The coastal transition at the mouth of a small mountainous river in Taiwan

The coastal transition at the mouth of a small mountainousriver in Taiwan

J . T. LIU, P. B. YUAN and J.-J . HUNGInstitute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan 804-24(E-mail: [email protected])

ABSTRACT

The Tseng-wen River is a small mountainous river in southern Taiwan that has

distinct dry and ¯ood seasons. Several lines of evidence have revealed that the

depositional system at the mouth of the river has transformed from a deltaic system to

an estuarine system. The evidence includes long to short-term shoreline changes,

geomorphology of the river mouth, mixed sediment sources inside the river mouth,

mixing and suspended sediment characteristics inside the river mouth, and the grain-

size distribution patterns on the river bed. Unlike the long-term evolution of many

other coastal systems, the transformation at the Tseng-wen River mouth is largely

caused by the building of a reservoir in the middle reaches of the river. The present

study provides an example that coastal environments can be in¯uenced by human

activities farther inland on a time scale much faster than what would occur naturally.

INTRODUCTION

Worldwide the major sediment source for mostcoastal depositional systems is ¯uvial. Factorsaffecting ¯uvial sediment discharges includeclimate, precipitation, river discharge, basin ge-ology, the size of the basin, and human impact(Milliman & Syvitski, 1992). Among major riversin the world, the greatest suspended sedimentdischarges by far, are associated with Asian rivers(Wright, 1985). Yet Milliman & Syvitski (1992)also point out the importance of rivers on high-standing islands such as Taiwan, to the globalsediment ¯ux to the oceans. A lot of these islandsare located on active tectonic belts, the complexinterplay between tectonic processes like uplift-ing, land subsidence, and riverine sedimentsupply, all in¯uence the development of theircoastlines. It is, therefore, important to under-stand how a particular coastal dispersal systemfunctions in this kind of setting.

Nowadays sediment loads for most rivers in theworld do not re¯ect their natural state, but rather,represent the results of human activities (Milli-man & Syvitski, 1992). Some of these activitiessuch as damming and diversion have dramatical-ly reduced the amount of sediment discharged by

rivers. The Nile is one of the better knownexamples. The erosion on the Nile Delta shore-lines increased greatly since the construction ofthe Aswan High Dam in 1964 (Fanos, 1995).Damming not only reduced the sediment load ofthe river, it also altered the content of the load(Palanques et al., 1990; Guillen & Jimenez, 1995).In the case of the Ebro River in Spain, dammingand building reservoirs not only greatly reducedthe sediment load, it also reduced the sandfraction in the sediments discharged by the river,causing intense reshaping of the nearshore deltaicarea (Guillen & Jimenez, 1995), and coarseningbeach sediments in the Ebro Delta (Guillen &Palanques, 1996).

Where a river discharges into the sea, two majorcategories of coastal depositional systems canoccur. An estuary, following the de®nition givenby Dalrymple et al. (1992), is formed if theseaward portion of the drowned river valley isbeing ®lled with both ¯uvial and marine sedi-ments (mixed source) (Dalrymple et al., 1992;Carter & Woodroffe, 1994; Perillo, 1995a). A riverdelta is formed when an estuary is ®lled, resultingin a prograding shoreline (¯uvial source domi-nates) near the river mouth (Dalrymple et al.,1992; Carter & Woodroffe, 1994).

Sedimentology (1998) 45, 803±816

Ó 1998 International Association of Sedimentologists 803

However, from the perspective of coastal evo-lution, estuaries and deltas occupy the oppositeends of a three-dimensional prism in an evolu-tionary classi®cation scheme of coastal environ-ments proposed by Dalrymple et al. (1992). Thelong axis of this prism represents relative timewith reference to changes of relative sea-level andsediment supply (Dalrymple et al., 1992; ®g. 2).The corners of the triangular cross-section of theprism represent the three dominant processes of¯uvial sediment input, wave energy, and tidalregime that control the morphology of river deltas(Galloway, 1975). However, in time, a coastal areawill slide back and forth through the prism at arate and by an amount, determined by the rate ofsea-level change, the sedimentation rate, and thebasin size (Dalrymple et al., 1992). In otherwords, the coastal evolution trend is reversible.Transgression will result in the development ofestuaries, and on the other hand, deltas willemerge due to progradation.

A tripartite zonation model for the facies of anideal estuary is also proposed by Dalrymple et al.(1992). The outer zone is dominated by marineprocesses in which coarse bed-load sedimentsmove up the estuary. The central zone is a low-energy area containing ®nest-grained bed-loadsediments in the estuary. The inner, river-domi-nated zone shows seaward movement of bed-loadsediments.

Sedimentation processes in an estuary (refer-ring to Pritchard's (1967) de®nition) are largelydetermined by the estuarine circulation, which isthe result of the mixing characteristics due to therelative strength of the river discharge and tide(Nichols & Biggs, 1985). Three basic types ofestuaries have been recognized on the basis ofmixing characteristics. They are salt wedge,partially mixed, and fully mixed estuaries (Prit-chard, 1955). A turbidity maximum exists inmany estuaries in the world, particularly partiallyin mixed estuaries despite of their sizes andshapes (Nichols & Biggs, 1985). It can be formedby three major different mechanisms, which aredescribed by three simple models. In the ®rstmodel, the turbidity maximum is associated withentrapment of suspended sediments of both¯uvial and marine origins by the estuarinecirculation (Nichols, 1977). The location of theturbidity maximum in this case, is near the tip ofthe salt wedge intrusion, a site of rapid shoaling,which corresponds to the null zone (Nichols &Biggs, 1985; Kostaschuk et al., 1992). In thesecond model, the turbidity maximum is locatedseaward of the limit of salt intrusion, which is

attributed to intertidal input and tidal scour(Buller et al., 1975). In the third model, theturbidity maximum is situated landward of thesalt intrusion at the head of tides due to tidaltrapping (Allen et al., 1980). In addition to thetidal trapping, long-term sedimentation can alsobe related to the co-oscillation of tides in amacrotidal estuary (Castaing, 1989).

Most existing models for estuarine sedimenta-tion are based on studies in large coastal plainestuarine systems in temperate latitudes. Al-though there have been reports on sedimentationprocesses of small rivers (Sondi et al., 1994;Sondi et al., 1995), and subtropical rivers havingsteep hinterland, high sediment yields, andseasonally variable discharge (Cooper, 1993,1994), rivers in these categories are under repre-sented in the literature. The estuaries of smallrivers are especially unique environments. Theyamong other things, provide insights into thekinetics of land/sea interactions (Sondi et al.,1995). In this paper, we present some evidence ofthe long-term coastal geomorphological trend andshort-term sedimentation processes at the mouthof a small mountainous river.

STUDY AREA

The most important sediment source for thecoastal plains along the west coast of Taiwancomes from the ¯uvial sediment of many riversoriginating in the Cental Mountain Range and thewestern Foothills of Taiwan whose peak eleva-tion exceeds 3000 m. Subsequently, the westcoast of Taiwan is often referred to as alluvial(Wang & Aubrey, 1987), and most rivers inTaiwan can be classi®ed as mountainous riversaccording to Milliman & Syvitski (1992).

The Tseng-wen River is the second largest riverin southern Taiwan in terms of drainage area(1176 km2), estimated mean annual runoff(2á36 ´ 109 m3), and estimated sediment dis-charge (31á13 ´ 106 m3) (Water Resources Plan-ning Commission, 1992). It is �180 km betweenthe headwater and the mouth of the river, and theelevation difference is 2524 m. The average an-nual rainfall for the drainage basin is 2643 mm(Water Resources Planning Commission, 1992).Because of the in¯uence of the monsoon climate,the river ¯ow shows distinctive dry and ¯oodseasons. Approximately 84% of the annual riverdischarge takes place between June and Septem-ber. On average, the discharge in August is twoorders of magnitude greater than that of each of

804 J. T. Liu et al.

Ó 1998 International Association of Sedimentologists, Sedimentology, 45, 803±816

the four months in the dry season betweenNovember and February (Fig. 1). This great dis-parity between ¯ood and dry seasons implies thatsediment out¯ux from the Tseng-wen River isalso concentrated during the ¯ood season. Astudy of sur®cial grain-size distribution patternson the sea ¯oor off the mouth of the Tseng-wenRiver indicates that sediment discharged by theriver is one of the most important sedimentsources for the nearshore region around the rivermouth (Liu et al., personal communication).

The cuspate shoreline geomorphology near themouth of the river (Fig. 2a) puts its delta near thewave-dominated end-member based on the trian-gular delta classi®cation scheme originally pro-posed by Galloway (1975), and on another similarclassi®cation scheme proposed by Davis (1985).Immediately north of the river mouth, the shore-line orientation turns sharply from NW±SE trend-ing to NNE±SSW trending (Fig. 2a). A multi-inletlagoon/barrier system exists to the north of theshoreline re¯ection point. The shorelines bothnorth and south of the mouth of the Tseng-wenRiver consist of dissipative sandy beaches withand without well developed dune ®elds.

The drainage basin of the Tseng-wen River iscomposed of sedimentary rocks ranging in agefrom Late Miocene to Pleistocene (Fig. 2b). Therocks were deposited in upper slopes to shallowmarine environments and were folded during aPliocene orogeny. The major rock types are

mudstone and sandstone, with local conglomer-ates. The Tseng-wen River is one of the few riversthat do not extend far into the metamorphic belt inthe east. The lack of metamorphic rock fragmentsin river bed sediments distinguishes the Tseng-wen River from those ¯owing through the meta-morphic terrain. In recent years, two intertidal tosubtidal spits (bars) have developed across theriver mouth as seen in a most recent satelliteimage (Fig. 3). These two spits resemble whatDalrymple et al. (1992) called river mouth bars fortypical wave-dominated estuaries. Therefore, oneof the objectives of this study is to determine thetransitional nature (delta to estuary) of the presentdepositional system of the river mouth of theTseng-wen River. Tides off the river mouth areaare mixed, with semidiurnal tides dominating.The tidal rage varies between 0á5 and 1á5 m. Themean tidal range is less than 1 metre. A recentobservation of tidal currents at about 1 km outsidethe river mouth (Liu et al., 1998) shows that theyare essentially alongshore, having a slightly on-shore tendency during the ¯ood (northward cur-rent) and offshore tendency during the ebb(Fig. 4). Flood currents dominate over ebb cur-rents in terms of magnitude and duration. Thewave ®eld in the study area is primarily dominat-ed by monsoon winds. The mean wave height isless than 60 cm. The combined in¯uence of theprevailing wave and tidal energy puts the mo-rphodynamic distinction of the study area in themixed energy but wave-dominated category(Fig. 5), according to the classi®cation schemeproposed by Nummedal & Fisher (1978) andHayes (1979) for mixed energy coasts worldwide.The coastal deposition of ¯uvial sediments dis-charged by the river has been the major factor forthe coastal development in the area (Huang & Liu,1995). In the past three hundred years, accompa-nying the southward shifting of the river mouth,sediments discharged from the river gradually®lled a large lagoon called Tai-jian Bay (periodbetween 1723 and 1735, Fig. 6). Yet, barrierislands remained to be the major coastal charac-teristics north of the river mouth (Fig. 6). With thegrowth of the subaerial portion of the river delta,the shoreline at the river mouth continued toprograde, resulting in the present orientation. Thedelta growth, lagoon in-®lling, and river mouthmigration is further illustrated by shoreline chan-ges between 1926 and 1973 (Fig. 7). However, theaccretionary trend of shoreline changes began toreverse since the completion of a reservoir in theupper reaches of the river in 1973 (Fig. 8). The1976 and 1984 shorelines near the river mouth

Fig. 1. The monthly discharge of the Tseng-wen Riverbased on a 10-year average (from Water ResourcesPlanning Commission, 1992).

Coastal transition 805


Fig. 2. (a) The coastline morphology of the study area. The shoreline was ®rst electronically traced from a geo-referenced SPOT satellite image taken onAugust 21 1993 and then digitized. (b) The geological map for the upper and middle reaches of the Tseng-wen River drainage basin and its two majortributaries (modi®ed from Lin, 1991). The insert shows where the study area is located on the island of Taiwan.

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indicate landward migration of a barrier islandnorth of the river mouth. By 1990, the barrier hascompletely welded onto the shore and disap-peared (Fig. 8). Since then, the shoreline aroundthe river mouth has been shifting landward. In theprocess, the geomorphology of the river mouth isalso gradually losing the distinct cuspate shape.The shoreline retreat at the river mouth is easilyrelated to the reduced ¯uvial sediment supply.Since 1974, over 80 000 m3 of bed-load sedimentshave accumulated in the reservoir (Fig. 9). Also,Liu et al. (personal communication) recentlyfound that during the ¯ood season, over 60% ofthe bed-load ¯uvial sediments are mud, which donot contribute to the building of the coast.Therefore, in this study, we will characterize thedeposition processes in the river mouth in the

light of long and intermediate-term coastal chan-ges observed in the past.

METHODS

Seasonal sampling of estuarine waterand analysis

Estuarine water samples were collected up-stream from the river mouth on board a small

Fig. 3. A Landsat satellite image of the river moutharea taken on June 27 1996. The arrows point to tworiver mouth bars.

Fig. 4. Stick diagram for the observed currents 1 km off the river mouth. Northward direction is toward the top of the®gure.

Fig. 5. Classi®cation of coastal morphology based onthe relative energy of waves and tides. The full squarerepresents the study area (modi®ed from Nummedal &Fisher (1978) and Hayes (1979)).



boat during the dry season (January) and ¯oodseason (June) of 1995. At each sampling station,the water samples were collected sequentially atdesired depths by using a home-made samplingsystem consisting of a manual peristaltic pumpand a silicon tube. The salinity of each samplewas measured in situ with a portable salinometer(Hydro-Bios), and later was precisely determinedwith an Autosal (Guildline 8400B) in the labo-ratory. Four litres of each water sample wasstored in a polyethylene (PE) bottle and broughtback for the determination of the suspendedparticulate matter (SPM) concentration. In thelaboratory, each water sample was ®lteredthrough a preweighed membrane ®lter (Nucleo-pore PC, 142 mm) driven by a peristaltic pump.After ®ltration, the residue on the ®lter waswashed with deionized distilled water to remove

sea salt. The washed ®lter was dried in an ovenat 60°C and then re-weighed with an electronicbalance (Mettler AT20) to determine the concen-tration.

Basin-wide sampling of river bed sedimentsand analysis

Sediment deposits on the river bed along theupper reaches, middle reaches, lower reaches,around the river mouth, and along the entirelength of two tributaries were taken (Fig. 10).Samples from the lower reaches, the river mouth,and offshore were taken during the ¯ood seasonin 1995. Other river bed samples were takenduring the course of two years in 1994 and 1995.The mineralogical and rock fragment composi-tions of these samples were analysed by using a

Fig. 6. Schematic maps showinglong-term shoreline changes in thestudy area since 1723. Some ofthese maps are based on historicalmaps of questionable accuracy.Consequently, these maps can onlybe used as indicators for the pastcoastal evolutionary trends (modi-®ed from Shih, 1979).



polarizing microscope on sediment thin sections.For appropriate comparison, sediments of thesame size range (1 F to 2 F or 0á50±0á25 mm) wereseparated from each sample for the making of thinsections. For each thin section, 300 points werecounted in order to obtain the percentage of thefollowing items: quartz, chert, orthoclase, plagio-clase, fossil shell fragments, and fragments ofsandstone, siltstone, silty claystone, claystone,slate, and volcanic rocks.

Because spatial grain-size distribution patternshave been shown to be effective tracers to indicate

long-term sediment transport pathways and sed-iment sources (Liu & Zarillo, 1990; Liu & Hou1997; Liu et al., personal communication), sam-ples from the lower reaches, the river mouth, andoffshore, were also analysed for grain-size fre-quency distributions using a rapid sedimentanalyser and recorded at quarter phi intervals, amethod similar to the one used by Liu & Hou(1997). We need to point out that in the sampleprocessing procedure, all the ¯occulations werebroken. Consequently, the grain-size analysisresults do not include the effect of ¯occulation.

Fig. 7. Past coastal changes in the vicinity of the mouth of the Tseng-wen River. The arrows indicate the speculatedorientation of the river channel at the mouth and the dotted lines indicate the outlines of intertidal and subtidalshoals (modi®ed from Shih, 1979).

Fig. 8. Recent shoreline changesaround the mouth of the Tseng-wenRiver. The 1993 shoreline is thesame as in Fig. 2(a). The 1996shoreline was obtained from aLandsat satellite image using thesame method as for Fig. 2(a). Rest ofthe shorelines were digitized from1:5000 aerial photographic mapspublished by the Ministry ofInterior.



RESULTS

Salinity and suspended particulate matter(SPM) concentration pro®les

The salinity and SPM data points from bothseasons were contoured by hand along the riverchannel starting landward at the river mouth to

render two-dimensional presentations of the lon-gitudinal distributions. It is clear that during thedry season, there is salt intrusion up the river to atleast 22 km landward from the river mouth(Fig. 11a). However, the null zone probably islocated around 15 km from the mouth as indicat-ed by the more strati®ed 13 p.s.u. isohaline(Fig. 11a). Seaward of this isohaline, the water

Fig. 9. Sediment accumulation inthe Tseng-wen Reservoir since 1974(data obtained from the Tseng-wenReservoir Management Bureau).

Fig. 10. Locations of river bed samples in the entire watershed of the Tseng-wen River.



column is vertically mixed. Landward from thislocation, the water column tends to becomestrati®ed farther upstream. Judging by the geom-etries of all isohalines, the mixing characteristicsof the river during the dry season represent apartially mixed estuary. A turbidity maximum isalso clearly present corresponding to the nullzone (Fig. 11b). Comparison of SPM concentra-tions on both sides of the turbidity maximumshows that SPM concentrations are higherupstream than downstream of the turbidity maxi-mum. The horizontal concentration gradient in-creases towards the turbidity maximum fromupstream, and decreases away from it towardsthe river mouth. This phenomenon suggests thatmost of the land-derived suspended sedimentsare trapped by the estuarine circulation in theturbidity maximum, only small amount escape tothe sea.

During the ¯ood season, seawater is pusheddownstream and con®ned to the lower part of thewater column (Fig. 12a). Consequently, the upper

1 m of the water column is vertically strati®ed yetshowing seaward increase of salinity gradient(Fig. 12a). The characteristics of the isohalinessuggest a salt-wedge estuary. However, the land-ward tip of the salt wedge is landward beyond thesurveyed area. The longitudinal SPM distribu-tions do not indicate the existence of a turbiditymaximum (Fig. 12b). In contrast to the dry sea-son, the SPM gradient ®rst decreases downstreamto a minimum of 37 mg/L and then increasesagain towards the river mouth. The SPM concen-tration increases by one order of magnitude nearthe mouth having a mid-depth maximum. Theshapes of SPM isolines suggest that the out¯ux ofsuspended ¯uvial sediments are carried by theout-going lighter water in the upper layer. Therapid increase of SPM concentrations near theriver mouth could be caused by a number ofreasons, which include the in¯ux of marinesediments, resuspension of bottom sediments bystrong currents and waves, and trapping of ¯uvialsediments by estuarine circulation.

Fig. 11. Longitudinal transects of (a) salinity and (b)suspended particulate matter (SPM) from the rivermouth landward during the dry season.

Fig. 12. Longitudinal transects of (a) salinity and (b)suspended particulate matter (SPM) from the rivermouth landward during the ¯ood season.



River-wide variations of sediment composition

Sediment composition analysis of river bed sed-iment samples resolved 11 constituents includingminerals and rock fragments. As quartz is rela-tively more resistant to mechanical abrasion andchemical weathering during downstream trans-port than other constituents, its absolute abun-dance tends to decrease more slowly than miner-als and rock fragments. When the abundance is ina relative sense (counting 300 points), the abun-dance of quartz actually increases downstream(Fig. 13). Consequently, the abundance of eachconstituent is plotted against that of the quartzwithin each sample. This is graphically analo-gous to the normalization by the quartz contentwhich represents the downstream transport pro-cess. Subsequently, if one ignores the existence ofthe two tributaries, meanwhile assuming homo-geneous river bed geology and all sediments onthe river bed come from the upstream, the idealpoint spread of the above normalization plotwould show data points from the upper reachesoccupying the top left side of the spread, thosefrom the lower reaches and river mouth occupy-ing the bottom right side of spread, and thosefrom the middle reaches falling in between.Conversely, if the normalized abundance valuesof a constituent downstream have higher valuesthan those upstream, it is very likely that thisconstituent is also introduced into the ¯uvialsystem from somewhere else. However, in reality,things are not clear cut. The existence of the twotributaries and the heterogeneous river basingeology will most de®nitely make the picturecomplicated. Also, constituents on downstreambeds might represent lag deposits from theupstream. Nevertheless, the sediment composi-tion plots still reveal valuable information. InFig. 14, different symbols are designated to dif-ferentiate geographic sections along the river

system. The geographic comparisons have yield-ed the following: (1) Distribution patterns of someconstituents show typical trends of downstreamtransport attrition from the upper reaches downto the river mouth. These include sandstonefragments and silty claystone fragments (margi-nal). (2) For volcanic rock fragments and slatefragments, their presence only in the lowerreaches and the river mouth are most likelyattributable to a marine origin since they showlittle presence in the drainage basin. (3) Becauseof folding in the drainage basin, the two tributar-ies and the middle reaches of the Tseng-wen ¯ow

Fig. 14. Plots of the abundance of sediment constitu-ents (vertical axes) against that of quartz (horizontalaxes) of the same sample. Different symbols are desig-nated to different geographic sections of the river andtwo tributaries.

Fig. 13. The abundance of quartz in various segmentsof the Tseng-wen River basin. It is absent in the upperreaches of the river.



through the same petrographic belts, such as theChutouchi Shale (Fig. 2b). This complicates theprovenance of many constituents. For example,orthoclase and chert only appear substantially inthe two tributaries, the lower reaches, and theriver mouth. By and large, the sediment compo-sition study has shown that river mouth sedi-ments not only come from the ¯uvial system, butalso come from the marine environment.

Variations of grain-size compositionin the lower reaches and river mouth

For each sample from the lower reaches, the rivermouth, and the offshore, 16 grain-size classeswithin the sand fraction plus the content of mudwere resolved from the grain-size analysis. Thegrain-size frequency distribution of each sampleis plotted, and all the plots are stacked up to forman array with the offshore sample on the bottom,and the upstream-most sample on the top(Fig. 15). At the ®rst glance, all the samples canbe readily distinguished into two groups. There isa sharp contrast between those which have high(over 40%) mud content, and those which do not(less than 10%). These two groups are alsoseparated geographically. Samples with low

mud content are all (except for TWRI-8, whichwill be discussed later) located in the immediatevicinity of the river mouth and offshore (Fig. 10).Samples having high mud content are all locat-ed upstream from the ones having low mudcontent.

One can easily designate the low mud contentsample as being inside the marine dominatedzone of a wave-dominated estuary, which ischaracterized by higher total energy level (waves,tidal and river currents) (Dalrymple et al., 1992).Because of the agitation of wave orbital motions,the mud is winnowed out of the river bed, leavingthe sediment deposits with low mud content. Onthe other hand, the high mud content is anindication of the river dominance, since most®ne-grained materials are of terrestrial origin inthis particular ¯uvial/marine system (Liu et al.,personal communication). Therefore, the boun-dary between the zone of marine dominance andriver dominance on the river bed is locatedapproximately between 0á5 and 0á7 km from theriver mouth (Fig. 15). The mean grain size of thesand fraction of each sample is also marked by aninverse triangle on the horizontal axis of each plot(Fig. 15). We suspect most longitudinal variationsin the geometry of grain-size curves and the mean

Fig. 15. Grain-size compositionplots of sediment samples takenseaward of the river mouth(TWN9±3), inside the river mouth(TWR-1, 2, 3, 4, 5, 6, and 8), and inthe lower reaches of the river(TWRI-4, 5, 6, and 8). The plots arearranged in a fashion according tothe longitudinal locations of thesamples (see Fig. 10) such that theoffshore sample (TWN9±3) isplaced on the bottom of the arrayand the landward-most sample(TWRI-4) is placed on the top ofthe array. The number in the pa-renthesis next to the sample num-ber is an estimate of the distance(positive landward) from the rivermouth in kilometres.



are caused by geographic variations of all thesampling sites except for TWRI-8. This particularsample has coarser sand fraction and much lessmud content than samples immediately up anddownstream. We speculate that this sample rep-resents the tidally-averaged location of the tur-bidity maximum during the ¯ood season.

DISCUSSION

From long-term coastal trends (Fig. 6) and theriver mouth morphological changes on an inter-mediate time scale (Fig. 8), one can deduce thatup until the mid-1970s, the ¯uvial/marine systemat the mouth of the Tseng-wen River displayedthe behaviour of a prograding river delta. Sincethe mid-1970s, shoreline changes around theriver mouth (Fig. 8) began an erosional trend incoastal evolution. This possibly means that theTseng-wen River mouth has ceased to become adeltaic system based on the progradation criterionfor the de®nition of a river delta (Dalrymple et al.,1992)., Dalrymple et al. (1992) also argued that anet long-term landward movement of sedimentsoutside the estuary mouth is one of the primaryfeatures that distinguish estuaries from deltas. Inour study, from the evidence of sediment com-positions, we found that volcanic and metamor-phic rock fragments in the river bed sedimentdeposits inside the river mouth did not originatefrom the ¯uvial system. This leaves only onepossibility that they come from marine sources.Thus the mixed sediment source criterion is alsomet for the de®nition of an estuary.

Therefore, we assert that a coastal transition hastaken place at the Tseng-wen River mouth from ariver delta to an estuary. This is illustrated on thetriangular classi®cation scheme for coastal envi-ronments proposed by Dalrymple et al. (1992)(Fig. 16). The transitional change that we proposeherein, is different from the coastal evolutionconcept illustrated by the triangular prism inDalrymple et al. (1992). First of all, the time scaleinvolved in the change of the Tseng-wen Rivermouth (decades) is much shorter than the coastalevolutionary model requires (centuries). Second-ly, the mechanisms are different. Long-termsedimentation rate associated with the eustaticsea-level change is the primary cause for coastalevolutionary changes. A common scenario hasoften been discussed in the context of coastalevolution is the ephemeral nature of estuaries(Perillo, 1995b). On many occasions when estu-aries became ®lled, they evolved into deltas.

However, in our case, the direction of change isthe opposite; from delta to estuary. The time scaleof change is much too short to include the effectof sea-level change. Therefore, the cause ofchange is largely anthropogenic, as the result ofretention of ¯uvial sediments by the reservoir.

From the dynamic point of view, the sedimen-tation processes inside the Tseng-wen Rivermouth are controlled by estuarine circulation inboth dry and ¯ood seasons. Because of thepresence of the two layered ¯ow, we suspect theimport of marine sediments takes place in boththe low and dry seasons as indicated by the saltintrusion. Yet the sediment trapping ef®ciency ishigher during the dry season when the estuary ispartially mixed (Schubel & Carter, 1984). Thediscrepancy between the spatial scales of thehydrographic data and the grain-size data regard-ing the location of the turbidity maximum duringthe ¯ood season needs explanation. Because thegrain-size distribution patterns are indications fortime-averaged sediment transport process (Liu &Zarillo, 1990; Liu & Hou, 1997), our data probablyrepresent the average scenario of the ¯ood season.The hydrographic data, on the other hand, re¯ectmore on the time scale of events, and the spatialscales do not necessarily coincide. Furthermore,the estuary to which the hydrographic data referis not the estuary from which the grain-size dataare taken. This is due to different perspectivesfrom which the estuary is de®ned (Perolli, 1995b).Nevertheless, in the present study, the mouth of

Fig. 16. A schematic presentation of the identity changeof the Tseng-wen River mouth from a deltaic system to anestuarine system based on the coastal classi®cationscheme proposed by Dalrymple et al. (1992).



the Tseng-wen River is an estuary no matterwhich de®nition for estuary is applied.

CONCLUSION

Several lines of evidence have pointed to thesame determination that the depositional systemof the Tseng-wen River has transformed from adeltaic system to an estuarine system. The evi-dence include shoreline changes, geomorphologyof the river mouth, mixed sediment sourcesinside the river mouth, mixing and suspendedsediment characteristics, and the grain-size dis-tribution patterns on the river bed. Unlike thelong-term evolution of many other coastal sys-tems, the transformation at the Tseng-wen Riveris largely caused by the building of a reservoir.Therefore, the present study also provides anexample that coastal environments can be in¯u-enced by human activities farther inland on atime scale much faster than what would occurnaturally.

ACKNOWLEDGMENTS

The funding for this study was provided by theNational Science Council under the grant num-bers NSC 84-2621-M-110-001 and NSC 85-2621-P-110-006 to J. T. Liu; NSC 84-2621-M-110-010 andNSC 85-2621-P-110-001 to P. B. Yuan; and NSC84-2621-M-110-011 and NSC 85-2621-P-110-003to J.-J. Hung. The Tseng-wen Reservoir Manage-ment Bureau provided the sediment accumula-tion data. Satellite images were provided by theCenter for Space and Remote Sensing Research,National Central University. We thank Jong-shangHuang, Hwa-young Chang for the assistance inthe ®eld and laboratory work.

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Manuscript received 4 February 1997; revisionaccepted 7 October 1997.



The coastal transition at the mouth of a small mountainous river in Taiwan

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