Table 1. Examples of channel pattern 1091 1. Average sediment and channel characteristics . 1092 2. Relationship between sinuosity and width- 2. Data for rivers with comparable mean annual depth ratio 1092 discharge 1095
very sinuous stream, the straight stream, and INTRODUCTION the stream that contains islands and to neglect Rivers are commonly classified according to the transitional patterns. This paper examines pattern into three major categories: meander- the sinuosity as well as other characteristics ing, straight, and braided (Leopold and Wol- of some stable alluvial rivers of the Great man, 1957). However, as with most classifica- Plains. A theory for the development of river tions of natural phenomena the grouping is patterns of varied sinuosity will be presented, arbitrary and tends to focus attention on the based on geology rather than hydraulic theory.
Geological Society of America Bulletin, v. 74, p. 1089-1100, 6 figs., September 1963 1089
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Median grain size and the percentage of silt ACKNOWLEDGMENTS and clay in the samples were obtained from This paper refers to one aspect of an in- the grain-size curves. Per cent silt-clay is vestigation into the effects of sediment charac- defined as that percentage of the sample passing teristics on fluvial morphology. R. W. Lichty the 200-mesh sieve or that portion smaller than of the U. S. Geological Survey assisted during 0.074 mm. most of the field work and made helpful sug- Weighted mean per cent silt-clay, calculated gestions both in the field and during prepara- by giving the bank and bed material a weight tion of the report. Lichty, R. F. Hadley, also equivalent to their exposure in the perimeter of the U. S. Geological Survey, and Prof. J. of the channel, is related to channel width- Hoover Mackin of the University of Texas depth ratio (Schumm, 1960). The writer has read and criticized the manuscript. been criticized for using a weighted mean, which seemed to bias the data; however, the DESCRIPTION OF STREAMS composite samples, which included both bed Data were collected at 50 cross sections along and bank material, were found to contain a a number of western streams. Almost all loca- percentage of silt and clay close to that ob- tions are within the Great Plains province and tained by calculating a weighted mean from are near U. S. Geological Survey gauging the bed and bank samples. Thus the weighted stations. At all cross sections the streams are mean per cent silt-clay is truly representative flowing in channels formed of alluvium; bed- of the percentage of silt and clay in the rock is not exposed at any of the cross sections. perimeter of the stream channel as suggested Aerial photographs show that at some reaches previously (Schumm, 196la). In this paper the the streams impinge on the edge of the valley. weighted mean per cent silt-clay is used as a Bedrock is probably exposed in these reaches, parameter descriptive of the sediment forming but the distance from the measured sections the stream channel. was such that any effects of bedrock on most The sinuosity of the rivers near the surveyed of the sampled cross sections would be minor. cross sections was studied on aerial photographs. However, at seven locations the stream pattern To obtain information on sinuosity a 5-mile was controlled by bedrock and possibly by segment of the river valley was selected, which structure. The data from these seven cross included the location of the cross section, and sections will not be used in this discussion. the length of stream channel within this reach Evidence from the gauging station records was measured. The sinuosity, expressed as the and observations made in the field indicate ratio of stream length to valley length, was that the streams are not aggrading or degrad- calculated. A straight stream has a sinuosity ing. The data for the 43 sections, therefore, of 1.0, and this number increases as the stream should be representative of stable alluvial departs from a straight line. Sinuosity has been streams that contain less than about 10 per used recently to describe stream patterns by cent coarse gravel and larger sediment in the Lane (1957), and Leopold and Wolman (1957). perimeter of the channel. CHANNEL PATTERNS METHODS OF INVESTIGATION A study of river patterns suggested that the A representative reach of each stream was qualitative classification of straight and mean- selected near a gauging station, and the channel dering channels could be expanded to include cross section was surveyed. The gradient of the five classes. For example, it was apparent that stream was measured in the field as well as there were three types of meanders—tortuous, from maps and aerial photographs. A compari- irregular, and regular. There was also a transi- son of gradients measured in the field with tional channel type between meanders and the those computed from maps and photographs straight channels. As Figure 1 shows, the tortu- for the same location indicate that the two ous pattern is very irregular. The meander methods yield similar results. bends are deformed, and the smoothness typical Samples of bank and bed material were of the ideal meander curve is absent. Irregular collected at each cross section, and at most meanders are irregular only with respect to the sections a composite sample was collected smoothly curved regular meandering pattern. along the perimeter of the channel. A grain- In some cases the irregular pattern seems to size analysis of the sediment samples was ob- consist of a meander pattern of low amplitude tained by sieving and hydrometer techniques. and wave length superimposed on a larger
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pattern. This condition may be similar to the double meanders described by Hjulstrom PROPERTIES OF SINUOUS (1949). He states that the small meanders are AND STRAIGHT STREAMS fashioned by low perennial flow and the larger Each of the streams was classified according by higher flows related perhaps to the mean to the five types of pattern. Table 1 gives the annual flood. The regular meandering pattern average channel and sediment characteristics needs little discussion. One should note, how- for each pattern. It is apparent that certain properties of the rivers are associated with each A Tortuous pattern. As sinuosity decreases from the tortu- ous to straight channels, the width-depth ratio of the channel increases (Mackin, 1956), the percentage of silt-clay in the banks decreases, B Irregular the per cent silt-clay in the perimeter of the channel decreases, and mean annual discharge increases. The other variables, median grain size of channel sediment, gradient of the stream, and gradient of the valley, show no C Regular progressive change with sinuosity. The average data for six straight channels that contained islands are also listed. Except for a larger mean annual discharge, the charac- ter of the channels with islands is very similar to that of the straight channels. Islands were not present at any of the sections where data were collected in the field. The relationships suggested by the average values of Table 1 are shown on Figures 2, 3, and 4. The relationship between channel E Straight width-depth ratio and sinuosity is shown on Figure 2. The regression line was fitted graphically to these data and to the data of Figure 4 by the method described by Searcy (1960). The relation between width-depth Figure 1. Examples of channel pattern. A, ratio (F) and sinuosity (P) is as follows: White River near Whitney, Nebraska 27 (P — 2.1); B, Solomon River near Niles, P=3.5F- (1) Kansas (P = 1.7); C, South Loup River Relatively wide and shallow channels tend to near St. Michael, Nebraska (P = 1.5); D, be straight, whereas relatively narrow and North Fork Republican River near Benkle- deep channels depart from a straight course. man, Nebraska (P = 1.2); E, Niobrara River near Hay Springs, Nebraska (P On Figure 3 the relationship between silt- = 1.0) clay in the banks of the streams is plotted against sinuosity. The banks of sinuous channels are composed of fine sediments and have a high ever, that it is not as regular as one is ordinarily percentage of silt-clay; however, the bank ma- led to believe. The transitional pattern is terial of straight channels varies greatly, and characterized by very flat curves which tend some straight and transitional channels have to repeat like typical meanders. The straight bank sediments containing 50-38 per cent pattern is not truly straight, but the minor silt-clay. Thus, although sinuous channels bends show no regularity. apparently must have resistant banks, the In spite of this detailed descriptive classifi- presence of large amounts of silt-clay in the cation of channel patterns, it was difficult to banks is no guarantee that the channel will be assign some of the channels to a specific pattern sinuous. because there are transitional types. This sug- The relationship between weighted mean gests that a continuum of channel patterns does silt-clay (M) in the perimeter of the channel exist, as Leopold and Wolman (1957, p. 63) and sinuosity (P) is better developed than that suggested. for bank material alone and indicates that as
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TABLE 1. AVERAGE SEDIMENT AND CHANNEL CHARACTERISTICS OF CHANNEL PATTERNS Weighted Median Mean Number Stream Valley Width- means Silt-clay grain annual Channel of slope slope depth silt-clay bank size discharge pattern channels Sinuosity ft/ft ft/ft ratio per cent per cent mm cfs tortuous 10 2.3 .00095 .00223 5.2 43.4 89 .42 70 irregular 9 1.8 .00062 .00116 19.0 14.0 82 .71 149 regular 4 1.7 .00077 .00132 25.5 8.8 72 .74 209 transitional 7 1.3 .00154 .00193 56.0 4.9 54 .45 255 straight 11 1.1 .00145 .00175 43.0 3.4 41 .35 370 straight (islands) 6 1.1 .00148 .00170 52.0 4.1 45 .37 421
the silt-clay content of the channel as a whole been shown to be a distinguishing characteristic increases the sinuosity of the channel increases between braided and meandering streams (Fig. 4). The relationship is described by the (Lane, 1957; Leopold and Wolman, 1957, p. equation 59). Figure 5 shows that for a given annual P = .94 M-25 (2) discharge the less sinuous streams generally have the steepest gradient. The scatter of the data in Figures 2, 3, and As Figure 5 shows, the more sinuous streams 4 can partly be explained by considering the are not among the rivers with highest annual variability of sinuosity along one stream. For discharge, which suggests, as do the data of example, along one of the streams studied, the Table 1, that discharge may have an inverse sinuosity has decreased recently from 1.9 to effect on sinuosity. However, an analysis of 1.6 by meander cutoffs. Thus, along any river variance reveals that a significant relationship the sinuosity may vary somewhat with time does not exist between sinuosity and mean depending on the formation of new bends or annual discharge, because the variation of dis- cutoffs. However, over a long period sinuosity charge for a given sinuosity is great. That should average that value indicated by the discharge only affects the dimensions of the regression lines of Figures 2 and 4. meanders can be demonstrated by noting that Because of the increased length of channel the sinuosity of the Mississippi River between per unit length of valley a sinuous stream is Mellwood, Arkansas, and Lake Providence, generally associated with a low gradient and a Louisiana, is 2.1. Average discharge between straight stream with a high gradient. Slope has these two points is on the order of half a million
WIDTH-DEPTH RATIO Figure 2. Relationship between sinuosity and width-depth ratio. Standard error is 0.064 log units. Correlation coefficient is — .89.
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10 • • • • • • A • • • •» • • • • •. • • • • • M «• •• • • • • • 1.0 0 20 40 60 80 OO SILT-CLAY IN BANKS IN PERCENT Figure 3. Relationship between sinuosity and silt-clay in stream banks
cfs. Table 2 shows that Red Willow Creek, the differences in the other properties of rivers with a sinuosity also of 2.1, has an average as sinuosity varies. discharge of only 42 cfs. Discharge therefore One additional reason for the lower dis- seems to have little effect on the sinuosity charges of the sinuous rivers is that data were of rivers. However, a change in discharge may collected only for rivers that could be waded cause a modification of sinuosity through its into. For a given discharge the wide shallow effect on the type of sediment load transported streams could be waded into, whereas the through the channel. Table 2 presents the data narrow deep streams could not. from four pairs of rivers. In each pair the dis- A review of the data indicates that a sinuous charge is comparable but the sinuosity differs. channel has the following properties: low The data are representative of the type of width-depth ratio; high percentage of silt rivers studied and illustrate for individual cases and clay in the perimeter of the channel; high
5.0
3.0
m >.«*» o 2.0 z
Q6_ SILT-CLAY IN PERCENT (M) Figure 4. Relationship between sinuosity and silt-clay in perimeter of channel. Standard error is 0.059 log units. Correlation coefficient is 0.91.
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percentage of silt-clay in the banks, although arnount and character of sediment load trans- banks of straight channels may also contain ported by these streams may be the dominant large amounts of silt-clay; a lower gradient factor determining channel sinuosity. than straight channels with the same mean The sediment forming the perimeter of these discharge. channels should be representative of the type of load transported through the channels. INFLUENCES OF SEDIMENT Median grain size, however, appears not to be LOAD ON SINUOSITY related to sinuosity. Indeed, the data presented The relationships presented heretofore afford in Table 2 show that the median grain size may the basis for an explanation of variations in be larger for the more sinuous streams. sinuosity of the Great Plains streams. A transi- The one parameter of sediment character
\-\
— — • 4 » • A • • • + * 1 <• • .001 + t f • . _ + -- • t
» * T 4- EXPLANATION + 1.0 1.4 t 4 OSITY J + = 1.5 2.0 -~ 1 A - > 2,0
.0001 1 10 OO MEAN ANNUAL DISCHARGE (eta) Figure 5. Relationship between stream gradient and mean annual discharge
tion between straight and meandering patterns that shows a good correlation with sinuosity exists, but for simplification only the end mem- is the percentage of silt-clay in the sediment. bers of this series, straight and sinuous or me- Previously it was demonstrated that the shape andering channels, will be discussed. of a stream channel (width-depth ratio, F) is Stable channels do not have an excess of related to the weighted mean per cent silt-clay energy or erosive ability, and their characteris- (M) as follows: tics must be considered with reference to the F=255M-1-08. (3) equivalence of energy available and energy expended in the channel. The channel shape A high percentage of silt-clay is associated with and gradient have adjusted to provide transport a relatively narrow and deep channel, and the of water and sediment without progressive writer has suggested (1960) that a high content aggradation, degradation, widening, or narrow- of cohesive materials causes a channel to be ing of the channel. Therefore, only the dis- resistant to widening. It appears also that the charge and sediment characteristics of these silt-clay content of the channel may be repre- stable alluvial channels are independent varia- sentative of the type of sediment load trans- bles (Mackin, 1948, p. 471). It has been sug- ported by a stream. It seems possible that the gested herein that discharge does not affect per cent silt-clay in the channel may be related the sinuosity of the channel, and the data of to the average percentage of total sediment Table 2 support this. Although the dimensions load transported in suspension. Conversely the of these stable alluvial channels are related pri- percentage of sediment coarser than 0.074 mm marily to discharge (Schumm, 1962), the found in the perimeter of a channel may be
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related to the average percentage of total sedi- sL? O O go 0 0 -- O ment load transported as bed load. .S i; ~ Tt-
Locatio n in suspension and still be considered bed-load."
Willow , The term bed load as used above is synony- Benkleman , Nebrask a Re d Whitney , Nebrask a Locate , Montan a Niles , Kansa s Norton , Stamford , Nebrask a Haigler , Nebrask a mous with the term bed-material load, which c is defined as that part of the sediment load rt _• ii which consists of grain sizes represented in the u, (J U Z3c be ^ bed (Einstein, 1950, p. 4). However, this . s CU 1 • o o p^
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amount of bed material transported by sus- instances valley and stream gradients are almost pension increases. Nevertheless, bed-material identical. If tectonic factors have not modified load differs from wash load, that part of the the slope of a valley, the gradient of the alluvial sediment load not significantly represented in surface should be at just that slope required for the bed (Einstein, 1950, p. 7), because at low the movement of the water-sediment mixture flows bed-material load is stationary or moves through the valley. The fact that the surface on the bed; wash load, however, is always in of the alluvium is too steep in many cases re- suspension as it is washed through the channel. requires an explanation. That explanation de- In the Great Plains rivers bed-material load is pends on an understanding of the changes in composed of sand, whereas wash load is com- stream regimen following the Pleistocene. posed of silt and clay. To avoid confusion the Most rivers of the Great Plains flow on the terms bed-material load and wash load will be upper surface of alluvium which fills valleys used in the following discussion. cut into bedrock. The deep valleys and the An opportunity to observe the effect of associated alluvium are a result of the changes changes in the proportion of wash load and bed- in base level, climate, and runoff during and material load along a river is afforded by the following the Pleistocene. A classic account of Smoky Hill-Kansas River system. In western the formation of such a valley and its filling by Kansas the sinuosity of the Smoky Hill River alluvium is Fisk's (1944; 1947, p. 19-22) is about 1.2, the per cent silt-clay forming the description of the post-Pleistocene changes in channel is about 5 per cent, and width-depth the Mississippi River valley between Cairo, ratio is about 85. Near the junctions of the Illinois, and the Gulf of Mexico. The lowermost Saline and Solomon rivers with the Smoky Hill part of the alluvium in this valley is composed the sinuosity increases to about 2.5, per cent of coarse sands and gravels which were trans- silt-clay is 20, and width-depth ratio decreases ported on the steep slope of the incised river. to about 10. The Saline and Solomon rivers As sea level rose with the melting of the ice introduce large amounts of fine sediments or sheets, gradient was decreased, and the coarser wash load into the Smoky Hill River. Below the sediment was deposited. As sea level continued junction of the Republican River with the to rise, progressively finer sediment was de- Smoky Hill River, sinuosity decreases pro- posited, until finally the river was transporting gressively to 1.1 at Topeka; per cent silt-clay only the finest fraction of its previous load. decreases to about 3, and width-depth ratio According to Fisk (1944) the river was braided increases to about 45. The Republican River during deposition of the coarser sediments. It is has only about 4 per cent silt-clay in its channel now meandering over the greater part of its above the junction and carries a large propor- course, and its load is predominantly silt, clay, tion of bed-material load into the Kansas and sand. Probably the same sequence of events River. These changes in sediment load down- occurred in the valleys of the Great Plains, for stream along the Smoky Hill-Kansas River logs of wells, which were drilled into the illustrate the importance of the proportions alluvium filling these valleys, show an upward of bed-material load and wash load to river decrease in sediment size from gravel and coarse sinuosity. sand at the base ot the deposit to either fine sand or silt, clay, and fine sand at the present EFFECT OF VALLEY HISTORY valley surface. Great variability of sediment ON SINUOSITY type occurs in these fills, but all available infor- In spite of the previous explanation of river mation shows a decrease in size of the alluvium sinuosity as a function of the components of toward the surface of the fill (Wenzel and total sediment load, another major problem Waite, 1941, p. 19; Williams, 1944, p. 38 and remains. If the shape, gradient, and dimensions Fig. 3; Latta, 1949, PI. 1; Davis and Carlson, of a channel adjust to discharge and sediment 1952, p. 229, PI. 3; Leonard, 1952, p. 47, PI. 3; load, why is the valley gradient too steep for Bjorklund and Brown, 1957, p. 190, well the discharge and load of sinuous streams? B10-48-1 lac). This relationship is shown in the Sinuosity reflects not only the ratio of channel generalized sketch of Figure 6. length to valley length but also the ratio of Depending on the geology of a drainage valley slope to channel slope. The data on basin, the load of a river might change from sinuosity plotted on Figures 2, 3, and 4 reveal gravel and sand to sand alone or from silt, that in some cases valley gradient is 2.5 times clay, sand, and gravel to predominantly silt and as great as stream gradient, whereas in other clay during aggradation. Thus in streams drain-
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ing areas underlain by sandstone, the caliber Undoubtedly during valley alluviation both of the load decreased, but the proportion of the dimensions and shape of the channels bed-material load to wash load probably changed in response to a decrease in discharge changed little. Streams draining areas of sand- and changed sediment-load characteristics. As stone and shale underwent not only a decrease Figure 2 indicates, an increase in sinuosity in the caliber of sediment load but also a re- would be accompanied by a decrease in width- duction in the ratio of bed-material load to depth ratio. Channels which are still straight wash load (Fig. 6). As a result the streams drain- underwent only a decrease in width and depth ing areas of mixed sediments, after deposition as discharge decreased. of the coarser fraction of the sediment load, If the preceding scheme is correct then recent were flowing on alluvium with a gradient in changes in the sediment load of rivers might excess of that required for transport of the be expected to have similar effects. For ex- predominantly wash load. A reduction of ample, when aggradation occurs in ephemeral gradient by degradation could be only partly streams, the coarser sediment is deposited, and the fines continue downstream. In streams transporting predominantly sand the down- stream effects are minor, but in streams trans- porting a mixed load of silt-clay, sand, and gravel, the lower reaches of the streams tend to narrow (Schumm, 196lb, p. 47, 63). This is probably the first phase of conversion to a meandering stream. Figure 6. Hypothetical cross section of a Many modern channels have changed from valley showing change in shape of stream meandering to straight. For example, a deple- channels toward top of alluvium as sediment tion of vegetational cover on hillslopes has becomes progressively finer. Shape of chan- caused an influx of coarse sediment into the nel is related to sinuosity (Fig. 2). channels of some New Zealand rivers. The result is a change from a narrow meandering effective, for with incision the stream encoun- channel to a wide straight one (Grant, 1950). tered the coarser sediments transported during It is not improbable that an improvement in a previous regime, and the development of an vegetative cover would decrease the proportion armor of coarse sediment could prevent further of bed-material load, which in turn would degradation. The formation of a sinuous course cause a reversion of the channel to its former appears to have been the only alternative. An condition. increase in sinuosity would not only decrease Another example of the conversion of a the gradient of a stream, but it would also in- meandering river to a straight course is fur- crease the frictional resistance to flow within nished by the Cimarron River in Southwestern the channel (Dryden and others, 1956, p. Kansas. Prior to 1914 the Cimmaron in Kansas 482). Streams draining areas of relatively flowed in a narrow, deep, meandering channel, coarse or sandy sediments were less affected but during and following a major flood in 1914, by the change in caliber of sediment load and the valley was gutted, and the underlying continued to flow on a gradient which is today sands were exposed. The banks collapsed, and essentially that of the valley itself, because they a wide, sandy, essentially straight channel was have been transporting relatively large amounts formed (McLaughlin, 1947). The type of sedi- of bed-material load throughout their history. ment load transported through this channel An increase in sinuosity and the accompany- has undoubtedly changed greatly as the channel ing decrease in stream gradient reflect the changed. It seems probable that not much need to dissipate the energy, in excess of that bed-material load could have moved through expended in friction and sediment transport, the pre-1914 narrow, meandering channel. that became available as the proportions of Apparent exceptions to the conclusions given bed load and suspended load changed. The heretofore are plentiful. For example, streams maintenance of a straight channel during valley draining the mountain meadows of South Park, aggradation resulted from the need to utilize Colorado, are meandering, yet their beds are all the stream's energy in overcoming frictional composed of cobbles. A possible explanation is resistance to flow and in the transport of sedi- that the coarse sediment acts as an armor over ment through the channel. which the stream meanders. Under the present
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regimen these coarse sediments are not moved; forming the perimeter of the channel, and a therefore, the predominant sediment load is gentler gradient for the same discharge than transported in suspension. In addition, it ap- less sinuous streams. The bank materials of pears probable that, although a stream is sinuous streams are cohesive, but this property transporting fine sediments, the valley gradient is not restricted to the most winding rivers. may be so gentle as to inhibit meandering. Mean annual discharge does not influence the Examples of this may be the Mississippi River sinuosity of these rivers. below New Orleans (Fisk, 1947) and the The sinuosity of stable alluvial streams, Illinois River (Rubey, 1952). Both rivers flow which transport predominantly sand, silt, and on surfaces which slope very gently down- clay, appears to be determined by the propor- stream, and both rivers are essentially straight tions of wash load and bed-material load trans- although they transport fine sediments. ported. A relatively wide and shallow channel Some knowledge of the geologic history of a is associated with the movement of a high valley is important for an understanding of the proportion of bed-material load, whereas a modern river. Indeed, it may be critical in some narrow deep channel is associated with the cases. For example, if the lower course of the transport of a sediment load composed pre- Illinois River had not been drowned by the dominantly of fine material, wash load. rapid deposition in the Mississippi River valley The flowing of sinuous and straight streams following the Pleistocene (Rubey, 1952) its on valley fills of the same gradient may be valley gradient would be steeper, and the explained by the change in caliber of the sedi- modern Illinois River would meander. Modern ment load and in the ratio of bed-material load rivers need to be considered not solely with to wash load transported by these streams dur- respect to the present regimen of the stream ing post-Pleistocene valley filling. Recent but also with regard to geological perspective, changes of stream sinuosity and shape can be for it appears that the valley gradient may be explained by a change in the type of sediment an independent variable influencing the present introduced into the channel. The beginning of pattern of the alluvial river. meandering in the mature stage of the Davisian cycle of erosion may also be attributed to a CONCLUSIONS change in the ratio of bed-material load to Sinaous streams on the Great Plains are wash load transported by the streams as the characterized by relatively narrow and deep relief of a region is lowered. channels, a higher percentage of silt-clay
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MANUSCRIPT RECEIVED BY THE SOCIETY, JULY 31, 1962 PUBLICATION AUTHORIZED BY THE DIRECTOR, U. S. GEOLOGICAL SURVEY
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