Response of alluvial rivers to slow active tectonic movement

SHUNJI OUCH)! c/o Institute of Geosciences, Faculty of Science and Engineering, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112, Japan

ABSTRACT Rates of Quaternary surficial deformation without faults are considered to be < 10 mm/yr (Schumm, 1963; Kaizuka, 1967; Bandy and Marincovich, Alluvial rivers respond to valley-slope deformation caused by 1973). Rates of active tectonic movements during shorter time spans active tectonics in various ways depending on the rate and amount of should have a wider range than rates of Quaternary tectonic movements, surficial deformation and on the type of river. On the basis of experi- which are expressed as average values during long periods. Aseismic mental results and field examples, hypothetical models of river re- deformation detected by geodetic surveys, however, seems to have a range sponse to anticlinal uplift and synclinal subsidence were developed for similar to that of the longer-term Quaternary tectonic movement (Holdahl different types of alluvial rivers. and Morrison, 1974; Reilinger and Oliver, 1976; Brown, 1978). Rites as An experimental braided channel responded to anticlinal uplift much as 10 mm/yr may be a reasonable estimation for active aseismic across the channel with degradation and terrace formation in the deformation. This rate seems low as compared with the changes of alluvial central part of the uplift. With subsidence, aggradation in the central rivers. When a period of some decades or a hundred years is considered, reach was the main response. Transverse bars developed downstream however, surficial deformation of this rate may deform valley slopes of the subsidence axis. An experimental meandering channel re- enough to affect alluvial rivers. The deformation of valley slope, the slope sponded to slope steepening with a sinuosity increase. Bank erosion of the surface on which the channel is formed, will inevitably change and point-bar growth occurred downstream of the anticlinal axis and channel gradient, which is a dependent variable determined by water and upstream of the synclinal axis. Upstream of the uplift axis and down- sediment discharge and by sediment size. This change of channel gradient stream of the subsidence axis, where the slope was flattened, water will upset the equilibrium between channel slope and hydraulic properties flooded over bar!». of the stream. Local convexity in longitudinal profiles of the middle Rio Grande, Volkov and others (1967) indicated that scouring of the rivsr bed New Mexico, is considered to be formed by a domal uplift. Local occurred where rivers flow through uplifted areas in the European part aggradation and degradation could be explained by the effect of uplift. of the Soviet Union, but that the opposite situation occurred in subsided The San Joaquin River, California, which is now highly controlled, areas. Welch (1973) suggested that a decrease in sinuosity by bank erosion does not show clear adjustment to the rapid subsidence due to ground- on the inside of bends in the Red River, Manitoba, Canada, is related to water withdrawid. It shows, however, a channel-pattern adjustment decreasing valley slope due to isostatic rebound. Adams (1980) showed to active tectonic subsidence that has been occurring for a long time. remarkable positive correlations between tilt rates measured along valleys The San Antonio and Guadalupe Rivers in Texas both increase their and changes in the sinuosity of those reaches. Burnett and Schumm (1983) sinuosity significantly where monoclinal movements steepen valley observed channel changes across active uplifts in the southeastern United slopes. States, and they indicated that streams of different sizes are in different stages of adjustment to the same uplift. Nansen (1980) stated that the INTRODUCTION meandering Beatton River in British Columbia, Canada, has not yet com- pleted adjustment to tilting, which may have occurred some thousands of River morphology and channel behavior have been given much at- years ago. tention by geomoiphologists, who attempt to explain river morphology; by Changes in numerous factors affecting alluvial rivers can certainly geologists, who study sedimentary structures of river deposits; and by civil obscure the effect of slow movement of the Earth's surface. Recent activi- engineers, who try to control rivers. Although many studies have been ties of man that have had large direct and indirect impacts on alluvial done on the controlling factors and their effects, little attention has been rivers make it much more difficult to identify and determine the effect of paid to active tectonic movement as a factor influencing river morphology active tectonic movement. In this study, effects of local valley-slope de- and channel behavior, except in a few works, such as Tator (1958), Welch formation were studied to reduce the complication to a certain degree. (1973), Adams (1.980), Russ (1982), and Burnett and Schumm (1983). Larger-scale factors, such as climatic fluctuations, possibly can be elimi- Tectonic movement contemporaneous with the formation of modern river nated from the causis of local changes in river properties. morphology is here referred to as "active tectonic movement." This study A series of ex[>eriments was performed to obtain ideas about how examines the hypothesis that alluvial rivers respond and adjust to active streams respond to surficial deformation, and alluvial rivers flowing tectonic movement and describes the process of adjustment. through areas of active tectonic movement were examined to determine The main reiison why tectonic movement has largely been ignored as whether changes of plane forms and longitudinal profiles could be a factor influencing river morphology and channel behavior is its slowness. detected.

Geological Society of America Bulletin, v. 96, p. 504-515, 15 figs., 1 table, April 1985.

504

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EXPERIMENTAL STUDY OF THE EFFECT OF ACTIVE TECTONICS ON ALLUVIAL RIVERS

Equipment and Procedure

All of the experiments were performed in a wood-framed flume set at the Engineering Research Center, Colorado State University, Fort Collins, Colorado, that is 9.1 m (30 ft) long, 2.4 m (8 ft) wide, and 0.6 m (2 ft) high. The central 2.4 m (8 ft) of the flume has a flexible bottom, which is supported by a steel beam across the flume at the center, 4.65 m from the upstream end of the flume. Anticlinal uplift or synclinal subsidence was simulated by jacking up the steel beam and adding or extracting a certain number of shims between the steel beam and the concrete blocks on which the beam rests. The shims are aluminum plates 1.27 mm (0.05 in.) thick. The rate of uplift or subsidence was empirically set rapidly or slowly enough to allow observation of the response of experimental channels. The rate is extremely rapid compared with active tectonic movement. The experiment is not a scale model, however, but it should be considered as an idea-generating method. Water was introduced into the inlet box at the end of the flume upstream from the water pipe (braided channels) or recirculated by a small pump (meandering channels). A point gage was used for measuring eleva- tions of the sand surface along cross sections. Cross-section numbers indi- cate distance from the upstream end of the flume in metres. Time used in the experiment is expressed as hours of water flow from a certain initial time, excluding the time required for measuring cross sections. The effects of both uplift and subsidence on braided and meandering channels were examined. Different channel patterns were formed by changing initial slope and discharge, and by introducing suspended load.

Braided Channel Experiments 20 AO The initial channel, 8.9 cm (3.5 in.) wide and 3.8 cm (1.5 in.) deep, Cross Section No. was molded on a 2% slope, which was formed of a mixture of moderately Figure 1. Mean depth changes of the experimental braided chan- sorted medium sand and a small amount of kaolinite (—9:1). Additional nel during uplift. Datum lines indicate the depth at 0.0 hr. Cross- sand was fed into the head of the channel by a vibrating sand feeder. section measurements were made every 25 cm from 3.75 m to 6.0 m, After 20 hr of running with clear water (Q = 100 ml/sec), a braided and every 50 cm in the rest of the flume from 1.0 m to 7.5 m. pattern had developed. This was the pattern used as the initial braided channel of the experiment, and the measurement was started from this point (0 hr). ing height with uplift. The thalweg was fixed, and downcutting was accel- Uplift. Uplift was started at 6 hr, 1 shim at a time, and it was erated in this reach. At the same time, a multiple thalweg channel with continued every 2 hr until 48 hr except at 14,20, 30,42, and 46 hr. At 48 submerged bars, which indicates an aggradational trend, formed in the hr, 4 shims were added, to make a total uplift of 2.54 cm (1 in. or 20 upstream reach of the uplifted area. The terraces were gradually eroded by shims). The measurement of cross sections was made every 2 hr, except thalweg shift, whereas the height of the terrace surface increased with from 19 to 22 hr (1-hr interval) and after 46 hr. uplift, and they were destroyed by 32 hr. As the terraces were eroded, Bench marks were set on the sand surface, and their elevations were degradation migrated into the upstream reach (Fig. 1). A strongly braided measured before and after each small uplift. Movement of bench marks at pattern, also a result of aggradation, developed downstream from the 4.65 m indicated that the surface of sand was uplifted almost the same terraces due to excess sediment supply from the uplifted area. When the amount as the added shims. No lateral tilting occurred. terraces disappeared, the braided pattern in the downstream reach devel- In spite of the uplift, no significant local convexity in the area of uplift oped a fixed thalweg with alternate bars as a result of degradation caused appeared in longitudinal profiles of mean bed elevation. The braided by the decline of sediment supply (Fig. 2). After 48 hr, when 4 shims were stream responded to uplift (1.27 mm/2 hr over distance of ~1.2 m) with added, a similar process beginning with terrace formation was resumed. degradation rapid enough to offset the uplift. In the uplifted area, channel The difference in deformation rate did not seem to affect the trend of depth increased with fluctuations (Fig. 1) caused by changes in pattern adjustment. (Fig. 2) and by small-scale complex response. Degradation started near The degradation occurring in the uplifted area did not exactly corre- 5.0 m after the first uplift, where the slope was steepened the most, and it spond to the uplift. There were fluctuations and pauses in the degradation migrated upstream. Sediment produced by the degradation caused slight while the uplift continued. These fluctuations and pauses seemed closely aggradation downstream. As degradation continued, terraces were formed related to pattern changes. in the central to downstream reach of the uplifted area after ~18 hr. Subsidence. The channel existing at 68 hr in the uplift experiment Terraces were distinguished from bars by their fixed position and increas- was used as the initial stage (0 hr) for the subsidence experiment. Subsi-

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hours

Thalweg

O Bar O

Terrace

Figure 2. Pattern change of the experimental braided channel during uplift.

dence was started at 2 hr by extracting 5 shims (6.35 mm) at a time, and 4 thalweg pattern developed -150 hr after water (100 ml/sec) was intro- episodes (5 shims each) of subsidence were performed every hour for 5 hr duced from the inlei: with —30° deflection. This pattern was used as the (total 20 shims or 1 in.). Cross sections were usually measured every initial stage (0 hr in Fig. 5). Clay was mixed with the water (-1,000- 2,000 30 min. ppm) that was circulated by a pump. Some clay deposited and formed a The braided stream responded to subsidence by deposition, but it was thin layer on the wet perimeter of the channel. This clay cover stabilized not enough to offs5t the subsidence. The rate of subsidence (6.35 mm/hr) the pattern. No sand was supplied from the sand feeder. apparently was too high for the stream to adjust completely. This resulted Uplift. Uplift was started with 2 shims added at 8 hr and at 12 hr, and in a local concavi ty in channel profiles (Fig. 3). Deposition as the main then 1 shim was added every 4 hr for a total of 8 shims (1.02 cm). The response to subsidence occurred quickly in the upper central part of the measurement of thalweg elevation and position and of some cross sections subsided area, and. it migrated upstream. The aggradation, however, did was made every 4 hr until 40 hr. After 60 hr, water was kept running until not reach upstream to 3.75 m. 73 hr. Transverse bars migrated slowly into the downstream reach of the The main response of the meandering channel to uplift was the subsidence, where the flow, slowed by slope flattening, flooded the entire increase in thalweg sinuosity in the downstream part of the uplift, where channel width. In this "flooded" reach, all bed features were submerged slope was steepened. The first notable change after the uplift started was under water, and the reach looked like a pool with no distinguishable removal of the clay cover in the downstream parts of meandering bends in thalweg (Fig. 4). The transverse bars did not reach the downstream end of the downstream side of the uplift. As the slope was steepened, flow eroded the subsidence, and the deficiency of deposition resulted in gradient con- the outer bank on the lower half of a bend. Sediment produced here was cavity by subsidence alone. In contrast, slope convexity at the upstream deposited on the edge of the facing or the next point bar. The growth of end of the subsidence was removed by degradation (Fig. 3). Degradation point bars induced further bank erosion, and this process resulted in an started in the uppeimost reach of the subsided area, where a slope discon- increase in thalweg sinuosity (Fig. 5). tinuity was formed with subsidence, and it migrated upstream into the Thalweg elevations in the uplifted area increased with uplift as the reach, where no subsidence occurred. This degradation, which occurred change of valley slope was compensated for by increased sinuosity, and the mainly as bar destruction, provided sediment to the subsided area. As a convexity due to the uplift can be observed in projected thalweg profiles result of this increase in sediment discharge and slope steepening by subsi- (Fig. 6). In other words, no significant degradation nor aggradation oc- dence, a strongly braided pattern developed in the upstream part of the curred. The projected thalweg profile is a plot of thalweg elevation versus subsidence (Fig. 4). No significant movement of sediment through the distance from the upstream end of the flume. flooded reach was observed. Thalweg degradation with alternate bar de- The channel did not fully adjust its thalweg slope by sinuosity in- velopment occurred downstream from the subsided area. crease even at the end of the experiment. Thalweg slope increased with projected thalweg slope (valley slope) steepening, although the rate of Meandering Channel Experiment increase was lowered, by the sinuosity increase. Relatively stable banks probably prevented or at least delayed the complete adjustment of slope. A trapezoidal straight channel with a 4-cm bottom width was molded In the upstream part of the uplift, where slope was flattened, flow velocity on a slope of —0.8%. The sand was composed of moderately sorted me- was reduced, and water flooded over the point bar. Clay was deposited, dium sand and a small amount of kaolinite (~ 10% to 20%). A meandering and the thalweg became indistinct.

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hours 10

cm 11

Ii

hh\. I •:. j

O Bar Figure 4. Pattern change of the experimental braided channel during subsidence. 40 60 80 Cross Section No. m Thalweg elevations in the subsided area lowered with subsidence, Figure 3. Mean bed profiles of the experimental braided channel because no significant degradation nor aggradation occurred. The convex- during subsidence. Cross-section measurements were made every 25 ity in projected thalweg profiles, which remained from the uplift experi- cm from 3.75 m to 6.0 m, and every 50 cm in the rest of the flume. ment, disappeared with subsidence (Fig. 8). A slight concavity appeared in the subsided area at ~24 hr, and it persisted during the remainder of the Subsidence. The lower half of the meandering pattern was destroyed experiment (Fig. 8). by the end of the uplift experiment mainly due to the introduction of clear water from 56 to 73 hr. After suspended load was reintroduced and banks Summary of Experiment were reshaped, a meandering pattern developed again. This pattern was used as the initial stage of the subsidence experiment (0 hr in Fig. 7). Response of the experimental channels to uplift or subsidence is Subsidence was made by extracting 1 shim (1.27 mm) at a time every 4 hr summarized in Table 1. from 8 to 36 hr (total 8 shims or 1.02 cm). The main feature of braided-stream response to uplift was incision The main response of the meandering channel to subsidence was the and terrace formation in the uplifted area. Accompanied by terrace forma- increase in sinuosity in the upstream part of the subsided area. The sinuos- tion and destruction, which occurred in the central to downstream part of ity increase was similar to that which occurred in the downstream part of the uplift, there was deposition in the upstream part of the uplift zone, and the uplift. After the first subsidence, the flow attacked the outer bank of the aggradation occurred in the reach downstream from the uplift. This ap- lower half of the bend in the upstream part of the subsidence, where slope peared as the multiple thalweg channel with submerged bars (upstream) was steepened. The clay cover was washed away, and the bank and the and the strongly braided stream (downstream). When the terraces were edge of a point bar were slightly eroded, and a small amount of sediment destroyed, these features disappeared, too. The aggradational condition in was deposited on the downstream part of the point bar and the upstream the downstream reach became degradational because of the decline in part of the next point bar. As the subsidence continued, this small deposi- sediment transport from the uplifted area. After the uplift ended and the tion developed into a new narrow bar attached to the old bar, and the disturbance due to the uplift decreased, the channel pattern returned to one outer bank was eroded (Fig. 7). This resulted in a slight sinuosity increase similar to the original pattern. during the subsidence. The back-water effect of the central part of the The main feature of the response of braided channel to subsidence subsided area may have reduced the rate and amount of sinuosity increase. was aggradation in the upper central reach of the subsided area. The In the downstream part of the subsided area where slope was flat- degradation that occurred at the upstream end of the subsided area sup- tened, the flow was slowed, and it flooded over the point bar (Fig. 7). The plied some sediment to the central reach. This increase in sediment dis- point bar located approximately from 5.0 m to 5.75 m was almost com- charge and the steepened slope were manifested as a strongly braided pletely submerged by 36 hr. Clay was deposited over the channel width, pattern in the upstream part of the subsided area. Aggradation extended and the thalweg became indistinct. downstream into the downstream part of the subsided area (where slope

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„ „m 0 AO hrs. 05— . A* 1-0— i 1-5—

20—

25— M

3-0—

3-5— v 1 ¿0— l

4-5— 50— f,% 55— i 4-0 60 80 Cross Section No. m 60— Figure 6. Projected thalweg profiles of the experimental mean- J*' ' JA dering channel during uplift. Thalweg elevation was measured at every 65— point required to describe a detailed thalweg profile (from 8 to 40 hr).

7-0— \ vL was flattened by the subsidence, and water flooded over bars) in the form

75— • •/<• of a transverse bar. The meandering channel showed a type of response to both uplift and 80— m subsidence different from that of the braided channel. The main response 6~~ 50 cm Thalweg Bar was the increase in thalweg sinuosity in reaches where slope was steep- ened. This response started quickly, although the rates of uplift and Figure 5. Pattern change of the experimental meandering channel subsidence were slow. Lateral change may be a preferred way for streams during uplift. to adjust to changes in slope, if it is possible. When the lateral change

TABLE 1. RESPONSE OF EXPERIMENTAL CHANNELS TO UPLIFT AND SUBSIDENCE

Cross section 2.5 3.0 3.5 4.0 4.5 50 5.5 6.0 6.5 no. 1 1 1 I | 4.65 1 1 1 1 Reach A B 1 C I D

Aggradation Degradation Aggradation

Frequent thalweg Strongly braided;

Uplif t shift; submerged bars Terraces central bars channe l . 1 Degradation Aggradation - Hooded" Degradation

Braide d Bar Destruction Strongly braided; Transverse bars Alternate bus

Subsidenc e central bars 1 "Flooded" Sinuosity increase Indistinct thalweg; Bank erosion; Uplif t clay deposition point-bar growth channe l i g

Sinuosity increase "Flooded" Bank erosion; Indistinct thalweg; Meanderi i point-bar growth clay deposition Subsidenc e

Sou: the center of the deformation is at cross section 4.6S. Reach A: upstream reach from the uplifted or subsided area, where no significant uplift or subsidence occurred. Reach B: upstream half of the uplifted or suicided area (from cross sections 3.5 to 4.65). Reach C: downstream half of the uplifted or subsided are (from cross sections 4.65 to 5.75). Reach D: downstream reach from the uplitted or subsided area, where no significant uplift or subsidence occurred.

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68 hrs.

40 60 80 Cross Section No. m

Figure 8. Projected thalweg profiles of the experimental mean- dering channel during subsidence. Thalweg elevation was measured at every point required to describe a detailed thalweg profile.

The Rio Grande flows across the uplift approximately along its major 8°m axis. Late Pliocene alluvium of the ancestral Rio Grande is displaced 85 m 50 cm vertically in the uplifted area (Bachman and Mehnert, 1978). Terrace Figure 7. Pattern change of the experimental meandering channel remnants distributed along both sides of the Rio Grande in the area of during subsidence. uplift also show displacement by the uplift (Ouchi, 1983). Deformation of Tertiary deposits and Quaternary terraces indicates that the modern uplift is a part of long-term tectonic movement. Thalweg profiles of the middle Rio Grande show a large convexity in occurs as increased sinuosity, vertical adjustment may not occur. In the the uplifted area, between Belen and Socorro (Fig. 9). Happ (1948) reaches where slope was flattened, water flooded over point bars, and the pointed out that this "hump" has existed at least since the 1918 mapping. thalweg became indistinct. Clay deposition occurred in these reaches, but He suggested that it has been caused by excessive local aggradation by two it did not affect bed elevation. powerful tributaries, Rio Puerco and Rio Salado, which flow into the Rio How much a channel can adjust its slope by a sinuosity change is not Grande in this area. The fact that the center and the expanse of the determined solely by the rate of slope steepening. Many other factors, such convexity almost perfectly coincide with the uplift suggests, however, that as bank stability and bed erodibility, may play important roles. In the it can be related to the uplift. A small bulge on the larger convexity of the experiment, relatively stable banks probably prevented the complete ad- 1936-1938 profile, which apparently was formed by the major flood of justment of channel slope. 1929 from Rio Puerco and Rio Salado, was removed by 1944 (Fig. 9). This indicates that the Rio Grande seems to be able to remove the sedi- ACTIVE TECTONICS AND ment contributed by these ephemeral tributaries before it makes a large CHANGES OF ALLUVIAL RIVERS convexity in the profile. Rio Puerco and Rio Salado are considered to have delivered less sediment prior to the 19th century (Bryan, 1928). The excess Middle Rio Grande, New Mexico local sedimentation thus is not necessarily the main cause of the convexity in the Rio Grande thalweg profile. In the Rio Grande Valley, New Mexico, a rapid elliptical domal Although the experimental results indicate that a braided river can uplift has been detected from leveling data between Belen and Socorro compensate for slow uplift by degradation, the Rio Grande, which is (Reilinger and Oliver, 1976; Reilinger and others, 1980). The maximum braided in this reach, obviously could not maintain its profile. The sedi- uplift observed near the center is -20 cm relative to the periphery, with the ment dumped by Rio Puerco and Rio Salado into the Rio Grande at the average rate ~5 mm/yr (Reilinger and others, 1980). reach where degradation is supposed to start may have prevented full slope

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adjustment, and hence the local convexity due to the uplift may have the natural pattern formed before the artificial control. The neotec- remained in the pr ofile. tonic subsidence had occurred for a long time prior to the artificial co ntrol Aggradation, which has been a problem in the middle Rio Grande, in the same area as the subsidence due to ground-water withdrawal. The seems to have a close relationship with the uplift. Progressive aggradation San Joaquin River flows in a very sinuous course toward the axis of the has been the prevailing feature at least since 1918 in upstream and down- valley through the area where slope has been steepened by the neotectonic stream reaches of the uplift. In the central part of the uplift, no progressive (in this case, active tectonic) subsidence. Slowly increasing slope in section aggradation is observed. C of Figure 10 could have caused the river to increase its sinuosity. From Alternate bar; with a braiding tendency upstream of the uplift and a Mendota pool to Firebaugh (section D), the river has a less sinuous course, strongly braided pattern in the downstream part of the uplift can be perhaps owing to slojje flattening by the active tectonic subsidence. Down- observed on the aerial photographs taken in 1948 by the U.S. Bureau of stream from Firebaugh, in sections E and F, where the flood plain widens, Reclamation. sloughs develop along the main channel. In section G, the river becomes straighter with smaller bends, and distributary sloughs increase in number San Joaquin River, California and size. This gives the reach an anastomosing pattern. In section H, the main channel and other sloughs become very sinuous, and the pattern Extremely rapid land subsidence due to ground-water withdrawal looks reticulate. There are numerous swamps and oxbow lakes on the has occurred in some areas in the San Joaquin Valley, California, since the flood plain, which is very wide. mid-1920s, especially since World War II (Poland and others, 1975). The The Merced River fan growing from the east side of the valley may San Joaquin River flows through a corner of one of the subsided areas. have some effect on the river pattern in sections G and H; however, as in Topographic maps, (1:24,000) made in 1920-1921 (before the rapid sub- the case of the Tulare Lake interior drainage in the southern San Joaquin sidence due to ground-water withdrawal started) and in 1956-1962 do not Valley, the large area of anastomosing or reticulate channel patterns in show any significant effects of the subsidence on the river pattern. The sections G and H is more likely to be a feature formed near the clown- river is highly controlled, and it probably could not adjust to the change in stream end of the active tectonic subsidence. The Tulare Lake interior valley slope. drainage, which was; explained by a damming effect of the alluvial fans The San Joaquin Valley has been a region of nearly continuous of the Kings River and Los Gatos Creek developing from both sides of deposition during :he late Tertiary and the Quaternary (Miller and others, the valley, is now believed to be the result of the continued tectonic sub- 1971), and it has been a subsiding area for a long time. The lacustrine sidence (Davis and Green, 1962). Corcoran clay layer, which is the principal confining bed in the San Joaquin Valley, shows postdepositional structural warping (Frink and Texas Coast Kues, 1954; Bull a nd Miller, 1975). Although the trend of warping of the Corcoran clay agrees well with the modern subsidence, ~ 152 m of down- Long-term subsidence of the Texas coastal plain is well-known geo- warping cannot be explained by the ground-water withdrawal. If tectonic logically as the "Gulf Coast Geosyncline" (Barton and others, 1933; Wa- subsidence is still occurring, it is now overshadowed by subsidence due to ters and others, 1955; Bornhauser, 1958). Within this large tectonic ground-water withdrawal. structure, many smaller structures such as local folds, flexures, and faults Although the San Joaquin River does not show clear adjustment to the rapid subsidence, due to ground-water withdrawal, the pattern change from upstream to downstream (Fig. 10) indicates a possible relationship New Mexco between the river pattern and the long-term neotectonic movement identi- fied by the deformation of the Corcoran clay layer. The river is artificially fixed in some places, but it can be assumed that the river pattern shows

Figure 9. Local convexity appeared in longiltudinal profiles of the Rio Grande from Belen to Socorro, New Mexico (from Ouchi, 1983). Profiles from 1936 to 1972 are based on the data compiled by the U.S. Bu- reau of Reclamation (1967, 1972). The 1917-1918 profile was drawn from topographic maps surveyed b3' the State Engi- neer's Office in 1917-1918. 10 mi

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Figure 10. Subsidence zone (due to ground-water withdrawal) and course of the San Joaquin River. Equal subsidence lines are from Po- land and others (1975). This subsidence zone generally agrees with the tectonic sub- sidence zone.

WftV Outline of valley Line of equal subsidnce 1926-72 (ft)

Contour line (ft)

have developed (Bornhauser, 1958; Weaver, 1955; Waters and others, (Honea, 1956). These faults, however, are not consistently contempora- 1955; Colle and others, 1952; Shelton, 1968). One of the best-known neous with deposition. A fault may be contemporaneous in a certain examples is the Post-Vicksburg (Colle and others, 1952) or Vicksburg-Frio horizon and postdepositional in a different horizon. Such a fault is formed (Waters and others, 1955) flexure along the southern Texas coast by later movement along the same zone of weakness (Hardin and Hardin, (Fig. 11). Contemporaneous faults, which form while sediment is being 1961). Even long after the active deposition, the larger-scale tectonic deposited, are an important feature associated with the flexure (Born- movement of the Gulf Coast Geosyncline may reactivate the flexure zone. hauser, 1958; Hardin and Hardin, 1961). A series of faults associated with Weaver (1955) mentioned that a modern fault formed on the Post- the Post-Vicksburg flexure is the Sam Fordyce-Vanderbilt fault zone Vicksburg flexure zone. Many active faults reported in the areas of rapid

Figure 11. Index map showing location of Guadalupe and San Antonio Rivers and Post-Vicksburg flexure in the Texas coast region.

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Post-Vicksburg a meander cutoff occurs in this section, there possibly will be change in the Flexure entire channel pattern. Along the Guadalupe River, meander cutoffs and some degradation seem to have already occurred in the section of steep- ened slope. More sinuous abandoned channels are observed on the flood plain, which is >6 m (20 ft) above the channel bed. The response of the San Antonio River, which has more clay and less sand (Morton and Donaldson, 1978), may be slower than that of the Guadalupe River, which has more sand in both bed and banks. With the finer material, the San Antonio River may also be more able than the Guadalupe River to increase sinuosity.

CHANNEL PATTERN AND RESPONSE OF ALLUVIAL RIVERS TO ACTIVE TECTONIC DEFORMATION

Alluvial rivers respond to active tectonic movement in ways that are dependent on the types of deformation and rivers. The main observable effect of surficial deformation seems to appear first in the channel pattern, and degradation and aggradation as river adjustment to valley slope de- Figure 12. Longitudinal profiles (projected) of Guadalupe and formation also affect the channel pattern. San Antonio Rivers across Post-Vicksburg flexure zone (drawn from Schumm (1981) developed a classification of alluvial-channel pat- 1:24,000 topographic maps), p = sinuosity; Sp = projected channel terns, applying the concept of channel-pattern change with valley slope to 4 4 slope (x I0" ); sc = channel slope (x 10" ). the classification of alluvial-channel pattern with sediment load types. Although it is still hypothetical, this classification is pertinent to this study, because the valley slope is treated as an independent variable, and because it also appeared to be compatible with the observations reported here. subsidence due to ground-water withdrawal along the Texas coast are Using this classification, hypothetical models of alluvial-river response to essentially the result of reactivation of old faults cutting through unconsol- active tectonic movement (anticlinal uplift and synclinal subsidence) were idated sediments (Kreitler, 1976). developed for different types of alluvial rivers (Figs. 13,14,15). The term The Post-Vicksburg flexure or the Sam Fordyce-Vanderbilt fault "reticulate" is used here for a widespread multiple channel network that zone may still be active and may have some effects on alluvial rivers has an angular cross-channel development. The reticulate pattern is sup- flowing across it. Recent movement of the land surface in the Gulf Coast posed to form on flat and wide valley floors, and it is not necessarily a region detected from the National Geodetic Survey releveling data by suspended-load channel, whereas the anastomosing pattern is the multiple- Holdahl and Morrison (1974) generally supports this assumption. channel network representing the steepest suspended-load channel. The meandering Guadalupe and San Antonio Rivers flow across the For braided rivers, which are the most obvious bedload channels, a Post-Vicksburg flexure zone (Fig. 11). Figure 12 shows longitudinal pro- valley-slope increase may change the channel pattern from meandering- files of both riven projected to straight lines along valleys. The profiles are thalweg-braided to bar-braided. Also, a valley-slope decrease may cause a divided into sections according to slope and pattern characteristics. The change from a bar-braided or meandering-thalweg-braided pattern to an profiles indicate that the flexure has had some influence on the rivers. Both alternate-bar pattern. profiles have relatively steep (valley slope) sections (section B) —32-64 km In the case of anticlinal or domal uplift across a braided riv er, the (20-40 mi) upstream from the mouth, and the rivers have a highly sinu- bar-braided pattern will always be observed in the reach downstream from ous course in this section. These steep slopes occur where the flexure is the uplift, where the slope is steepened and sediment discharge increases expected to deform the land surface. Morton and Donaldson (1978) rec- (Fig. 13a). Terraces are formed in the central part of the uplift where ognized the gradient change between this reach and the downstream reach, degradation occurs. The braiding tendency in the reach upstream from the but they attributed the change to the difference between alluvial plain and uplift may be less than in the downstream reach, because the slope is delta plain. This interpretation, however, cannot explain the gradient in- flattened, and sediment discharge does not increase. Alternate bars with a crease from section C to section B of both rivers. The steep gradient of braiding tendency in the upstream reach, terraces and a degradational section B is very ¡likely a direct result of the flexure movement. The rivers trend in the central area, and a bar-braided pattern in the downstream increase their sinuosity remarkably in this section (Fig. 12). This is what is reach from the uplift will be the dominant features. expected in response to a local steepening of the valley slope. The channel- The main feature of braided-river response to synclinal or basinal slope increase of steepened reaches is offset by the sinuosity increase to a subsidence across the river is aggradation in the central to downstream certain degree. The Guadalupe River shows a reasonable change of chan- area of the subsidence. A straight channel with transverse bars may de- nel slope, when che general decrease in channel slope in a downstream velop in the downstream reach of the subsided area (Fig. 13b). In some direction is considered. In sections C and B of the San Antonio River, cases, frequent overbank floods and channel avulsions may form multiple channel slope does not seem to have adjusted as well. Slope steepening in channels. At the upstream end of the subsidence, where a convex slope section B may be too large to be offset completely by sinuosity increase, irregularity tends to be formed, degradation will occur. This degradation, and the sinuosity in this section (3.08) seems close to a maximum value. If which removes the convex slope irregularity, increases sediment load

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Slope deformation River adjustment

Profile

Figure 13. Adjustment of a braided river to (a) anticlinal up- Pattern lift and (b) synclinal subsidence across it.

bar-bra ided bar- braided or meanderi ng - talweg braided

b. Subsidence Profile

Pattern

alternate bars

downstream. On the upstream side of the subsidence, therefore, a bar- side of the uplift for a mixed-load river (Fig. 14a), and the anastomosing braided pattern will be dominant (Fig. 13b). pattern will develop for a suspended-load river (Fig. 15a). The convexity In the case of anticlinal or domal uplift across a meandering river, formed by the uplift will be reduced as the degradation proceeds, and the which is the most common mixed- or suspended-load channel river, swampy reach will be drained. For suspended-load rivers, which are more sinuosity increase will be observed on the downstream side of the uplift stable and can accommodate higher sinuosity than can mixed-load rivers, as the valley floor is steepened (Figs. 14a and 15a). On the upstream side the whole process will proceed more slowly. of the uplift, channel straightening can be expected, but the damming Sinuosity increase also occurs on the upstream side of subsidence effects of the uplift may be more apparent. As a result, there will be across a meandering river (Figs. 14b and 15b). In the downstream part of inundation of flood-plain and channel avulsions, and a swampy condition the subsidence, a condition similar to that occurring on the upstream side with deposition of fine material will occur. The reticulate (or in some of uplift is expected to occur; however, because slope adjustment by ag- cases, anastomosing) channel pattern will probably develop (Figs. 14a and gradation is a slow process even after the pattern threshold is exceeded on 15a). After the pattern threshold is exceeded by meander cutofls on the the oversteepened upstream side of the subsidence, the swampy condition steepened slope, full-scale degradation will start and migrate upstream. in the lower downstream part of the subsidence has a better chance to The sinuous- or island-braided pattern will develop on the downstream develop and remain than on the upstream side of uplift. The multiple-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/4/504/3444997/i0016-7606-96-4-504.pdf by guest on 30 September 2021 Figure 14. Adjust- ment of a mixed-load meandering river 1o (a) anticlinal uplift and (b) synclinal subsidence across it. Time sequence is expressed in the order from top to bottom.

Suspended-load meandering river a. Uplift Ù b. Subsidence f Slope deformation Slope deformation ana adjustment Channel pattern and adjustment Channel pattern

Figure 15. Adjust- ment of a suspended-load meandering channel to •reticulate (a) anticlinal uplift and SxjOSI w S reticulate: (b) synclinal subsidence across it. Time sequence is expressed in the order from top to bottom.

- cutoff — — cutoff —

* anastomosing anastomosing

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channel reticulate pattern most likely forms near the downstream end of Brown, L. D., 1978, Recent vertical crustal movement along the east coast of the United States: Tectonophysics, v. 44, p. 205-231. subsidence for suspended-load meandering rivers. As on the downstream Bryan, Kirk, 1928, Historic evidence on changes in the channel of Rio Puerco, a tributary of the Rio Grande in New Mexico: Journal of Geology, v. 36, p. 265-282. side of uplift, the sinuous-braided pattern for a mixed-load river (Fig. 14b) Bull, W. B., and Miller, E. R., 1975, Land subsidence due to groundwater withdrawal in the Los Banos-Kettleman City and the anastomosing pattern for a suspended-load river (Fig. 15b) will area, California—Part 1, Changes in the hydrologic environment conducive to subsidence: U.S. Geological Survey Professional Paper 437-E, p. El-71. develop with meander-bend cutoffs on the upstream side of subsidence. As Burnett, A. W., and Schumm, S. A., 1983, Alluvial-river response to neotectonic deformation in Louisiana and Missis- sippi: Science, v. 222, p. 49-50. the slope restoration by aggradation in the subsided area will take a longer Colle, Jack, Cooke, W. F., Jr., Denham, R. L., Ferguson, H. C., McGuit, J. H., Reedy, Frank, Jr., and Weaver, Paul, time than that by degradation in the case of uplift, widespread reticulate 1952, Sedimentary volumes in Gulf Coastal Plain of United States and Mexico—Part IV, Volume of Mesozoic and Cenozoic sediments in western Gulf Coastal Plain of United States: Geological Society of America Bulletin, v. 63, channels or lakes are likely to be a common feature in the area of p. 1193-1200. Davis, G. H., and Green, J. H., 1962, Structural control of interior drainage, southern San Joaquin Valley, California: U.S, subsidence. Geological Survey Professional Paper 450-D, p. D89-91. Frink, J. W., and Kues, H. A., 1954, Corcoran Clay—A Pleistocene lacustrine deposit in San Joaquin Valley, California: Changes in channel pattern caused by surficial deformation will ap- American Association of Petroleum Geologists Bulletin, v. 38, p. 2357-2371. pear as changes in sedimentary fades. Detailed studies will make it possi- Happ, S. C., 1948, Sedimentation in the middle Rio Grande Valley, New Mexico: Geological Society of America Bulletin, v. 59, p. 1191-1216. ble to detect facies changes of sedimentary layers from known tectonic Hardin, F. A., and Hardin, G. C., Jr., 1961, Contemporaneous normal faults of Gulf Coast and their relation to flexures: American Association of Petroleum Geologists Bulletin, v. 45, p. 238-248. movements and to detect slow contemporaneous paleotectonic movement Holdahl, S. R., and Morrison, N. L., 1974, Regional investigation of vertical crustal movements in the U.S., using precise from changes in sedimentary facies, as Slack (1981) attempted. In the case relevelings and mareograph data: Tectonophysics, v. 23, p. 373-390. Honea, J. W., 1956, Sam Fordyce-Vanderbilt fault system of southwest Texas: Gulf Coast Association of Geological of braided-river deposits, a strongly braided pattern will form horizontally Societies Transactions, v. 6, p. 51-54. Kaizuka, Sohei, 1967, Rate of folding in the Quaternary and the present: Tokyo Metropolitan University Geographical bedded longitudinal-bar facies downstream of anticlinal uplift and up- Report, no. 2, p. 1-10. stream of synclinal subsidence. Downstream of subsidence, transverse-bar Kreitler, C. W., 1976, Lineations and faults in the Texas Coastal zone: University of Texas, Austin, Bureau of Economic Geology Report of Investigations 85, 32 p. facies, the dominant feature of which is planar cross-stratification, will Miller, R. E., Green, J. H., and Davis, G. H., 1971, Geology of the compacting deposits in the Los Banos-Kettleman City subsidence area, California: U.S. Geological Survey Professional Paper 497-E, 46 p. form. Meandering river deposits may show a cyclic vertical facies change Morton, G. C., and Donaldson, A. C., 1978, Hydrology, morphology, and sedimentology of the Guadalupe fluvial-deltaic in response to slow contemporaneous tectonic movement. Downstream of system: Geological Society of America Bulletin, v. 89, p. 1030-1036. Nansen, G. C., 1980, A regional trend to meander migration: Journal of Geology, v. 88, p. 100-108. uplift and upstream of subsidence, point-bar growth will intensify deposi- Ouchi, Shunji, 1983, Effects of uplift on the Rio Grande over the Socorro magma body, New Mexico, in Chapin, C. E., ed., Socorro region II: New Mexico Geological Society, 34th Field Conference, Guidebook, p. 54-56. tion of point-bar sand. The point-bar facies will be interrupted by sinuous Poland, J. F., Lofgren, B. E., Ireland, R. L.,andPugh, R. G., 1975, Land subsidence in the San Joaquin Valley as of 1972: (or island)-braided or anastomosing channel facies, when the channel U.S. Geological Survey Professional Paper 473-F, 78 p. Reilinger, R. E., and Oliver, J. E„ 1976, Modern uplift associated with a proposed magma body in the vicinity of Socorro, pattern change occurs on the oversteepened slope. The point-bar deposi- New Mexico: Geology, v. 4, p. 583-586. Reilinger, R. E„ Oliver, J. E., Sanford, Allan, and Balazs, Emery, 1980, New measurements of crustal doming over the tion will resume after the slope is restored. Downstream of subsidence or Socorro magma body. New Mexico: Geology, v. 8, p. 291-295. upstream of uplift, a widespread reticulate channel pattern will enhance Russ, D. R., 1982, Style and significance of surface deformation in the vicinity of New Madrid, Missouri: U.S. Geological Survey Professional Paper 1236-H, p. 95-114. flood-plain deposition. Schumm, S. A., 1963, Disparity between present rates of denudation and orogeny: U.S. Geological Survey Professional Paper 454-H, p. Hl-13. — 1981, Evolution and response of the fluvial system, sedimentologic implications: Society of Economic Paleontolo- gists and Mineralogists Special Publication 31, p. 19-29. ACKNOWLEDGMENTS Shelton, J. W., 1968, Role of contemporaneous faulting during basinal subsidence: American Association of Petroleum Geologists Bulletin, v. 52, p. 399-413. Slack, P. B., 1981, Paleotectonics and hydrocarbon accumulation. Powder River Basin, Wyoming: American Association I wish to thank S. A. Schumm for his guidance during the study and of Petroleum Geologists Bulletin, v. 65, p. 730-743. Tator, B. A., 1958, The aerial photograph and applied geomorphology: Photogrametric Engineering, v. 24, p. 549-561. for reading the manuscript. 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