2007) 185–196www.elsevier.com/locate/geomorph

Geomorphology 85 (

River channel change during the last 50 years in themiddle Yangtze River, the Jianli reach

Luqian Li a,⁎, XiXi Lu a, Zhongyuan Chen b

a Department of Geography, National University of Singapore, Singapore 119260b State Key Laboratory for Estuarine and Coastal Research, East China Normal University, Shanghai, China 200062

Received 5 October 2004; received in revised form 20 March 2005; accepted 29 March 2006Available online 17 January 2007

Abstract

Intensive anthropogenic disturbances have affected the channel of the middle Yangtze River since the 1950s. This paper selectsthe Jianli reach as an example to examine human impact on channel change in the middle Yangtze River. 1:100,000 channeldistribution maps from 1951, 1961 and 1975 and 1:25,000 navigation charts from 1981 and 1997 were employed to reconstructchannel change in the study reach. The result indicates that the channel, under the constraint of levees along the riverbanks,underwent a minor widening but frequent bank failure due to susceptible bank structure and increase in water discharge. The bankfailure promoted bar growth in the channel. Cross-section changes and quantitative calculations of erosion and deposition based onthe DEM derived from navigation charts present a pattern of over-bank sedimentation and riverbed incision. The stage and durationof floods have increased following levee construction and bank revetment.© 2006 Elsevier B.V. All rights reserved.

Keywords: River channel change; Human impact; DEM; Yangtze River; Levee construction; Bank revetment

1. Introduction

The response of river channels to human interventionhas been well documented (Braga and Gervasoni, 1989;Hooke, 1995; Gurnell, 1997; Lane and Richards, 1997;Surian, 1999; Fuller et al., 2003; Rinaldi, 2003). Com-mon human disturbances in river catchments includeimpoundment construction, sediment mining, bank re-vetment and artificial cutoff. These activities initiatechanges in the hydrology regime and channel convey-ance ability, andmay reduce channel stability (Knighton,1984; Shield and Abt, 1989; Simon, 1992; Winterbot-tom, 2000; Simon et al., 2002; Yang et al., 2002; Fuller

⁎ Corresponding author.E-mail address: lvqianli@yahoo.com (L. Li).

0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2006.03.035

et al., 2003; Grant et al., 2003; Kesel, 2003; Rinaldi,2003). Human activities can sometimes induce channelchange more significantly than those induced by naturalevents such as floods, droughts and landslides (Petts andAmoros, 1984; Surian and Rinaldi, 2003). River channelchanges will result in various environmental and socialconsequences such as difficulties in navigation manage-ment, flood hazard and the alteration of aquatic andriparian ecosystems. A better understanding of humanimpact on river channels is of great importance for riverengineering and environmental management.

Human modification on the Yangtze River channelstarted with levee construction hundreds years ago, inorder to harness the inundation of the Yangtze fluvialplain (JLAC, 1991; Chen et al., 2001a,b). Currently,there stand 30,000 km levees along the middle andlower Yangtze River, protecting 12 mega-cities like

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Wuhan, Nanjing and Shanghai from floods with returnperiod of about 10–20 years (Wu, 2003). Since thefoundation of PR China in 1949, extensive interventionshave modified the middle Yangtze River, the mostmeandering stretch of the whole Yangtze River (Panand Lu, 1999; Yin and Li, 2001). For example, threeartificial cutoff events in the lower Jingjiang reach(between the end of the 1960s and early 1970s) short-ened the river length by 78 km, reducing the sinuosity ofthe reach from 2.83 to 1.93 (Pan and Lu, 1999; YWCC,2000). Failure along both riverbanks deteriorated afterthe cutoff projects (JLAC, 1991).

Apart from the levee construction and the cutoffs,impoundment construction also initiated changes in thewater discharge and sediment load in the Yangtze River.Over 40 thousands reservoirs with 119.5 billion m3

in total capacity have been constructed in the YangtzeRiver basin (Wu, 2003). Sediment retained by thesereservoirs in the upper Yangtze basin has increased from5×106 tons/year to 100×106 tons/year from 1950 to themiddle 1980s and amounted to 230×106 tons/year in1990 (Yang et al., 2002). Thus, sediment increase due todeforestation has been counterbalanced by sedimenta-tion in these reservoirs in the upper Yangtze basin(Lu and Higgitt, 1998). As a result, the Yichang gaug-ing station (the last station in the upper basin) hasexperienced a stable annual sediment flux and waterdischarge. However, due to the construction of theGezhouba Dam on the main stem of the river, theYangtze at Yichang station has experienced a decline inthe minimummonthly water discharge and an increase inthe maximum daily sediment flux (Lu et al., 2003; Lu,2004). In the middle and lower reaches of the YangtzeRiver, the changes in the water discharge and sedimentflux are more obvious due to the lake reclamation and thesevere sediment deposition in the lakes. Dongting Lake,which used to be the largest lake in the Yangtze Riverbasin, has reduced its water storage capacity from29.3 billion m3 in the 1950s to 17.8 million m3 in the1990s (Xiang et al., 2002). This huge reduction of waterstorage capacity has significantly altered the hydrolog-ical regime in the middle Yangtze River. For example,annual water discharge and sediment load at the Jianlistation has increased 20.2% and 6.7% from the 1950s tothe 1990s, respectively (Pan and Lu, 1999).

There is a growing concern about this channel. A fewstudies have identified the channel change in the middleYangtze River due to human intervention (Lu and Luo,1997; Yang and Tang, 1999; Pan and Lu, 1999). Due tothe difficulties of field survey on this mega river, suchwork focuses on examining hydrological regime changewith the available hydrological gauging data. Any holis-

tic research on river channel change in relation to humanactivities along the middle Yangtze River is lacking.

Historical maps and air photographs have been widelyapplied to the study of river channel change with 10–100-years time scales (Pišút, 2002; Khan and Islam, 2003).Digital Elevation Model (DEM) derived from maps andphotographic materials have often been employed toillustrate three-dimensional topographic change (Chap-pell et al., 2003; Fuller et al., 2003). The application ofDEMs enables accurate positioning and estimation oferosion/deposition in river channels or on fluvial plains(TenBrinke et al., 1998; Chappell et al., 2003; Fuller et al.,2003; Lane et al., 2003). The present study couples DEMsderived from navigation charts with hydrological surveydata to identity the river channel alterations in the middleYangtze River during the last 50 years in terms of changesin platform, cross-section and erosion/deposition patterns.The objectives of this study are (1) to examine riverchannel changes specifically due to levee construction; (2)to explore the factors responsible for the channel changes;and (3) to estimate the volume and location of erosion anddeposition in the river channel.

2. Study area

The 108 km long Jianli reach is located in the moststrikingly meandering section of the middle YangtzeRiver. The connection with Donting Lake is located atthe end of the reach (Fig. 1). The Jianli reach is regardedas the most dangerous reach along the Yangtze Riverduring flood seasons because of the susceptible bankconditions and the fragile foundation of levees along thebanks (JLAC, 1991). The capacity of Dongting Lakehas been reduced in volume due to reclamation over thelast 50 years leading to higher flood discharges andincreased sediment load to the Yangtze (Fig. 1; Pan andLu, 1999; Du et al., 2001). When big floods occur in themiddle Yangtze River, slow water flow at the junction ofDongting Lake and the Jianli reach leads to a longerflood retention time and a higher flood level.

Human activities in the Jianli reach during the last50 years include the Shangchewan cutoff event, ongoingconstructions of levees and bank revetments. In order tofacilitate navigation along the Jianli reach, a bend atShangchewan was artificially cut off at the end of the1960s and early 1970s. This shortened the river lengthby 29.2 km (YWCC, 2000). The floodprone Jianli reachis now protected by 255 km of levees, including theGreat Jingjiang Levee and the Yangtze major leveesfunded by the national government, and minor leveesfunded by local government. Prior to 1998, the leveeswere around 5–6 m above the ground and could contain

Fig. 1. The Jianli reach, located at the lower Jingjiang section, the middle Yangtze River.

187L. Li et al. / Geomorphology 85 (2007) 185–196

floods with return period of 10–20 years (JLAC, 1991).After the 1998 flood, the levee bodies were reinforced to10–12 m in width and another 1.5–1.7 m in height. Thelevees along the Jianli reach protect 5 million people and13,500 km2 farmland from flood disasters.

3. Data and methods

The 1:25,000 navigation charts in 1981 and 1997 and1:100,000 channel evolution maps in 1951, 1965 and1975 were used in this study. River boundaries, definedas the margin of waterfront at medium water stage (12 m

above the navigation reference plane), were extractedfrom the maps and charts. Channel planform changeswere investigated by comparing changes in the channelwidths and bar areas. Shifting of the navigation channelwas analysed with special emphasis on the anbrachingof the Jianli bend. Quantitative analysis on channel area,bar area and channel width was made on the basis ofinformation derived from the bigger scale navigationcharts in 1981 and 1997. In addition, three bank types(steep, collapsed and protected with boulders) and twolevee types (major levees funded nationally and minorlevees funded locally) were extracted from the 1981 and

Table 1Annual water discharge and sediment load in the study reach

Water discharge (m3 s−1) Sediment load (kgs−1)

1956–1965⁎ 1966–1975 1976–1981 1982–1997 1951–1965 1966–1975 1976–1981 1982–1997

Jianli 10207 11002 11333 12100 9820⁎ NA 13200 10220Qilishan 9912 8460 9260 8372 1910⁎⁎ 1310 1124Luoshan 19941 20178 19400 NA 13100⁎⁎ 15450 NA

Data with ⁎ are adopted from Yang and Tang (1999), and data with ⁎⁎ are averaged from 1951–1975.

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1997 navigation charts to illustrate bank erosion pro-cesses and the effects of levee construction.

Cross-section data surveyed by the Yangtze WaterConservancy Committee (YWCC) at the Jianli andLuoshan gauging stations from 1965 to 1981 wereavailable for consultation. Channel geometry change dueto the Shangchewan cutoff event was examined bycomparing four cross-sections near or across the cutoffbend. The navigation charts in 1981 and 1997 includedcontours and spot heights above and below the navi-gation reference plane, which were used for constructingDEMs. After digitising all the contours and spots heightsin the navigation charts, two DEMs were derived to

Fig. 2. Bank migration in the study reach from 1951 to 1997. Riverboundaries of 1951, 1965 and 1975 were extracted from channel distri-bution map (1:100,000), and those of 1981 and 1997 were from naviga-tion charts (1:25,000). A,b,c and d are locations of cross-sections in Fig. 6.

represent river topography in 1981 and 1997. Twentycross-sections with 5 km intervals and the longitudeprofile of the river were extracted from each DEM foranalysing topography changes along the study reachfrom 1981 to 1997.

Patterns of erosion and deposition were also analysedbased on the constructed DEMs. The differences bet-ween the 2D plane area and the 3D surface area indicatethe roughness and steepness of the surface. The volumeof the space between the surface and the navigationreference plane were also computed. Furthermore,lateral erosion and accretion were mapped by comparingriver areas in 1981 and 1997. The part of the 1997 riversurface area that was not included in 1981 was definedas the “lateral eroded area”, and the accretion river

Fig. 3. Navigation channel shift in the Jianli bend from 1951 to 1975.

Fig. 4. a) Bar development from 1981 to 1997. b) Bank failure from 1981 to 1997.

Table 2Bar growth (km2), 1981–1997

Mid-channel bar Side bar Entire channel

1981 0.51 59.43 144.11997 0.87 65.94 150

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surface in 1981 that was not in 1997 was identified as“accretion area”. Cut and fill analysis using ArcGISgave an indication of vertical erosion and deposition inthe channel during 1981–1997.

Annual data on water discharge, suspended sedimentconcentration and water stage at the Jianli, Luoshan andQilishan stations from 1951 to 1997 were used to explorethe factors affecting the river channel change (YWCC,1951–1997) (Table 1). Jianli station is located near thebeginning of the study reach, while the Luoshan stationis at its end. The Qilishan station is on the passage fromthe mouth of Dongting Lake to the Yangtze mainstream(Fig. 1). The history of levee construction in the studyreach, recorded in JLAC (1991), assisted in under-standing the impact of levee construction.

4. Results

4.1. Planform changes inferred from maps

Bank migration during the last five decades in thestudy area can be mapped from the reconstructed riverboundaries (Fig. 2). The most obvious planform changewas the artificial cutoff of the Shangchewan bend at theend of the 1960s. Rapid siltation occurred within the

abandoned river bend and it was not displayed onnavigation charts after 1981 (Fig. 2).

The shift of navigation channel in the Jianli bendfrom 1951 to 1975 is also clear (Fig. 3). In 1951, thenavigation channel in the Jianli bend was divided intonorth and south channels by a mid-channel bar. The barbecame attached to the south bank of the Jianli bendaround 1965 (Fig. 3). The north channel was used as themajor navigation channel until a cut-off event in 1971when this sand bar became an island in the channel(WNB, 1983). The north channel silted up and the southone in turn became the main navigation channel duringlow flow until 1975 (Fig. 3). Afterwards, the north oneagain became the navigation channel because of thegrowth of the side bar (Fig. 4a).

Although major channel shifts were impossiblebecause of the engineered levees being in place, asmall-scale minor channel widening trend occurred

Table 3Bank failure (km), 1981–1997

Collapsed bank Bank protectedwith boulders

Steep bank

1981 Left bank 24.62 4.06 22.17Right bank 11.93 8.85 9.61

1997 Left bank No data 7.08 36.87Right bank No data 7.45 24.47

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from 1961 to 1997. The channel width of the study reachincreased by 66 m on average during this period, eventhough the average channel width ranges from 900 m to1700 m. and the area of the river channel increased from144.1 km2 in 1981 to 150 km2 in 1997 (Table 2).

The bar development and bank failure along the Jianlireach are shown in Fig. 4. The bars tended to migratedownstream and develop at meander bends (Fig. 4a).The area of channel bars increased by 0.7 km2 from 1981to 1997 (WNB, 1997). The reach experienced severebank failure from 1981 to 1997 (Fig. 4b). The total lengthof the collapsed banks was around 36.5 km, or 33% ofthe length of the Jianli reach in 1981. The majority of thecollapses occurred on the left banks (Table 3). Some ofthe collapsed banks were then protected by bankrevetment (Fig. 4b). From 1981 to 1997, the length ofthe left banks under boulder protection increased from4 km to 7 km, while that of the right bank decreased from

Fig. 5. Cross-sections at Jianli and Luoshan from 1

8.8 km to 7.4 km. At the same time, the length of steepbanks increased significantly, by around 15 km on theeach side (Fig. 4b and Table 3).

4.2. Cross-section changes inferred from YWCC surveydata

The cross-sections at the Jianli and Luoshan stationfrom 1965 to 1981 are illustrated in Fig. 5. The deepestchannel for navigation at Jianli was near the right bank in1966, but it was near the left bank in 1975. The channelnear the left bank (north channel) silted up between 1975and 1981, when the south channel was scoured by aboutanother 3 m (Fig. 5a). The right bank retreated by around50m from 1966 to 1975 (Fig. 5a). The cross-section at theLuoshan station shows two troughs in the channelseparated by a raised shoal (Fig. 5b). The riverbed nearthe left bank storedmassive sediments from 1965 to 1975,but thiswas removed to a depth of 10m between 1975 and1981. During this period, about 12 m of sediment wasdeposited on the other side of the channel (Fig. 5b).

The four cross-sections near Shangchewan in Fig. 6show a general trend of deposition during the imple-mentation of the artificial cutoff project, especially in theabandoned bend at the cutoff (Fig. 6b). Cross-sections atthe both ends of the cutoff bend also displayed signs oferosion, especially in the central portion of the

965 to 1981, based on YWCC surveyed data.

Fig. 6. Cross-sections near the Shangchewan cutoff based on the YWCC surveyed data. Locations of cross-sections are shown in Fig. 2.

191L. Li et al. / Geomorphology 85 (2007) 185–196

uppermost cross-section during the first 2 years after the1971 cutoff (Fig. 6).

4.3. Cross-section changes inferred from DEM

Cross-sections other than those along Jianli, Luoshan,and Shanchewan were examined using the DEMs (Figs.7 and 8). The range of elevations along the Jianli reach in

Fig. 7. Topography and longitude profiles in the stu

1997 is larger (−43 m to 20 m) than that in 1981 (from−30 m to 14 m), suggesting a higher roughness of theriverbed in 1997 (Fig. 7).

Cross-sections at twenty sites were extracted from thetwo DEMs (Fig. 8). The deep thalweg in a meanderingbend is located along the outer concave bank, while thepoint bar is attached to the inner bank due to the inter-action between helical flow and primary longitudinal

dy reach in 1981 and 1997 based on DEMs.

Fig. 8. Cross-section changes from 1981 to 1997 derived from DEMs. Cross-sections start from left bank to right bank, facing downstream. Location of each cross-section was indicated in Fig. 7.

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Fig. 9. Topographic changes from 1981 to 1997. a) Lateral erosion and accretion during 1981–1997. b) Vertical erosion and deposition during 1981–1997.

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flow (Leopold and Wolman, 1960), as shown by thecross-sections 3, 4, 5, 7, 8, and 17. Riverbed scouringbelow the navigation reference plane happened at allcross-sections except at cross-sections 10 and 11 locatedat the Yanchuantao where the river is straight. Deposi-tions above the reference plane can be seen at the cross-sections 13, 15, 16 and 17 at meander bends. The channelgeometry at the Yanchuantao straight course was relative-ly stable as shown by cross-sections 10 and 11. Further-more, channel widening can be observed in many cross-sections, such as 3, 9, 12, 13, 14, 15, 17 and 18 (Fig. 8).

4.4. Erosion/deposition amounts estimated from DEM

The total area subjected to lateral erosion during1981–1997 in the Jianli reach was about 16 km2, while

Table 4Quantification of erosion and deposition in the river channel, 1981–1997

2D Area (m2) Surface

1981 above 0 m 62210758.28 6225161981 below 0 m 76018891.92 7608541997 above 0 m 67524374.97 6753991997 below 0 m 78243450.03 7832681997–1981 above 0 m 5313616.69 528831997–1981 below 0 m 2224558.11 22414

the area subjected to lateral accretion was around10 km2 (Fig. 9a). Lateral erosion, as expected, mainlyoccurred at the concave banks of meander bends, whileaccretion occurred at the convex banks, such as theXiongjiazhou bend (Fig. 9a). Fig. 9b illustrates thedominance of vertical erosion and deposition in thelaterally stable channels shown in Fig. 9a, from 1981 to1997. The depositional areas unexpectedly were mainlylocated at the outer banks of meanders, while theerosional areas were at the opposite banks. This mayresult from the local deposition of bank failure material.The Jianli and Damazhou bends were particularlydominated by aggradation, whereas the Xiongjiazhoubend was dominated by degradation (Fig. 9b).

The volume of vertical erosion and depositionderived from the DEMs, presents a pattern of overbank

area (m2) Volume (m3) SA-2DA (m2)

38.51 332453192.2 40880.2334.52 315750789 66542.657.19 382773092.9 15582.2247.37 371055054.2 83397.3418.68 50319900.68 −25298.0112.85 55304265.21 16854.74

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deposition and in-channel erosion. Using the navigationreference plane as benchmark, about 50 million m3 ofsediment was deposited above the reference plane from1981 to 1997, while about 55 million m3 of sedimentwas washed out from the channel below the referenceplane (Table 4). The difference between the 3D surfacearea and the 2D plane area (Table 4) indicates that thesurface below the navigation reference plane wasrougher and steeper than that above the plane both in1981 and 1997, and that the surface below the referenceplane became rougher and steeper from 1981 to 1997due to river-bed incision, while the surface above itbecame flatter and gentler due to overbank deposition.

5. Discussion

5.1. Impacts of hydrological regime changes

Hydrologic regime change is a major contributingfactor of hydraulic geometry change (Leopold et al.,1964; Knighton, 1984; Petts and Amoros, 1984; Merrittand Wohl, 2003). Water and sediment transportedthrough the Jianli station into the study reach increasedwith time, due to the reduction of lake capacity resultedfrom reclamation and sedimentation in Dongting Lake(Table 1). The higher water discharge in the Jianli reachfacilitated the scouring within river channel (Fig. 3), andinitiated severe bank erosion and channel widening(Figs. 2 and 4b). In conjunction, higher suspended sedi-ment concentration from the upper reach and sedimentsupply due to local bank collapse lead to the growth ofpoint bars in the channel (Figs. 3 and 4a), which will, inturn, reduce its capacity leading to channel erosion.

The shrinkage of Dongting Lake and the resultantincreased water discharge also contributed to the inten-sified floods in the Jianli reach. The flow from DongtingLake through Qilishan station is confluent with the flowthrough the Jianli reach (Fig. 1). If the converged flowcannot be discharged in time, the water stage rises faster(JLAC, 1991). The water stage in the 1998 flood washigher than that in the 1954 flood, despite the smaller totaldischarge during the 1998 flood (Li andNi, 2001; Yin andLi, 2001). The higher water levels also led to overbankflows in the middle Yangtze River. The YWCC statisticalrecords on the high water stages at the Jianli station showsthat in 25 out of 37 years (1949–1985), the stationsuffered higher water stages above the warning level(34 m), and the duration of this hazardous stage becamelonger (JLAC, 1991).

The Shangchewan cutoff event also disturbed thehydrological regime and affected channel changes in theJianli reach. The shift of navigation channel at the Jianli

bend resulted from the hydrological changes due to thecutoff event (Fig. 3). The cutoff accelerated bank fail-ures in the Jianli reach due to the resultant channelgeometry changes. Before the artificial cut-off event, thetotal length of the collapsed banks along the Jianli reachwas 18.6 km, or 14.8% of the reach length before theartificial cut-off event. These bank collapse mainlyoccurred at/near meanders such as the Jianli bend. Afterthe Shangchewan cutoff event, bank collapses occurrednot only at the meandering bends but also at the straightportion such as the Yanchuangtao course and the newchannel at the Shangchewan cutoff (JLAC, 1991).

5.2. Impact of bank failure

Bank composition determines the stability of bank(Knighton, 1984; Lawler et al., 1999). Composite bankstructure and helical flow in meanders facilitate bankfailure in the study reach, as the cohesive material at thebank toe is very susceptible to erosion (Yang and Tang,1999).

Severe bank failures contributed to the bank migrationat the meander bends in Baxinzhou and Guanyinzhou(Fig. 2), channel widening (Fig. 2), and bar growth(Figs. 3 and 4a). Among the meanders along the Jianlireach, the Jianli bend underwent the severest bank collapseportion. From 1969 to 1985, the bank along Xiaowuguiz-hou in the Jianli bendwas retreated to 600–1500m behindthe original bankline (JLAC, 1991). The inferred frequentbank failures are consistent with YWCC cross-sectionobservation data from 1954 to 1965. During this period,the channel in the Jianli reach widened by 113 m, or a rateof about 10 m/year (YWCC, 1951–1997).

Another contribution of bank failure is sedimentsupply to deposition in river channel or above riverbank.According to Simon et al. (2002), failed bank material inthe Missouri river was partly removed by river flow anddeposited as bed material or dispersed as suspendedload. These failed bank material and frequent overbankflows facilitated overbank sedimentation within theconstrained areas by the levees. The difference involume calculation derived from the two DEMs in thisstudy (Table 2) can be a credible indicator of thedeposition during the over-bank flows from 1981 to1997, including the large-magnitude flood in 1996.

5.3. Effect of bank revetment and levee construction

Bank protection structures along a meandering riveraffect channel morphology and dynamics by restrictingthe width of wandering belts (Xu, 1997). Since 1949,bank revetment has been carried out at the susceptible

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meanders such as the Jianli, Shangchewan and Gua-nyinzhou bends. The traditional bank revetment con-struction on the middle Yangtze River used boulders andlumber to strengthen the delicate riverbank, whicheffectively protected banks at meander bends fromfurther erosion. The decrease in collapsed banks from1981 to 1997 is a good indication of the success of suchmethods (Fig. 4b and Table 3). However, it also has itsnegative effects. According to Simon (1992), if therevetment material sinks in the river channel, the channelcapacity will be reduced and hydraulic geometrydownstream is affected. In addition, bank revetmentwill enhance the undercutting in the riverbed, becauseless sediment is entrained in water flow and less energywill be consumed during the friction with riverbanks(Winterbottom, 2000; Rinaldi, 2003).More stress will beadded on the riverbed, which is more vulnerable than theprotected riverbank. This may partly explain theobserved scouring in the river channel of the Jianli reach.

The intense levee construction also contributes to theerosion/deposition pattern in the study reach. The studyreach is almost completely controlled by the Yangtzemajor levee and local minor levees (Fig. 1), and thebankfull discharge would be amplified by the leveeswhich benefits overbank deposition and scouring in thechannel during floods. As the gradient of the riverchannel was reduced after cutoffs, flood flow cannotflush down immediately, and suspended sediment in theflow has a longer retention time, which benefits thesedimentation over the flood plain (Yin and Li, 2001).Thus, the levees on the banks play an important role ingeomorphic changes such as deposition on flood plain,and erosion in the channel.

6. Conclusions

This study uses the historical maps of differentperiods and hydrological data to reconstruct channelchanges in the Jianli reach during the last 50 years. Aminor channel widening was observed, although thestudy reach was controlled by the major and minorlevees. Bank failures were very common due to highwater discharge and frequent bank-full flow as a result ofgradual shrinking of the Dongting Lake. The collapsedbanks contributed to the minor channel widening, andthe collapsed materials contributed to bar development.

Twenty cross-sections derived from DEMs for 1981and 1997 illustrate that the study reach experiencedoverbank deposition and in-channel erosion. This patternconcurs with the sediment volume estimated from the twoDEMs, which further suggests scouring below thenavigation reference plane and deposition overbank.

Longitude profiles along the study reach indicate down-cutting in the riverbed. We believe that this phenomenonis related to levee construction and bank revetment.

Although some river channel changes in the studyarea have been detected, uncertainties remain due to theemployment of the small-scale maps. A full understand-ing of the river channel change in the study area requiresmore studies, especially after the closure of the ThreeGorges Dam, which is likely to cause further channelchanges in the Yangtze River.

Acknowledgments

This research was funded by National University ofSingapore (Grant No: R-109-000-054-112). The authorswould like to sincerely thank Mr. Joy Sanyal for his helpin DEM construction, as well as Dr. Zhanghua Wangand Dr. Maotian Li for many useful suggestions. Thanksalso go to Dr. Takashi Oguchi and another anonymousreviewer for their constructive comments and sugges-tions, which improved the quality of the paper.

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