Geomorphology 41Ž. 2001 143–156 www.elsevier.comrlocatergeomorph Sediment delivery to the three gorges: 1. Catchment controls D.L. Higgitt a,), X.X. Lu b a Department of Geography, UniÕersity of Durham, Science Laboratories, South Road, Durham, DH1 3LE, UK b Department of Geography, National UniÕersity of Singapore, 10 Kent Ridge Crescent, Singapore, 119260, Singapore Received 10 February 2000; received in revised form 27 July 2000; accepted 29 May 2001 Abstract The paper examines sediment yield and its response to catchment disturbance and environmental variables in the Upper Yangtze basin, where the attention of environmentalists has been drawn to the Three Gorges ProjectŽ. TGP . Information about the source and conveyance of sediment from the catchment area to the Three Gorges Reservoir has implications for management strategies. Methodologies for establishing the relationships between land cover, climatic and topographic variables with sediment yield are introduced. The analysis uses a sediment load data set, containing 250 stations with up to 30 years of measurement, a 1=1 km resolution land cover database and variables extracted from various geodatabases. The mean sediment load delivered from the Yangtze upstream of Chongqing is 318 Mt ay1, but the contribution from the Jialing tributary is higher in terms of specific sediment yield at 928 t kmy2 ay1. Long-term sediment yield at Yichang has not exhibited an upward trend despite the evidence for increased soil erosion within the basin. Examination of sediment response to catchment disturbance and spatial variability in relation to controlling variables has been undertaken in an attempt to predict future sedimentation impacts. Time series analysis illustrates that significant increases in sediment yield have occurred over about 8% of the catchment area while about 3% have experienced decreasing sediment yields. The latter are associated with major reservoir schemes on the tributaries of the Yangtze. When the spatial pattern of sediment yields within the basin is analysed, AnaturalB climatic and topographic factors explain most of the variability in the relatively sparsely populated western part of the Upper Yangtze basin, but do not afford very good prediction in the more populated eastern part. Incorporation of land cover information does not provide additional explanation of spatial variability. Examination of the response of sediment delivery to catchment disturbance and environmental variables provides an illustration which may have some lessons for the management of the sedimentation problem in the Three Gorges Reservoir and a basis for modelling future changes in sediment delivery. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Sediment yield; Sediment delivery; Geodatabases; Yangtze 1. Introduction is one of the key environmental issues that has focused attention on the dynamics of soil erosion and The potential impact of sedimentation on the op- fluvial sediment transport in the catchment of the eration and life span of the Three Gorges Reservoir Upper YangtzeŽ Gu and Douglas, 1989; Edmonds, 1992; Qian et al., 1993; Luk and Whitney, 1993. ) Corresponding author. Fax: q44-191-374-2456. There are two major issues of concern—whether E-mail address: [email protected]Ž. D.L. Higgitt . proposed reservoir regulation procedures are effi- 0169-555Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0169-555XŽ. 01 00112-X 144 D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 cient enough to control the conveyance of most of has proposed that the Langbein–Schumm model is the sediment-laden waters, and whether the long-term broadly applicable to explain national variations observed sediment delivery to the Three Gorges is within China. As the availability of gauging station representative of future trends. Investigation of the data has increased a more complex relationship with temporal and spatial variability of sediment transport climate and vegetation has emergedŽ Douglas, 1967; within the Upper Yangtze catchment leads to the Wilson, 1973; Jansen and Painter, 1974; Walling and following paradox. There is widespread evidence Webb, 1983; Jansson, 1988. , mainly because of the that the extent and magnitude of soil erosion across impact of human activity on natural vegetation cover. southern China has increased dramatically during the At a global scale, the importance of topography and last 30–40 yearsŽ Smil, 1993; Wen, 1993; Edmonds, the significance of relatively small mountainous 1994. There is no evidence from the sediment load catchments as major contributors to global continen- measurements at the Yichang gauging stationŽ down- tal sediment export has been notedŽ Milliman and stream of the TGP dam site. of a trend in sediment Syvitski, 1992; Summerfield and Hulton, 1994. The yield delivered from the Upper Yangtze catchment. difficulty of obtaining sufficiently detailed, spatially The average annual loadŽ for a catchment area of just distributed data on catchment characteristics has over 1 million km2 , is reported as 520 MtŽ Mason, hampered attempts to disentangle the various con- 1999.Ž . A question for catchment managers and geo- trols on sediment yields within large catchments. morphologists. is whether the observed spatial and However, the availability of global environmental temporal variability of both sediment production and data sets comprising description of hydroclimatic, conveyance can be explained adequately by a model biological and geomorphological characteristics of of sediment delivery for the Upper Yangtze. In the Earth offers a means of extracting catchment attempting to address this issue, the appropriateness variables for integration with sediment yield data. of methodologies and available data sets to analyse This approach has been used to examine global sediment delivery within large river catchments can variations in sediment yieldŽ Summerfield and Hul- be considered. The paper first provides a discussion ton, 1994; Ludwig and Probst, 1998. where individ- on approaches to modelling regional sediment yield. ual catchments are represented by a single sediment Second, background information about the Upper yield value. In the present study it is extended to the Yangtze catchment and the construction of data investigation of sediment yields within the Upper sources for examining sediment yields is described. Yangtze. Third, results focusing on estimates of sediment The prediction of sediment yields is complicated delivery to the Three Gorges reservoir are presented, by the interaction of controlling variables, human along with description of sediment responses to impact on the hydrological system, and by scale catchment disturbance and environmental variables. effects associated with different catchment sizes. The Evidence for recent changes in soil erosion and proportion of sediment eroded from catchment slopes sediment delivery to the reservoir from the area that is exported, decreases with catchment size in surrounding the Three Gorges is addressed in the most environments as opportunities for storage in- second paperŽ. Lu and Higgitt, this volume . crease downstreamŽ. Walling, 1983 . This proportion can be quantified by the sediment delivery ratio. The scale-dependency inhibits the direct comparison of 2. Regional sediment yields specific sediment yieldsŽ t kmy2 ay1. from catch- ments of contrasting sizes. In addition, sediment The geomorphological and hydrological literature delivery ratios may change over time, damping the contains many attempts to relate global or regional response of suspended sediment loads to the magni- sediment yields to controlling factors. Langbein and tude and extent of soil erosion. Time lags between SchummŽ. 1958 produced a model of sediment yield initial sediment mobilisation and export from the in relation to effective precipitation, reaching a maxi- catchment outlet may be considerableŽ Trimble, mum in semi-arid environments and declining as 1998. , and have significant effects on the manage- vegetation cover protects the land surface. XuŽ. 1994 ment of sediment laden waters. D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 145 3. The Upper Yangtze: characteristics and data marise measurements from a network of hydro- sources graphic stations throughout the Upper Yangtze. The original records for each station provide information 3.1. Catchment characteristics on the station co-ordinatesŽ. latitude and longitude , The Upper Yangtze catchment, with the exception catchment area, mean monthly and annual water of the extreme west, experiences a subtropical mon- discharge and sediment load, and the magnitude and soon climate. Precipitation varies from -250 mm date of occurrence of the maximum daily discharge. on the northern edge of the Qinghai–Xizang Plateau The use of historical gauging station data raises the to )1000 mm in the east of the catchment. Popula- question of data quality. The monthly sediment loads tion densities range from -10 persons kmy2 in the are based on discrete rather than continuous mea- mountainous west to )500 persons kmy2 in the surements of suspended sediment at daily to weekly Sichuan Basin. The Upper Jinsha, Yalong, Dadu and intervals. As such, the frequency of sampling does Min principally drain the mountainous areas to the not ensure that all ranges of flow were sampled. The west of the catchment. To the east, the Tuo, Fu, sampling intervals and the exclusion of the bedload Jialing and Qu flow through areas of high population