Yilan slants towards the east and the Piyanan Fault cuts diagonally through from northeast to southwest. Except for where the Lanyang River originates from Siyuan Pass, runs parallel to the fault upstream, and encounters fewer bends, large falls are common elsewhere along the rivers because of the fault. Formation of alluvial fans occurs at midstream. Flat low lands downstream tend to create alluvial deposits and meanders are created as a result. The river runs underground during the dry season to become an ephemeral river and again bursting into a flood in the rain season.
