The Periyar is an 8th order stream with considerable development of tributary network. The average drainage density of the basin is 2.46 km/ km2 with a total number of first order streams of 15,773. The total stream length inclusive of all orders in the basin is . The river originates from the high hills of the Western Ghats and the influence of tectonism can be evidenced from the stream pattern and presence of structural valleys. Forests occupy nearly (28%) of the basin area whereas cardamom and settlement with mixed tree crops occupy an area of and respectively. The basin is ecologically sensitive. Nearly 80% of the total area situated in the high ranges are susceptible to erosion and mass movements.