河中的水流加速时,它可以接更多的沉淀物下游的河床和运输。另外,当流动减慢,运输能力下降和沉积物悬浮沉积在河床上。当河流进入湖泊,它从河中水流湍急的政权过渡到一个非常缓慢的流在湖里。因此,沉积物沉积的放缓,形成一个三角洲。逆发生在出口的湖,那里的水加速。因此,沉积物往往是下游和运输,不允许形成的三角洲。然而,这只是部分回答你的问题,因为经常湖泊周围的地形平坦,一个将可能期望看到很多河流湖泊被多个排水的例子。这个问题的答案是,多个并行的河流是不稳定的:如果一个河流变得略大,它将会有更多和更快的流动。因此,它将增加其运输能力沉积物,挖掘其床和更大的增长。如果它生长大,泥沙运输将进一步增加,这条河会变得更多。 This positive feedback leads to the grow of the largest outlet at the expense of the smaller ones, until you have only one river outlet. However, it takes time for one outlet to monopolize the flow. And in very flat and changing terrain you can find lakes drained by a delta. One example of this is the proglacial lake of Kaskawulsh glacier in the Yukon Territory, Canada: [![enter image description here][1]][1] This lake is in a divide. And it is drained not by one, but by two deltas. Interestingly, the Slims river in the top of the picture has reversed its flow direction in historical times. In [this NASA Earth Observatory article][2], you can find out more about this interesting river and some great images, plus comparisons of how the flow regime has changed in the past. [1]: https://i.stack.imgur.com/naP0U.png [2]: https://earthobservatory.nasa.gov/images/90116/river-piracy-in-the-yukon