Before Lake Oyamada was formed, there had been mountains (古琵琶湖山脈) in the current location of Lake Biwa; the Suzuka Mountains had not yet risen, and the rivers in the southeast of the present-day Lake Biwa flowed out to Ise Bay.
大山田湖以前、現在の琵琶湖の位置には山（古琵琶湖山脈）があり、鈴鹿山脈は未だ隆起せず、今日の琵琶湖東南部の河川は伊勢湾へ流れていた。