Only the upper reaches of Tachia Creek retained its original, level topography and lower temperatures, making it suitable for the Taiwan trout. Later, construction of a dam prevented the passage of the fish to the sea, turning it into a fresh-water trout.
唯獨大甲溪流域上游維持平緩的地形，保存了台灣鱒的生活環境；但中、下游水溫高，中游在今天的德基水庫到谷關間又形成一小斷層，阻止了鱒魚遊回大海，台灣鱒遂變成「陸封性」鱒魚。