Response of Alluvial Rivers to Slow Active Tectonic Movement
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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 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 RESPONSE OF RIVERS TO TECTONIC MOVEMENT 505 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.