There is very little evidence of thick submarine fans and major slope deformation features in the mouth of Lanyang River, and around it the seashore line and the contour lines beneath the sea level are very straight and smooth that shows the river doesn’t deposit the obvious sediments and indicates it is a young river.
There are two nature prominent embankments beside Suao Bay, and all the contour lines bulge out from it at the influence of Black Current northwards, and there are several sand bars outside Suao Bay, including a submarine elliptic sand bar about more than 2.5 km long, indicating that the Lanyang River once has flowed out from Suao Bay since long time ago.
Lanyang Plain is a vast triangular plain, about 800 km2, but Lanyang River is only a clear stream, which is 75 km in length, low in sand and limited flow. The amount of depositing sediments in Lanyang River is only about 8 million metric tons every year that can not be the sources of earth in Lanyang Plain. How did the vast triangular Lanyang Plain form?
The great change of Lanyang River course should come from a big landslide of northern Snow Mountain in the east hillside. The place of landslide formed Lanyang Plain directly. The great amount of earth and stone sank into the sea, and then there was no obstructer to make a turn to Suao Bay. So, it pour into the sea directly, and formed the new Lanyang River course and a new river mouth that proved the fact of big landslide once took place in northern Snow Mountains.