With construction of the Three Gorges Dam in the Changjiang River, some deep-digging engineering was carried on the base of the river beds at site of the Three Gorges Dam, so that two of deepest troughs (No. 76 and No. 77) were exposed and show completely the landforms. Thanks the huge engineering, we have first time to observe Changjiang trough bottoms and have good evidences to study the geomorphology of the troughs. This paper is to document the No. 76 and No. 77 troughs of the Changjiang River and use the morphology to study geomorphological development of troughs.

The observation data was obtained by channel-depth map in the scale of 1:5000, in which the data were regularly measured between November of 1978 and May 1979 along the river channel and the map was printed in May 1980. The map reveals that there are more than 90 troughs at the Three Gorges channel in the Changjiang River, which the troughs are all over 40 m deep below the lowest river level along the longitude sections. Sum of the trough's length take 45% of the total length of the river channel. Troughs of No.76 and No. 77 are located at the site of the Three Gorges Dam. The deepest base of the river bed is at the elevation of 10.6 m a.s.l., while the deepest bottom of the deep-tough is at the elevation of -10.7 m a.s.l.

The troughs have geomorphological features as follows:

1. The trough with vertical and steep walls
   This trough is located in the Sandouping channel with 400 m wide in the river level, 325 m wider in the shallow channel of less than 30 m deep, but ca 40 m wide in the deep trough of more than 40 m deep. The trough profile section shows its U-shaped with vertical walls in the two sides.
2. Left side of trough wall, which was experienced by strongly abrasion processes
   There are numerous of abrading galleys on the vertical walls of the river troughs. He abrading galleys indicate the directions of water currents. The depths of galleys are normally several decades of centimeter, but a few reaches to more than a half of meter. We also found many narrow galleys along the fissures of the granite rocks. Some surfaces of the granite rock become flower-shaped, indicating the surface experienced gravel-abrasion under the whirling currents.
3. The top of trough face, which was experienced by strongly hitting and abrading erosions
   Some walls of the trough are very smooth, suggesting that these experienced strongly abrasion on the rock fissure faces. On the top of rock, there are a great of huge rock fragments. We found many hitting holes on the rock surface with several centimeters in the diameter.
4. Right side of trough wall with vertical abrading galleys and grounding hollows
   It suggests the trough experienced current down-cutting processes. A great of cone rocks left on the trough base, indicating an undergoing abrasion processes so that the base rock can keep its sharp shapes.
5. Left side of trough wall with opened-cracks and colluvial lump
   It suggests that the week tectonic sites are easier developed trough walls than the hard rocks.
6. The deepest base of the trough with grounding hollow
   The hollow is -5 m deep on the trough base, while a deeper hole formed on the -10 deep below the trough base. The hollows were developed under the eddy-current abrasions with rocky gravels.

According to morphology of the trough, we attempted to interpret causes of the trough in geomorphology as the following:

1. The troughs were formed by fluvial down-cutting erosion along tectonic cracks, when the river bed was moved to the current trough site and stood over there stablely .
2. Deep-cutting points are normally located at the deepest base (i.e. deep hollow or deep pond) of the trough, when intensive tectonic broken zone with joint-points of cross-broken and longitude fissures focus on.
3. When fluvial down-cutting occurred in Changjiang bed, jet current (particular eddy current) would be carried lots of pebbles and eroded on the bottom of the river bed, leading to trough deepening, such as scouring landforms on the right and left walls of the troughs. Meanwhile, water current with gravels and pebbles is of hitting, abrading, grounding processes in vertical walls.
4. After trough deepening, the released fracture on trough bank would induced more opened cracks happened. This process would increase more rocky walls broken and full down.

In order to estimate ages of the trough, a few of samples from sediments in the trough bottoms were radiocarbon-dated. Two of the earliest dates are at 33,800±1600 yr B.P. (No. 9924) and 38,200±2400 yr B.P. (No. 9923), suggesting that the trough was formed at least between 35,000-40,000 yr B.P. Recent study has reconstructed that there was much warmer period between 30,000-40,000 yr B.P. than the present day (Shi et al., 1999), when the temperature was 2-4^oC higher and 40 percent of precipitation than today. High rainfalls would create frequently floods and high velocity of fluvial in the river channel and river valley, leading to various intensive erosion processes of striking, abrading and under-cutting, and finally forming troughs up to 80m deep in the Changjiang River.