In river valley reservoir projects, the gravity of the siltation problem induced due to catchment degradation is serious and needs to be suitably addressed.
For this purpose, the Ministry of Environment and Forests, Government of India have made it obligatory for the project authorities to implement physical engineering and biological measures in the catchment area of the project to be taken up pari passu (concurrently) with the implementation of the Hydropower project.
This activity involves several works defined under the 'Catchment Area Treatment' (CAT) plan.

For evolving the CAT plan, the status of the reservoir catchment was analysed.
The reservoir catchment consists of five types of forests namely, the East Himalayan Sub-Tropical wet hill forests (elevation range of 800–1,800 metres (2,600–5,900 ft)), East Himalayan wet temperate forests (elevation range of 1,800–2,700 metres (5,900–8,900 ft)), Oak and Rhododendron forests (elevated above 2,700 metres (8,900 ft)), mixed coniferous forests (in elevation range of 2,700–3,750 metres (8,860–12,300 ft)) and alpine scrubs/pastures (above 3,750 metres (12,300 ft)).
There are 35 reserve forests in the catchment, out of which 29 are in West Sikkim district and six (6) are in South Sikkim district.
The entire catchment area was analysed in detail to assess the degraded areas to be treated under the CAT plan to reduce siltation problems.
An area of 13,075 hectares (32,310 acres)) was identified for implementing engineering and biological treatment measures.
These measures were implemented, starting with 1995-96, by the Forest Department of the Government of Sikkim involving engineering treatment measures (included agricultural land, forest land and water land) and biological treatment measures.
15 nurseries were established covering an area of 18.5 hectares (46 acres)) to provide saplings/seedlings for plantation in the sub-watersheds of the catchment identified for treatment.
In river valley reservoir projects, the gravity of the siltation problem induced due to catchment degradation is serious and needs to be suitably addressed. For this purpose, the Ministry of Environment and Forests, Government of India have made it obligatory for the project authorities to implement physical engineering and biological measures in the catchment area of the project to be taken up pari passu (concurrently) with the implementation of the Hydropower project. This activity involves several works defined under the 'Catchment Area Treatment' (CAT) plan.
For evolving the CAT plan, the status of the reservoir catchment was analysed. The reservoir catchment consists of five types of forests namely, the East Himalayan Sub-Tropical wet hill forests (elevation range of 800–1,800 metres (2,600–5,900 ft)), East Himalayan wet temperate forests (elevation range of 1,800–2,700 metres (5,900–8,900 ft)), Oak and Rhododendron forests (elevated above 2,700 metres (8,900 ft)), mixed coniferous forests (in elevation range of 2,700–3,750 metres (8,860–12,300 ft)) and alpine scrubs/pastures (above 3,750 metres (12,300 ft)). There are 35 reserve forests in the catchment, out of which 29 are in West Sikkim district and six (6) are in South Sikkim district. The entire catchment area was analysed in detail to assess the degraded areas to be treated under the CAT plan to reduce siltation problems. An area of 13,075 hectares (32,310 acres)) was identified for implementing engineering and biological treatment measures. These measures were implemented, starting with 1995-96, by the Forest Department of the Government of Sikkim involving engineering treatment measures (included agricultural land, forest land and water land) and biological treatment measures. 15 nurseries were established covering an area of 18.5 hectares (46 acres)) to provide saplings/seedlings for plantation in the sub-watersheds of the catchment identified for treatment.
Rangit Dam