Silent Valley National Park a total area of 89.52 km² is located in the Nilgiri Hills, Palakkad district, Kerala, in South India. The park is one of the last undisturbed tracts of South Western Ghats montane rain forests and tropical moist evergreen forest in India. The park is contiguous with the proposed Karimpuzha National Park (225 km²) to the north and Mukurthi National Park (78.46 km²) to the north-east. It is the core of the Nilgiri International Biosphere Reserve (1,455.4 km²) and is part of The Western Ghats World Heritage Site, Nilgiri Sub-Cluster (6,000+ km²) under consideration by UNESCO.The visitors' center for the park is at Sairandhri.
Silent Valley is rectangular, 7 km (east-west) X 12 km (north-south). It is separated from the eastern and northern high altitude plateaus of the (Nilgiris Mountains) by high continuous ridges including Sispara Peak (2,206 m) at the north end of the park. The park gradually slopes southward down to the Palakkad plains and to the west it is bounded by irregular ridges. The altitude of the park ranges from 658 m to 2328 m at Anginda Peak, but most of the park lies within the altitude range of 880 m to 1200 m. Soils are blackish and slightly acidic in evergreen forests where there is good accumulation of organic matter. The underlying rock in the area is granite with schists and gneiss, which give rise to the loamy laterite soils on slopes.
The Kuntipuzha River drains the entire 15 km length of the park from north to south into the Bharathapuzha River. Kuntipuzha River divides the park into a narrow eastern sector of width 2 kilometers and a wide western sector of 5 kilometers. The river never turns brown and is always crystal clear, perennial and wild. The main tributaries of the river, kunthancholapuzha, Karingathodu, Madrimaranthodu, Valiaparathodu and Kummaathanthodu originate on the upper slopes of the eastern side of the valley. The river is uniformly shallow, with no flood plains or meanders. Its bed falls from 1,861 m to 900 m over a distance of 12 km, the last 8 km being particularly level with a fall of only 60 m. Kuntipuzha is one of the less torrential rivers of the Western Ghats, with a pesticide-free catchment area.
Silent Valley gets copious amounts of rainfall during the monsoons, but the actual amount varies within the region due the varied topography. The mean annual rainfall ranges from over 5000 mm in the Neelikal area in the west to around 3200 mm on the eastern side of the park. The park being completely enclosed within a ring of hills, has its own micro-climate and probably receives some convectional rainfall, in addition to rain from two monsoons. In general the rainfall is higher at higher altitude and decreases from the west to east due to the rain shadow effect. Eighty per cent of the rainfall occurs during the south-west monsoon between June and September. It also receives significant amount of rainfall during the north-east monsoon between October and November.
The mean annual temperature is 20.2 °C. The hottest months are April and May when the mean temperature is 23 °C and the coolest months are January and February when the mean temperature is 18o C. Because of the high rainfall, the relative humidity is consistently high (above 95%) between June and December.
Valley areas of the park are in a Tropical and subtropical moist broadleaf forests Ecoregion. Hilly areas above 1,000 m are in a South Western Ghats montane rain forests region. Above 1,500 m, the evergreen forests begin to give way to stunted forests, called sholas, interspersed with open grassland. Both are very important to naturalists, biologists and other researchers because the rich biodiversity here has never been disturbed by human settlements. Several threatened species are endemic here. New plant and animal species are often discovered here. Throughout human history about 10% of the genetic stock found in the wild has been bred into palatable and higher yielding cereals, fruits and vegetables. Future food security depends on the preservation of the remaining 90% of the stock through protection of high biodiversity habitats like Silent valley.
Six distinct tree associations have been described in the valley. Three are restricted to the southern sector: [(Cullenia exarillata & Palaquium ellipticum)], [(Palaquium ellipticum & Mesua ferrea(Indian rose chestnut)] and [(Mesua ferrea & Calophyllum elatum)]. The remainder are confined to the central and northern parts of the Park: [(Palaquium ellipticum & Poeciloneuron indicum)], [(Calophyllum elatum & Ochlandra sp.)] and [(Poeciloneuron indicum & Ochlandra sp.)].
A study of natural regeneration of 12 important tree species of Silent Valley tropical rain forests showed good natural regeneration of all 12 species. The species studied were Palaquium ellipticum, Cullenia exarillata, Poeciloneuron indicum, Myristica dactyloides, Elaeocarpus glandulosus, Litsea floribunda, Mesua nagassarium, Cinnamomum malabatrum, Agrostistachys meeboldii, Calophyllum polyanthum, Garcinia morella and Actinodaphne campanulata.
Recent selective felling of three trees per acre, has led to the cutting of 48,000 m³ of timber from about 20 km². In 1996, Impatiens sivarajanii, a new species of Balsaminaceae was discovered in the park. There is a huge hollow Kattualying tree here which can fit 12 people inside.
On June 6, 2007 the Kerala cabinet approved the buffer zone proposal. The new 147.22 km² zone will include 80.75 km² taken from Attapady Forest Range, 27.09 km² from Mannarkkad Forest Range and 39.38 km² from Kalikavu Forest range and consolidated to form a new range, Bhavani Forest Range, of 94 km² and 54 km² would be brought under the existing Silent Valley Range of the National Park.