A poorly defined ridge that runs from near Chakkupallam (north of Kumily) on the border between Kerala and Tamil Nadu extending southwest to near Vallakkadavu segregates the upper reaches of Periyar from the rest of the Cardamom Hills. Immediately downstream of Mullaperiyar dam, Periyar cuts through this ridge. North of this ridge, the Cardamom Hills extend up to the high Munnar saddle. Here, east of the river channel, the undulating plateau rises up to the watershed line of the Western Ghats. This section drains into the Periyar through the Perinjankutty Ar and the drainage channel is essentially from south-east to north-west. This portion of the Cardamom Hills is on an average high. The northern edge of the Cardamom Hills bordering the High Range Plateau on the other hand slopes steeply to the south and is drained by Panniyar. The Panniyar joins the Muthirapuzha, which drains the High Range Plateau section of the Periyar Basin in the north. The crestline of the Cardamom Hills in this section along the border with Theni and Kambam, in comparison with the rest of the main watershed line of the Cardamom Hills and the High Ranges, is low, reaching up to only about . East of Chakkupallom, a small part of the adjacent Vaigai basin protrudes from Tamil Nadu into Kerala in this section. This portion of the Periyar basin is drier than all other west sloping parts of the Western Ghats. It has no significant extent of natural forests remaining anywhere. At the turn of the 20th century, it was fully covered by evergreen forests.