The term physiography refers to the actual existing conditions of the land, depending upon the structures, formational changes and natural agencies. Chennai aquifer system comprises interdependent river basins of Araniar, Kosathalayar, Cooum and Adyar. All these 4 rivers are west to east flowing rivers. Buckingham canal which is a man-made canal runs from north to south. Araniar and Kosathalayar originate from Andhra Pradesh whereas the Cooum and Adyar originate from surplus courses of Cooum tank in Tiruvallur Taluk and Chembarambakkam Tank in Sriperumbudur taluk. Nearly 60% of the region is covered by plains. These are smooth surfaces which experienced erosion and form the inland topography, where sediments like laterite, limestone have been washed away by sedimentation. Flood plains consisting of sand clay are found along the boundaries of Arabia and Kosasthalaiyar rivers. Agricultural land occupies 61% of the Chennai aquifer system.
Drainage Map, Chennai aquifer system
Adyar River originates at the confluence (Thiruneermalai) of the two streams that drain the upstream area of Chembarambakkam tank (Gayathri, 2014). The river flows year-round with an average discharge of 89.43 MCM/ year at Kathipara causeway. It drains the southern part of the district and remains flooded during monsoon. During high tide, the backwaters from Bay of Bengal enter inland up to 3-4 km. Cooum is the other river flowing through the middle part of the district and carries only drainage water which is highly polluted. The flow of the Cooum River at Korattur is 40.2 MCM/year for an average 31 days in a year. Otteri Nullah is another small stream flowing in the northern part of the city. Buckingham canal was the main man-made channel used for navigational purposes in the area north of Ennore, but currently acts as sewerage carrier in the city.