长江中下游自城陵矶至江阴1,160公里内,江中有大小沙洲80多个,汊道100余处。分汊河道按其分汊的数目多少可分为双汊和多汊。从平面形态上分析,双汊型河道江中夹有一个沙洲,多半为顺直分汊。如长江马鞍山河段的江心洲左、右汊;小黄洲左、右汊。

SYNOPSISThere are in general two different types of braided stream. One has large num-bers of cross-channels with an ever shifting thalweg from place to place. The chan-nels can easily be silted up and abandoned by the flow, but new channels will be formed in some other places at will. The lower reaches of the Yellow River is typical of this type of braided stream. On the other hand, there are braided streams which assume a relatively stable pattern with not many cross-channels. Although periodic evolutions are noticed among these channels, but they seldom lose their existence as a result of sediment accretion. The middle and lower reaches of the Yangtze Eiver belong to this group.Based on experimental studies as well as on field observations, this paper deals with the characteristics of the flow, the effects of the nodes along the banks on the fluvial processes, the geometric shape of the river channels and the geomorphology of the bed of the braided Yangtze River. The results can be summarized as follows:1. The flow conditions of the braided stream are unique for the existence of a zone of division of the flows and a zone of re-union of the cross-channels. The zone of division can be classified into two catagories, one as division through a relatively straight channel and the other through a curved one. In the former case, the surface flow is directed towards the main channel with bottom flow moving towards the side channel. The opposite is true for the latter case. Evolution of the channel patterns will lead the division through straight channel towards the type of division through curved channel. The junction of two channels or more can be distinguished in the zone of re-union. When two curved thalwegs with opposite direction join together, two opposite spiral flows towards the middle of the channel are created. If more channels take place in the re-union, then a correspondingly increased number of spiral flows will be established.2. The nodes along the river banks are the important boundary condition for the formation of braided stream. Once the nodes are exposed by the flow, alternative wide and narrow reaches along the river course will be developed. Experimental re-sults indicate that as the width and length of the expanded reach become twice of and six times respectively the width of the reach with nodes, bars and islands will be formed in the expanded reach which will lead eventually to the creation of a braided stream. If there are nodes along both banks of the river and the reach is so long such that its length equals to its width, then the main flow will be kept in the middle of the reach and the effect of directing the flow towards the opposite bankby the nodes will be lessened. If the length of the reach with’ nodes along both banks is smaller than its width, then the flow coming from the left side upstream of the reach will be directed towards the right side of the downstream reach, and vice versa.3. A complex braided channel system with a plain view look like "the head of a goose" is often a combination of straight channel, channel with slight curves, cur-ved channel and loop with the shape of a reversed "S". In the historical past, dif-ferent single channel patterns can be mutually converted, and there is always a channel the evolution of which plays a dominant role. The fluvial processes of the braided channel system as a whole depend on the changes which take place in that channel.4. Islands appeared in the braided system are often built up from the central bars. There are many ways in. which central bars can be created. If the river as-sumes a form with alternative wide arid narrow reaches due to the existence of nodes along the banks, then upstream of the nodes, the flow will be dammed up in the flood season ,and central bars will be developed due to the accumulation of sediment. Downstream of the nodes, the diversion of flow is also beneficial for the accretion of the channel and the formation of the central bars. In some times central bar is developed in consequence of the cuttin