蘭陽溪流域上游，分布許多沖積扇階地，最高的本流側階地崖可達約60-100公尺，大多全部露出礫石層。這些沖積扇的發育可分為三期。第一期是大約在距今 2000 年至1500 年間的米磨登沖積扇期。 第二期是在距今約 1100 年至 700 年間的夫布爾沖積扇期。夫布爾沖積扇發育後，米磨登溪、夫布爾溪和蘭陽溪本流開始大規模下切至大約現在的位置。 第三期則是在最近約 400 年來的小規模沖積扇發育期，陸續形成美羅溪沖積扇、逸久溪溪谷沖積扇和茲那谷溪沖積扇。 影響沖積扇發育的原因可能有三，其一是區域構造抬升與流域回春；其二是氣候變潮溼或降雨型態改變；其三主要是全球海水面上升。前兩項因素引起流域剝蝕潛能增加，故對環境變化反應較快而集中產生大量沈積物的支流發育沖積扇；第三項因素造成本流搬運力減弱，所以曾集中輸出大量沈積物的支流發育沖積扇；河川的再下切則均與沈積物減少有關。至於各項因素的相對重要性，有待進一步研究。

　　　　　Many alluvial fan terraces, which are about 60 to 100 meters higer than the present riverbed, developed along the upper reach of Lanyang River. The present study divides the development of alluvial fan into three stages. The first stage, Mimodeng fan stage, is considered during 2000-1500 yr. BP. The second stage, Fubuhel fan stage, is considered during 1100-700 yr. BP. After the second stage, Lanyang River was downcut to the level near the present riverbed. The third stage is the recent 400 years, Meiluo, Ijeou, and Tzynahgu fan were formed sequentially. There are three possible reasons, tectonic uplift, climatic change and ultimate base level rising, considered as causes of the growth of main alluvial fans. The former two factors might have accelerated erosion. The alluvial fans were developed by tributaries that were sensitive to environmental changes. The third factor resulted in the aggradation of main river valley. The aggradation decreased the transportation ability of main stream, that in turn lead to the development of alluvial fans.