The key factor was the sediment the river dropped as its current slowed at the entrance to the sea.

关键因素是河水在入海口处流速减慢时掉落的泥沙。

In a river, the strength of the water's flow depends on how far the river drops from a high point to a low point.

在河流中，水流的强度取决于河流从高处下沉到低处的距离。