It is supposed that, before construction of Heian-kyo was completed, natural water flows existed in the former Karasuma valley (古烏丸谷) (around present Karasuma-dori Street) and in the former Horikawa valley (古堀川谷) (around present Horikawa-dori Street).
平安京造営以前には、古烏丸谷（現在の烏丸通付近）や古堀川谷（堀川通付近）に自然の河川が流れていたと推測されている。