Yangtze River of China: historical analysis of discharge variability and sediment flux

Abstract

Hydrological records (covering a 100-year period) from the upper, middle and lower Yangtze River were collected to examine the temporal and spatial distribution of discharge and sediment load in the drainage basin. The Yangtze discharge, as expected, increases from the upper drainage basin downstream. Only an estimated 50% of the discharge is derived from the upper Yangtze, with the rest being derived from the numerous tributaries of the middle and lower course. However, the distribution of sediment load along the Yangtze is the reverse of that observed for discharge, with most of the sediment being derived from the upper basin. A dramatic reduction in sediment load (by ∼0.8×10⁸ tons/year) occurs in the middle Yangtze because of a marked decrease in slope and the change to a meandering pattern from the upper Yangtze rock sections. Considerable siltation also occurs in the middle Yangtze drainage basin as the river cuts through a large interior Dongting Lake system. Sediment load in the lower Yangtze, while significantly less than that of the upper river, is somewhat higher than the middle Yangtze because of additional load contributed by adjacent tributaries. A strong correlation exists between the discharge and sediment load along the Yangtze drainage basin during the dry season as lower flows carry lower sediment concentration. During the wet season, a strong correlation is also present in the upper Yangtze owing to the high flow velocity that suspends sand on the bed. However, a negative to poor correlation occurs in the middle and lower Yangtze because the flow velocity in these reaches is unable to keep sand in suspension, transporting only fine-grained particles downstream.

Hydrological data are treated for 30 years (1950–1980), when numerous dams were constructed in the upper Yangtze drainage basin. At Yichang and Hankou hydrological stations, records revealed a decreasing trend in annual sediment load, along with slightly reduced annual discharge at the same stations. This can be interpreted as the result of water diversion primarily for agriculture. Sediment load at Datong further downstream is quite stable, and not influenced by slightly reduced discharge. Furthermore, sediment concentration at the three hydrological stations increased, which can be attributed to sediment loss in association with intensifying human activity, especially in the upper drainage basin, such as deforestation and construction of numerous dams. Mean monthly sediment load of these 30 years pulses about 2 months behind discharge, implying dam-released sediment transport along the entire river basin during the high water stage.

Introduction

Runoff and sediment load play an important role in modifying river morphology and patterns through time and space, and supporting riverine ecosystems along adjoining fluvial surfaces Hupp and Osterkamp, 1996, Gupta et al., 1999, Lu and Higgitt, 1998, Lu and Higgitt, 1999, Miller and Gupta, 1999. The physical processes driving these two important components result from the interplay among the source of water and sediment, sediment flux, fluvial geomorphology and atmospheric circulation Milliman and Syvitski, 1992, Gupta and Asher, 1998.

Over its geological history, China's Yangtze River, which flows from west to east to debouch into the East China Sea, has served as a link between nature and people. Its abundant fluvial resources, operating under the eastern Asian monsoon were vital for early Chinese Neolithic civilization >10,000 years B.P. (Chang, 1986). The huge Yangtze drainage basin, which is more than 6300 km in length and has a catchment area of 1.94×10⁶ km², can be divided into the upper, middle and lower Yangtze reaches, primarily on the basis of geology and climate, and secondarily on the basis of the resulting geomorphology of the river.

The upper Yangtze is more than 4300 km long from the source to Yichang, and has a total drainage area of about 100×10⁴ km² (Tong and Han, 1982). The mainstem (the Jingshajiang) is jointed by four major tributaries in this river section: the Yalongjiang, Mingjiang, Jialingjiang and Wujiang (Fig. 1,

). These rivers originate on the Tibetan plateau where the elevation is generally over 4000 m, and the rivers are deeply incised into rocky canyons with >1000 m in elevation difference between the riverbeds and mountain peaks (Fig. 2a). The channels are about 0.5–1.5 km wide, 5–20 m deep, and vary in slope from 10–40×10⁻⁵, reaching a maximum of 450×10⁻⁵ (Fig. 3).

The middle Yangtze is 950 km in length, and has a total drainage area of about 68×10⁴ km² (Tong and Han, 1982). Three large inputs join the main stream in this section: the Dongting Lake drainage basin, Hanjiang River and Poyang Lake drainage basin (Fig. 1;

). This section of the river starts from Yichang, at the end of the Three-Gorges reach, to Hukuo, an outlet of the Yangtze on Poyang Lake (Fig. 1). In the middle Yangtze, the slope decreases dramatically to 2–3×10⁻⁵, but at some locations slope can be as high as 6–8×10⁻⁵. Locally negative bed slope occurs in the middle Yangtze where the two interior Dongting and Poyang Lakes are located Fig. 1, Fig. 3. The channel of middle Yangtze is wider and deeper than the upper course, with the width between 1 and 2 km and the depth between 6 and 15 m. A typical meandering river pattern with many cutoffs prevails in this river reach, where the river exits from the upper rock-confined valley into the Jingjiang fluvial plain (Fig. 2b). The well-known Jingjiang dike has been constructed along the river and elevated over time to 12–16 m in different places above the ground surface (Fig. 2c). This is the most vulnerable region for flood hazards Changjiang Hydrological Committee of Hydrology Ministry, 1997, Li et al., 1999. The Three-Gorges Dam will be completed in 2009 in the Yichang reach to serve as the flood control (Fig. 2d). Several major meander cutoffs on the Jingjiang plain occurred during the early to mid-20th century. In the 1960s and the 1970s, a set of cutoff projects were used to manage the fluvial environment (You, 1987). This stabilized the migration of the river channel although the course of the river was shortened by approximately 78 km. This straightening of Jingjiang has resulted in tremendous siltation downstream in the river channel, bypasses, and interior lakes (You, 1987). Consequently, river and lake beds were raised to increase flood hazard potentials (Yin and Li, this volume).

Below the Hukuo, the final 930 km constitutes the lower Yangtze River with a total drainage area of 12×10⁴ km². Several large interior lakes, such as the Chaohu and Taihu (Fig. 1;

and

) in association with many tributaries, drain into the lower Yangtze. This segment of the river wanders among plains and hills, on which a high-stage water level about 8 m above mean water level, is clearly marked (Fig. 2e). The slope of riverbed decreases to 0.5–1.0×10⁻⁵, and the channel widens to 2–4 km and deepens to 10–20 m. The river channel can however, be wider than >15 km and as shallow as ∼6 m in its estuarine region (Fig. 2f). The lower Yangtze drainage basin, including its large estuarine system, benefits largely from upstream discharge and sediment accumulation (Chen et al., 1988). These are vital natural resources for the intensive agriculture, fisheries, waterfowl production, vegetable irrigation and domestic consumption in the lower basin. Large estuarine islands (up to 40 km wide and 100 km long) and vast coastal wetlands, primarily due to rapid sediment progradation seaward during the last 2000 years B.P. (Chen et al., 2000) have been used for agriculture, housing and industry. On average, Southwest Pacific typhoons effect the lower Yangtze basin directly or indirectly one to three times each summer. On some occasions, typhoon-generated storms have met the annual Yangtze flood, raising sea level along the estuary by 2–3 m (Shao et al., 1991).