The Lanyang River and its branches made an united alluvial plain in the shape of an equilateral triangle. The triple points are Toucheng, Sanxing, and Su'ao. Every side of it goes for about 30 kilometers; and, its square measure is about 320 square kilometers. The nor thwest side from Toucheng to Sanxing is the Xueshan Mountain Range. It is the junction area of the cliff and the plain. Below the Sanxing- Su'ao line was the Central Mountain Range. Due to the streams flowing out of the valleys, the line of ranges goes ragged. From Toucheng to Su'ao is an indentation of the coastal line. Lanyang River empties into the Pacific Ocean between Donggang of Zhuangwei Township and Qingshuei of Wujie Township. The water recourse is abundant. The fertile soil of Lanyang Plain is the major living space for people in Yilan.
The upper Lanyang River’s steep slopes and rushing water has strong erosion effect that carries piles of sands. Whenever it goes out of the mountain root line, sands and pebbles silt up as an alluvial fan due to its sudden gentle slope and overwhelming water. With the river cutting the higher part of the ground and with the sands and pebbles silting up, the water paths of the alluvial fan are mesh-like. Whenever the great rain comes, floods devour the plain. The water channels are inconsistent and intertwining, robbing, and assailing; these are the basic features of a flood plain.
The middle and lower par t of the Lanyang River gradually slows down, and the silt settles down. After a long silting up the Lanyang Plain was formed in the shape of a nearly equilateral triangle. The broad and flat plain made,transportation so convenient that drew great populations in towns and cities. Thus Lanyang Plain became the essential part of Yilan County.