The existence of water sources, especially rivers, is one of the most important factors in the site selecting. But from the past until now, the establishment of settlements by the rivers, despite countless blessings and benefits, had also some risks. In this research, using the study of sub-basins in the southeast of the Caspian Sea, firstly, the relationship between the settlement pattern of ancient sites and drainage networks has been investigated, then, by estimating the flooding risk of the sub-basins based on the linear, areal and relief aspects, for the first time, the relationship between the flood risk and the settlement pattern of the ancient sites has been analyzed. The results demonstrate the importance of proximity to rivers in the site selection and based on this, the distances close to rivers with lower stream order are the most ideal areas for establishing settlements. The relationship between flooding risk and settlement pattern of the sites showed a decrease in the frequency of settlements next to the higher stream orders and on the other hand, an increase in the frequency and density of ancient sites near the first stream orders and at further distances from the rivers, along with the increasing the flooding risk in the sub-basins. these results demonstrate the understanding of the ancient inhabitants of the southeastern areas of the Caspian Sea sub-basins about the danger of flooding and, as a result, taking this into account in site selecting.