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Active Tectonics: Studies in Geophysics 5 Alluvial River Response to Active Tectonics STANLEY A.SCHUMM Colorado State University ABSTRACT Alluvial rivers are influenced by changes of valley floor slope, and therefore deformation of the valley floor by active tectonics can cause pattern change, aggradation, and degradation. Canals have been abandoned owing to uplift in Iran, and major floods and avulsive shift of the Indus River has been attributed to earthquake-generated dams and tilting of the Indus Valley. Streams crossing the Monroe uplift in Louisiana and the Wiggins uplift in Mississippi show pattern, gradient, and depth changes that indicate that these uplifts are active. The Mississippi River shows effects of the Lake County and Monroe uplifts. Slow deformation of a valley floor will eventually affect channel stability. This poses hazards to structures on or near the river banks as well as to bridges and pipeline and powerline crossings. Navigation may be impaired, and the variability of overbank flooding along the river can cause legal problems. INTRODUCTION Alluvial rivers, those that flow between banks and on a bed composed of sediment that is transported by the river, are sensitive to changes of sediment load, water discharge, and variations of valley floor slope (Schumm, 1977). Therefore, in addition to the dramatic effects when stream channels and terraces are offset along faults (Wallace, 1967; Stevens, 1974), other more subtle effects should be recognizable when deformation is vertical, slower, and aseismic. Many of the major rivers of the world follow structural lows and major geofracture systems (Potter, 1978). In fact, Melton (1959) suspected that streams that have adjusted to tectonic activity are numerous, and the lower Mississippi River and Rio Grande are clearly in areas of structural instability as are the lower Amazon, Niger, Tigrus, Euphrates, Rhine, and Indus, among others. The high discharge of these rivers should permit them to maintain their courses in spite of active tectonics. However, large rivers, because of their low gradients, may, in fact, be the most significantly affected by the minor changes in slope caused by active deformation. In spite of the practical significance of active tectonics, only a few investigators have considered its effects on alluvial rivers (Tator, 1958; Schumm, 1972, 1977; Adams, 1980; Russ, 1982; Burnett and Schumm, 1983). It is possible that this situation exists because variations of channel morphology and behavior can also be attributed to downstream variations of discharge and to the quantity and type of sediment load; therefore, the effects of active tectonics are difficult to detect. In ad-
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Active Tectonics: Studies in Geophysics dition, the attention of geomorphologists and photogeologists generally has been concentrated on the identification of geologic structures that are assumed to be quiescent (Howard, 1967; Ollier, 1981, p. 180) rather than on the effect of ongoing deformation on alluvial channels. Nevertheless, geomorphologists working in petroleum exploration, e.g., DeBlieux (1951, 1962) and Tator (1958), indicated that fluvial anomalies, such as local development of meanders or a braided pattern, local widening or narrowing of channels, anomalous ponds, marshes or alluvial fills, variations of levee width or discontinuous levees, and any anomalous curve or turn, are possible indicators of active tectonics. In addition, active tectonics can produce nickpoints, convexities or concavities of the longitudinal profile, channel depth variations, and, of course, either aggradation or degradation. Another problem is that active tectonics takes several forms. Deformation can be along faults (shear, normal, or reverse in a downstream sense) or pairs of faults (horst and graben). These faults should have the same effect as a monocline, dome, or basin. In addition to local structural features, the entire valley may be tilted upstream, downstream, or laterally. The possibilities are great, but, in reality, the primary effect of tectonics will be local steepening or reduction of gradient or cross-valley tilting. In addition to these primary influences on channel and valley gradient and configuration, there will be secondary effects, as the rivers respond to the changed gradient (aggradation or degradation), and there will be tertiary effects, as decreased or increased sediment loads influence reaches downstream of the deformed reach and as aggradation or degradation in the deformed reach progresses upstream. In addition to tectonic effects, there can be similar influences as a result of differential compaction of sediments (draping) over buried topographic highs. TECTONIC EFFECTS Where tectonism has been persistent for long periods of time active deformation will produce a channel response that will be superimposed on the long-term tectonic effects. Major valley deformation or total disruption of the river system can be the result of long-term tectonism. Valley Change The most commonly cited evidence for deformation is the warping of alluvial terraces in a valley. If the deformation has persisted the oldest terrace is the most deformed by uplift (convex) or subsidence (concave), and it will show the greatest offset by faulting (Machida, 1960; Zuchiewicz, 1979, 1980). Where there has been uplift or subsidence, terraces are warped upward or downward, and the extent of the displacement can be determined by comparison with the longitudinal profile of the present river if, indeed, the river has adjusted to the past deformation. Machida (1960) assumed that the longitudinal profile of a terrace is described by a negative exponential function in a downstream direction and that deviations from this curve indicate deformation. Valley-floor deformation can also be indicated by depth to bedrock. Alluvium will be thickest over downfaulted or downfolded zones and thinnest over areas of uplift (Kowalski and Radzikowska, 1968). River Changes When uplift is too rapid to be accommodated by a river there will be disruption of the drainage pattern (Sparling, 1967; Twidale, 1971, pp. 133–136; Ollier, 1981). For example, Freund et al. (1968) have evidence that movement along the Dead Sea rift in Israel disrupted streams that formerly drained from Jordan and crossed what is now the Dead Sea Valley to the Mediterranean Sea. Portions of the channels of these rivers are now displaced about 43 km as a result of movement along the boundary faults of the Dead Sea rift. The Murray River on the Riverine Plain near Echuca (Victoria, Australia) is an impressive example of channel modification by tectonic activity (Bowler and Harford, 1966). The Cadell Fault block has converted the Murray River from a single channel to an anastomosing system of channels that surround the obstruction. The abandoned segment of the Murray River is preserved on the dipslope of the fault block. A particularly active tectonic area is the eastern side of the East African rift valley near Lake Victoria. Doornkamp and Temple (1966) described the formation of lakes in some valleys, as a result of gradient reduction, and this could be the first stage of drainage disruption. In many cases, incising channels will encounter resistant strata, which may retard or prevent the maintenance of an antecedent condition. Incised meander patterns can also indicate deformation because the alluvial river pattern is affected by the deformation before it becomes fixed by incision into bedrock (Gardner, 1975). EVIDENCE FOR ACTIVE TECTONICS The preceding discussion reviews evidence for past deformation but not necessarily for active tectonics. Modern river and valley-floor morphology and geodetic
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Active Tectonics: Studies in Geophysics surveys provide the best geomorphic evidence of ongoing deformation. In those areas where the rate of deformation is low, active tectonics is reflected by geomorphic features that react to small changes of slope, for example, the gradient of terraces and stream channels and meander characteristics (Radulescu, 1962; Neef, 1966). One of the most sensitive indicators of change is the valley-floor profile and longitudinal profile of the stream (Bendefy et al., 1967; Zuchiewicz, 1979). Degradation and aggradation can be evidence of active tectonics, and these processes will be accompanied by changes of channel morphology (depth and width) as the channel incises or as it aggrades. Therefore, progressive changes of thalweg or water-surface elevation can provide information on vertical changes of the channel. When a long gaging-station record is available, it can be used to determine water-surface change with time. When the gage height or water-surface elevation for a specific discharge [e.g., 100 cubic feet/second (cfs)] is plotted against time, any rise or fall of the specific gage height indicates a change of either channel capacity (area) or elevation (see Figure 5.16 below). For example, on the axis of an active uplift both the river bottom and water surface at a given discharge should be lowered, as the channel scours in response to increased slope (Volkov et al., 1967). When the water surface and river bottom rise through time, either very rapid uplift or a decrease of channel capacity by aggradation can be the cause. Channel Patterns Stream patterns are sensitive indicators of valley-slope change (Schumm, 1972, 1977; Schumm et al., 1972). Adams (1980) demonstrated a relation between measured tilt rates and downstream changes of sinuosity for the Mississippi River between St. Louis and Cairo and for the lower Missouri River. That is, in order to maintain a constant gradient a river that is being steepened by a downstream tilt will increase its sinuosity, whereas a reduction of valley slope will lead to a reduction of sinuosity. However, Twidale (1966) reported that both the Flinders and Leichardt Rivers have changed to a braided pattern as a result of the steepening of their gradient by the Selwyn Upwarp in northern Queensland. Another cause of meander growth and river shift is the lateral or transverse tilting of the valley. In general, a river should shift in a downtilt or a downslope direction and concentrate its attack on the valley side that has been downtilted (Cotton, 1941). Nanson (1980) showed that the south-flowing Beatton River in British Columbia is affected by isostatic tilt to the east, which caused a deviation from the normal downstream migration of the meanders. The tilting has augmented the easterly directed flow velocities, thereby resulting in an easterly bias to channel migration. However, although meander loops tend to migrate toward the east, frequent channel cutoffs leave the channel close to the west wall of the valley. The Beatton River is confined within a valley and is not free to shift laterally for an appreciable distance. This is unlike the situation on the Hungarian Plain, where the Tiza River has shifted laterally for long distances, as a result of deformation of the surface of the plain (Mike, 1975). River Types It is not possible to predict consistently the pattern and channel changes because different types of alluvial channel will respond differently to active tectonics; therefore, the characteristics of alluvial channels must be reviewed before their potential response can be evaluated. This can best be done by discussing a simple classification of alluvial channels that is based on type of sediment load (bed load, mixed load, suspended load) and pattern (Figure 5.1). Five basic channel patterns exist (Figure 5.1). These are straight channels with either migrating sand waves (pattern 1) or with a sinuous thalweg and alternate bars (pattern 2). There are two types of meandering channels, a highly sinuous channel of equal width (pattern 3a) and channels that are wider at bends than in crossings (pattern 3b). The meandering-braided transition FIGURE 5.1 Channel classification based on pattern and type of sediment load with associated variables and relative stability indicated. From Schumm (1981).
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Active Tectonics: Studies in Geophysics (pattern 4) and a typical braided-stream (pattern 5) complete the sequence. The relative stability of these channels in terms of their normal erosional activity and the shape and gradient of the channels, as related to relative sediment size, load, velocity of flow, and stream power, are also indicated on Figure 5.1. It has been possible to develop these patterns experimentally by varying the gradient, sediment load, stream power, and type of sediment load transported by the channel (Schumm and Khan, 1972). The range of channels from straight through braided forms a continuum (Figure 5.1), but experimental work and field studies have indicated that the pattern changes between braided, meandering, and straight occur at river-pattern thresholds (Figure 5.2). The pattern change takes place at critical ranges of valley slope, stream power, and sediment load (Schumm and Khan, 1972). Observed rivers can be placed within the five general categories. However, within the meandering stream group there is considerable range of sinuosity (1.25 to 3.0), which is the ratio of channel length to valley length. In addition, in the braided-stream category there are bar-braided and island-braided channels. Islands are vegetated bars. There are also multiple channel patterns termed anastomosing, anastomosed, or anabranch channels (Schumm, 1977, p. 155; Smith and Smith, 1980). In fact, it has been suggested that 14 channel patterns can be recognized (Figure 5.3). Experimental studies and field observations confirm that a change of valley-floor slope will cause a change of channel pattern and dimensions. The change will differ, however, depending on (1) where the channel lies on a plot such as that of Figure 5.2, (2) the type of channel (Figures 5.1 and 5.3), and (3) the amount and rate of deformation. FIGURE 5.2 Relation between valley (flume) slope and sinuosity (channel length/valley slope or valley slope/channel slope) during experiments at constant discharge. Sediment load, stream power, and velocity increase with slope, and a similar relation can be developed with these variables. From Schumm and Khan (1972). FIGURE 5.3 The range of alluvial channel patterns for the three channel types shown in Figure 5.1. A, Bed-load channel patterns; B, mixed-load channel patterns; C, suspended-load channel patterns. From Schumm (1981).
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Active Tectonics: Studies in Geophysics Slight increases of valley slope will shift river patterns from left to right on Figures 5.2 and 5.3, as the river adjusts its gradient by pattern change. With greater changes of valley slope, incision may produce sufficient sediment to cause a change from one type of channel to another with a metamorphosis of a mixed-load channel to bed-load channel (Figure 5.3). In addition, significant reductions of slope or greatly increased sediment loads will produce aggradation and very likely a braided channel, as a result of sediment deposition. Braided channels are the result of high bed-load transport on steep gradients or of deposition. Therefore, a braided channel can be in equilibrium or unstable. The anastomosing pattern (pattern 14 of Figure 5.3) is still an enigma. It may be a relatively steep gradient suspended-load channel analogous to the bed-load and mixed-load braided channels (Figure 5.3), or it may be the suspended-load equivalent of the unstable braided channel that forms where overbank flow produces multiple channels in a valley. In an effort to determine the effects of active tectonics on alluvial channels experimental studies were performed by Ouchi (1983, 1985) and Jin (1984) in a large flume (8.5 m×2.4 m), the center section of which could be raised or lowered by hydraulic jacks. Figure 5.4 summarizes the results for braided and meandering channels during uplift and downwarping. Because the braided channel could not change its pattern, as a result of uplift, it degraded forming terraces. The sediment produced by the incision caused aggradation downstream, and the reduced gradient upstream also caused aggradation. During downwarping the experimental braided channel degraded in the upper steepened reach, and it aggraded downstream. Adjustment was much slower during subsidence because during uplift channel incision is concentrated in a channel, whereas adjustment by aggradation requires deposition not only in the channel but over the valley floor. The aggradation in zones B and C reduced downstream sediment loads and induced degradation in zone D (Figure 5.4). Adjustment of the meandering channel was as expected with increased overbank flooding upstream and an increase of sinuosity on the steeper reaches during uplift. Jin’s (1984) results also show clearly the meandering-channel response to uplift (Figure 5.5). Note that in each case (Figure 5.4) the secondary response to the primary deformation causes tertiary effects in zones A and D both upstream and downstream of the zones of deformation (B and C). Figure 5.4 is presented to illustrate the complexity of the channel response to active tectonics. In each case if the experiment had continued without further deformation there FIGURE 5.4 Effect of uplift and subsidence on braided and meandering experimental channels. Zones of active deformation (B, C) are separated by the axis of deformation. Zone A upstream and Zone D downstream from the zones of active deformation show tertiary effects of valley slope change. The position of the descriptive term indicates generally where the process or channel form is present. Modified from Ouchi (1983). would be additional channel adjustment. For example, in the uplift experiments degradation, which was concentrated at the axis of uplift (Figure 5.4), would have extended upstream to at least the boundary between zones A and B. Alluvial channels are sensitive indicators of change. However, they adjust to changes of hydrology and sediment load as well as to active tectonics. Therefore, it may be difficult to determine the cause of channel change because man’s activities and climatic variations both act to alter discharge and sediment load during historic time. Channel pattern change alone is not sufficient evidence for active tectonics, rather it is one bit of FIGURE 5.5 Meandering channel pattern change, as a result of uplift in center of figure. A shows meandering channel developed after 400-h run time, B shows effect of uplift on this 3.5-m reach of experimental channel after an additional 100 h. Upstream reach shows evidence of overbank flooding, development of multiple channels, and aggradation. Downstream reach shows increase of meander amplitude, wavelength, and sinuosity as well as degradation and a cutoff. Flow is from left to right. Stippled pattern represents sand on floodplain. Vertical-line pattern represents silt and clay deposits on floodplain. From Jin (1984).
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Active Tectonics: Studies in Geophysics evidence that must be supported with other morphologic evidence of aggradation, degradation, or survey data. In many areas the evidence will be circumstantial. Nevertheless, anomalous reaches that are not related to artificial controls or to tributary influences may reasonably be assumed to be the result of active tectonics. EXAMPLES Examples from the Near East, Pakistan, and the lower Mississippi Valley illustrate the impact of active tectonics on the effective utilization of rivers and canals. Near East Where man has built long-lasting structures, active-tectonic effects are recorded by displacement of these features. For example, in the southeastern corner of Iran, the Shaurn anticline forms a range of low hills. Folding began in late Pliocene, and it still continues (Ambraseys, 1978). In the first or second century A.D. two canals were cut across the anticline in order to lead water from a canal system on the northeast flank to the more extensive and fertile plains on the southwest. These channels afford a unique opportunity for measuring the uplift of the anticlines since the canals were built. One canal still carries water, but, where it crosses the anticline it has cut down about 3.5 m below its original bed. The other canal has been abandoned. An accurate survey along its alignment shows that, along the anticlinal axis, the bed of the canal has risen at an average rate of approximately 1 m per century (Lees, 1955). In the Tigrus and Euphrates Valley, where there is active tectonics (Lees, 1955; Adams, 1965; Mirjayar, 1966), canals have also been abandoned. They show reversed gradients and incision. Indus Valley There are numerous active faults in the Indus Valley (Kazmi, 1979). The most spectacular effect of active faulting is due to the Rann of Cutch Fault zone in the lower Indus Valley. In 1819 a severe earthquake resulted in the 6-m uplift of a 16-km-wide and 81-km-long tract of alluvial land. This feature was locally known as Alah Bund (Oldham, 1926), and it blocked an eastern branch of the Indus River. The channel at that time was dry, but flow was re-established during a flood in 1828. Lyell (1857, p. 462) stated that “for several years after the convulsion of 1819, the course of the Indus was very unsettled, and at length, in 1826, the river threw a vast body of water into its eastern arm, forcing its way in a more direct course to the sea, burst through all the artificial barriers that had been thrown across the channel, and at length cut right through the Alah Bund.” For discussion of recent history of the Indus see Holmes (1968), and for an interesting hypothesis concerning the decline of an Indus civilization see Dales (1966), who suggested that one ancient Indus valley city (Mohenjodaro) was flooded as a result of major tectonic activity forming a dam in the Indus Valley. This is possible as a result of valley-floor warping, but not as the result of a major natural alluvial dam (Lambrick, 1967). Finally, the westward shift of the Indus River during the last few thousand years suggests major avulsive changes owing to westward tilting of the Indus River valley (Wilhelmy, 1969). Mississippi Valley Lake County Uplift The great 1811–1812 earthquakes near New Madrid, Missouri, have created considerable concern about the possibility of a recurrence. Therefore, extensive studies have been carried out in this area, and the literature relating to the geophysics and geology of the Mississippi Embayment between Memphis and Cairo is abundant (McKeown and Pakiser, 1982). The area of deformation near New Madrid is referred to as the Lake County Uplift (Figure 5.6). The surface of the uplift is as much as 10 m above the general level of the Mississippi River Valley. The deformed area has a maximum length of about 50 km and a maximum width of about 23 km. Its relief is uneven, and the surface is dominated by two elongated bulges. The Lake County Uplift consists of part of four different geomorphic surfaces, the modern Mississippi meander belt and three separate Mississippi River braided-stream terraces. Lateral migration of the Mississippi River during and following the most active periods of deformation has eroded a considerable amount of the uplifted surface. During great floods of the past, the as yet uneroded portions of the Lake County Uplift existed as islands on the Mississippi River alluvial plain. Russ (1982) cited the following evidence of active deformation of the Lake County Uplift: (1) profiles of the Lake County Uplift reveal that the structure is significantly higher than the natural occurring landforms of the modern meander belt (Figure 5.7A); (2) the longitudinal profiles of abandoned river channels and natural levees have been significantly warped, some to the extent that the original river flow direction has been reversed; (3) the modern floodplain is also warped (Figure 5.7B); and (4) the Reelfoot scarp vertically offsets abandoned Mississippi River channels, which once flowed across the area.
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Active Tectonics: Studies in Geophysics FIGURE 5.6 Map of New Madrid region showing the Lake County Uplift. From Russ (1982). FIGURE 5.7 Longitudinal profiles between Mississippi River miles 845 to 930. Locations are shown on Figures 5.6 and 5.8. A, Natural-levee profile and low-water profiles; B, floodplain profile. From Russ (1982).
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Active Tectonics: Studies in Geophysics An examination of Figure 5.7 reveals that all the profiles have a similar shape, suggesting that they may be the result of the same events. The profiles are convex upward, a configuration that is commonly associated with uplift, and Russ (1982) concluded that this shape may be due to recent and even current deformation. Russ also stated that several aspects of the meander pattern of the Mississippi River suggest control by tectonic processes. Above the uplift axis, between Cairo, Illinois, and Hickman, Kentucky, the river is currently relatively straight. From Hickman south to Arkansas, however, it is sinuous (Figure 5.8). It is possible that the river straightened its course to increase its gradient in an area where tilting is reducing it, whereas downstream the high sinuosity reflects steepening (Figures 5.2 and 5.3). The river is constrained by the topographically high Sikeston Ridge to the north and the Tiptonville Dome to the south. The position of the Mississippi River course within its meander belt also suggests the possibility of tectonic influence. Between Cairo, Illinois, and Hickman, Kentucky, and in general between Blytheville, Arkansas, and Memphis, Tennessee, the river flows along the eastern edge of its meander belt (Figure 5.8). However, between Hickman and Blytheville the river shifts to the west. It is conceivable that the river has been deflected to the west as a result of the uplift. However, old maps FIGURE 5.8 Map of Mississippi Valley between Memphis, Tennessee, and Cairo, Illinois. indicate that the position of the river in 1765 is similar to that of today. Thus, any significant tectonic deflection must have occurred before 1765. The effect of the New Madrid earthquake on the Mississippi River provides an extreme example of the tertiary effects of active tectonics on a major river during a long period of time. For example, Walters and Simons (1984) studied the history of the river, and they summarized as follows: From 1765 to the winter of 1811–1812 the lower Mississippi was a graded river, and there were four neck cutoffs during that period. Beginning on December 16, 1811, and continuing intermittently through February 1812, the New Madrid earthquake shocks caused bank caving, which introduced tremendous quantities of sediment into the channel. The most severe caving occurred in the reach from the confluence of the Ohio and Mississippi Rivers to below Blytheville, Arkansas (Figure 5.8). The increased sediment load caused excessive shoaling, enlargement of islands, and at some locations new islands and point bars were formed. During the years following the earthquake (1818–1874), the sediment began to move gradually downstream. This increase in sediment load caused reduction in meandering, and the number of cutoffs doubled. From 1875 to 1932 the number of neck cutoffs decreased. Above Osceola the introduction of sediment was almost instantaneous, and the response was the formation of a wider aggrading channel. Below Osceola the response was a steepening of the gradient by meander cutoffs and sinuosity reduction. From the examination of gaging-station records and especially specific-gage relations, Walters and Simons (1984) concluded that the lower Mississippi River channel from above New Madrid, Missouri, to Red River Landing, Louisiana, was aggrading after about 1880, perhaps as a result of the sediment introduced by the New Madrid earthquakes. Monroe Uplift The Monroe Uplift, the extent of which is defined by deformed Cretaceous and Tertiary strata, is a dome approximately 120 km in diameter. It is situated mostly in northeastern Louisiana (Figure 5.9), but it extends into southeastern Arkansas and west central Mississippi (Wang, 1952). Its eastern-most extension includes the Mississippi River between Greenville and Vicksburg. Geologic evidence that the Monroe Uplift was active since the Tertiary has been presented by several authors. Veatch (1906) discussed the existence of two active linear structures, which pass through the uplift. He further claimed that recent movement along the west end of the flexure has resulted in the formation of a series of shoals
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Active Tectonics: Studies in Geophysics FIGURE 5.9 Index map showing location of Monroe (M) and Wiggins (W) Uplifts. FIGURE 5.10 Index map of Monroe Uplift. From Burnett and Schumm (1983). on the Sabine and Angelina Rivers and swamping of an area in the Angelina River Valley in eastern Texas. The uplift consists of the Boeuf and Tensas Basins as well as Macon Ridge and the Mississippi River from mile 450 to 530. The basins are composed of large back-swamp areas crossed by several old Arkansas River channels with well-developed natural levees. Macon Ridge in the center of the uplift is composed of five mid-Wisconsin glacial outwash terraces (Saucier and Fleetwood, 1970), which form a narrow, north-south trending, elongated area of high ground. Several streams—Ouachita River, Bayou Bartholomew, Boeuf River, Big-Colewa Creek, Bayou Macon, and Deer Creek (Figure 5.10)—that cross the Monroe Uplift in northeastern Louisiana and generally parallel the Mississippi River were studied by Burnett (1982). They flow generally to the southwest across the uplift, and they locally occupy old abandoned courses of the Arkansas River. The Mississippi River has a highly irregular thalweg profile through the Monroe Uplift (Winkley, 1980). The thalweg slope is significantly reduced or even reversed in part of the uplift zone. At river mile 485, the mean thalweg slope is −0.00004, but it increases downstream to +0.0001. This suggests that deformation is occurring and that it is affecting a major river. Evidence of recent surface movement on the Monroe Uplift is indicated by precise geodetic surveys. The geodetic surveys do not cross the uplift axis, but they suggest uplift of the southern part of the area between 1934 and 1966 (Burnett, 1982). The longitudinal profiles of Pleistocene and Holocene terraces show convexities, which are due to uplift. If the Monroe Uplift is still active today, the modern stream and valley-floor profiles should exhibit the effects of the uplift similar to those shown by terrace profiles. Indeed, valley profiles of the Monroe Uplift streams shows an obvious zone of upward convexity (Figure 5.11). Comparing the amounts of vertical deformation of the terraces and floodplains in the Monroe Uplift area to their ages of formation provides a means of estimating contemporary rates of uplift in this area. By this method, the convexity in the oldest and highest Macon Ridge terrace profile (Qtb1) indicates that about 3.8 m of vertical deformation has occurred at the uplift axis since this terrace was formed (Figure 5.11). Saucier (1970) estimated the age of this terrace to be 33,000 yr old. Therefore, the maximum rate of uplift is estimated to be about 1.0 mm per year during the last 33,000 yr. For the other profiles the rates vary from 0.01 to 1.4 mm/yr. Sinuosity for the five rivers is plotted with reference to
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Active Tectonics: Studies in Geophysics FIGURE 5.11 Longitudinal valley profiles of streams crossing the Monroe Uplift, and Macon Ridge, which is a remnant of a Pleistocene Mississippi River terrace. From Burnett and Schumm (1983). the position of the uplift axis (Figure 5.12). In each case sinuosity increases as the axis is approached or crossed. Where sinuosity is high above the axis, there is a decrease of sinuosity as the axis is approached (Bayou Bartholomew). The results are as suggested by Figures 5.2 and 5.3. To examine the variability of the channel-bed elevation and also changes in the bank height along the streams that cross the Monroe Uplift, channel thalweg elevations were plotted in relation to the valley distance along Boeuf River and Big Colewa Creek (Figure 5.13). These projected channel profiles are not affected by changes in sinuosity because the thalweg elevation (or low water elevation) at a given location is plotted with reference to valley distance rather than channel distance (Burnett, 1982). In Figure 5.13, the difference in the elevation of the projected channel profile and that of the valley profile at a given location represents the depth of the channel below the valley surface at that location. The reaches with large differences in elevation between the valley surface and channel (high banks) are those where the channel has downcut or degraded. Also, where average bank height is small the channel has not degraded, or it has, in fact, aggraded. The projected channel profiles do not parallel the valley profiles, indicating that varying amounts of degradation or aggradation have occurred along the channel. The Boeuf River has apparently com FIGURE 5.12 Variations of sinuosity along six streams crossing the Monroe Uplift. Distance above and below uplift axis is shown. Downstream is to the right. Arrows indicate axis of uplift, and cross hatching shows zones of modern uplift. From Burnett (1982). pensated for the uplift, but the Big Colewa Creek profile contains a major convexity (Figure 5.13). At the axis of the uplift and in the downvalley zone of the uplift, the average bank heights are, in general, high (11 to 13 m for the Boeuf River and 6 m for Big Colewa Creek). These observations indicate that degradation has occurred at and below the uplift axis, but above the axis FIGURE 5.13 Valley and projected-channel profiles of Boeuf River and Big Colewa Creek. From Burnett (1982).
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Active Tectonics: Studies in Geophysics FIGURE 5.14 Effect of Monroe Uplift on Big Colewa Creek. A, Change of channel depth. Three reaches show different degrees of response to uplift. B, Change of channel gradient and slope of valley floor. C, Change of sinuosity. From Burnett and Schumm (1983). FIGURE 5.15 Vertical benchmark movement along a National Geodetic Survey route between Jackson, Mississippi, and New Orleans, Louisiana. From Brown and Oliver (1976) with permission of the American Geophysical Union. degradation has yet to affect the profile of Big Colewa Creek. The Big Colewa Creek channel can be used to summarize the effect of uplift on an alluvial channel. It can be divided into three zones of activity along its valley (Figure 5.14A). The lower zone, from the mouth to valley km 32, has a high average bank height of about 6 m. The middle zone, from valley km 32 to 65, shows a clear upstream decrease in the average bank height from 6 to 2 m. The upper zone, above valley km 65, has a constant low average bank height of 2 m. Degradation may have occurred in the lower zone, whereas entrenchment is still in progress in the middle zone, and in the upper zone entrenchment has not yet occurred. Figure 5.14B shows changes in valley slope and channel thalweg slope with valley distance. The valley slope remains high from the mouth to valley km 40, and then it suddenly decreases. The break in slope, at valley km 40, defines the apparent location of the uplift axis. The thalweg slope is high from the mouth to valley km 55, and then it also suddenly decreases. The fact that the two curves do not coincide suggests that the channel has incised through the axis of uplift. Sinuosity is approximately 1.2 in the upstream stretch of Big Colewa Creek (Figure 5.14C). Downstream between valley km 50 and 55, sinuosity increases to about 1.7, and then it gradually decreases to 1.5 at the mouth. In summary, numerous relations between the morphology of the streams and terraces and the underlying Cretaceous and Tertiary structures of the Monroe Uplift indicate that the area is still active tectonically. The patterns of changes and the present stream morphology provide information on the response of streams to active uplift within their valleys. Wiggins Uplift In contrast to the Monroe Uplift, active displacement of the Wiggins Uplift (Figure 5.9) is clearly displayed by geodetic surveys (Figure 5.15). Bogue Homo Creek is analogous to Big Colewa Creek in this area, and it displays similar morphologic differences. Above the axis of uplift the channel is anastomosing, and the main channel is relatively straight. Immediately below the axis the channel has incised below the former floodplain to form a low terrace, and sinuosity is higher. Numerous cutoffs have occurred, and locally braided reaches have developed as a result of increased sediment loads resulting from incision. A gaging station on Tallahala Creek provides evidence of channel incision at an average rate of 12 mm/yr since 1940 (Figure 5.16). This is three times the rate of measured uplift; however, a channel probably adjusts episodically to continuous uplift (Ouchi, 1983). A factor that makes comparison of channel behavior
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Active Tectonics: Studies in Geophysics FIGURE 5.16 Specific gage plot for Tallahala River near Runnels-town, Mississippi. The water-surface elevation was determined for a specific discharge of 78 cfs for each year of record. This discharge is base flow and reflects change of bed elevation. From Burnett and Schumm (1983). difficult is the size or the energy of the channel. Small channels such as Big Colewa Creek and Bogue Homo Creek have been unable to keep pace and incise across the uplift axis. Streams of intermediate size such as Tallahala Creek have incised across the axis, but their long profile still shows a convexity. Large rivers such as the Pearl River (19,900 km3) have been able to keep pace with the uplift, and its projected-channel profile is relatively straight, although terraces and the valley floor are deformed (Figure 5.17). Above the axis of uplift the Pearl River is not anastomosing, but it has developed a new floodplain below the axis of uplift, and the former floodplain is a low terrace. Changes of channel morphology can frequently be attributed to tributary contributions of water discharge and sediment load, but on the Monroe and Wiggins Uplifts, the patterns of channel change are related to active FIGURE 5.17 Longitudinal profiles of terraces and floodplain and a projected-channel profile of the Pearl River, a major river crossing the Wiggins Uplift. A projected-channel profile is a plot of channel-bed elevation against valley distance, which eliminates the effect of sinuosity of the channel long profile. From Burnett and Schumm (1983). tectonic deformation of the alluvial valley of streams crossing the uplifts (Burnett, 1982). Mississippi River In the preceding discussion of the Mississippi River Valley, three areas of active tectonics have been identified, the Lake County Uplift, the Monroe Uplift, and the Wiggins Uplift. Russ (1982) demonstrated an effect of the Lake County Uplift on the Mississippi River gradient and pattern. Maps (Fisk, 1944) of the old Mississippi River meander courses show that the Mississippi River has maintained very high sinuosity values in this area during the last 2000 to 6000 yr (Figure 5.18). Winkley (1980) discussed the possible effects of the Monroe Uplift on the past and present morphology of the Mississippi River in the vicinity of Greenville Bridge (mile 531.3), where the Monroe Uplift has exposed Tertiary bedrock in the Mississippi River channel near Greenville. Meander loops have grown and cut off at the same location several times, and the sinuosity has been consistently high near Greenville (Figure 5.18). In this sinuous zone, the Mississippi River has shifted laterally across the Yazoo Clay, being unable to cut through it. Watson et al. (1984) suggested that the Wiggins Uplift has increased sinuosity farther south near Natchez (Figure 5.18). As the average slope of the Mississippi River low-water surface is about 56 mm per river kilometer, with even slow uplift rates of from 3 to 5 mm/yr the effects on this great river can be significant. Studies are under way to determine the extent of the effects of active tectonics on this major commercial artery. Obviously if there is a significant influence on the river, it will have implications for navigation, flood control, and the taxpayer. SIGNIFICANCE Some alluvial rivers are currently adjusting to active tectonics, and this must be considered as an additional explanation for river instability. Furthermore, geomorphic studies provide independent support for the results of the National Geodetic Survey resurveys because river response to uplift conforms to that expected from field and experimental studies of river morphology elsewhere. As noted above, active tectonics can be responsible for aggradation, degradation, channel avulsion, and pattern change, both downstream and upstream of the deformed reach. Therefore, the net result of active tectonics is unstable river reaches that are characterized by incision, deposition, bank erosion, meander cutoffs, or the development of meandering, braided, or anastomosing patterns (Figure 5.4).
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Active Tectonics: Studies in Geophysics FIGURE 5.18 Variations of Mississippi River sinuosity below Cairo, Illinois, as determined from maps prepared in 1764, 1820–1830, 1881–1893, and 1930–1932. Note consistently high values of sinuosity in reaches affected by Lake County Uplift (Cairo, Illinois-Caruthersville, Missouri); Monroe Uplift (Greenville, Mississippi); and Wiggins Uplift (?) below Natchez, Mississippi. From Schumm et al. (1982). Navigation can be affected by aggradation and the development of bars. Bank erosion, incision, and pattern change can impact riparian use and cause loss of valuable structures (bridges, loading docks) as well as agricultural land and homes. The frequency of overbank flooding will be increased in reaches of aggradation and reduced gradient. Changes in the frequency of overbank flooding through a zone of active tectonics will change the position of “ordinary high water,” which is usually a legal boundary, and this may lead to confusion and litigation concerning the location of the river bank. If rivers are affected by active tectonics then obviously canals will be. Canals are usually constructed to carry relatively clear water on gentle slopes. As in the Middle East, a slight warping can seriously affect the efficiency of the canal (Leary et al., 1981). The evidence of river response to active tectonics can be used to evaluate the tectonic stability of hazardous waste-disposal sites. For example, in addition to the criteria presented earlier, studies of salt domes in eastern Texas suggest that the Oakwood Salt Dome may be active because channels on the central dome are incised up to 4 m, and there are three abandoned channel reaches that suggest lateral movement of the channels away from the dome (Collins et al., 1981). Furthermore, as in the past, fluvial evidence can be used to identify those areas most favorable for exploration for gas and oil. The geomorphic techniques can be used to aid in planning geophysical surveys and the selection of drilling sites. Active tectonics in some areas has had a disastrous effect. Earthquakes in the upper Indus Basin have triggered massive landslides that impounded vast amounts of water that eventually overtopped the natural dam and caused catastrophic flooding downstream. In addition, valley-floor deformation could lead to major avulsive shifts of a river. The effects discussed herein are primarily related to aseismic deformation, but it is this slow deformation that has been ignored when river behavior is studied. Clearly more detailed studies of the effects of active tectonics on alluvial rivers are needed to establish the within-channel hydraulic changes that can be expected and the engineering response that is required to mitigate the detrimental effects on river stability and use. ACKNOWLEDGMENTS The field work was carried out by Adam Burnett in Louisiana and Mississippi and was performed under research grants from the National Science Foundation (EAR-7727573) and the U.S. Army Corps of Engineers, Potamology Division, Vicksburg. I thank Larry Lattman and David Russ for their helpful reviews. REFERENCES Adams, J. (1980). Active tilting of the United States midcontinent: Geodetic and geomorphic evidence, Geology 8, 442–446. Adams, R.M. (1965). Land Behind Baghdad, University of Chicago Press, Chicago, Ill., 187 pp.
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Representative terms from entire chapter: