Limnology report on the Teesta River

by Rohan Nath

Introduction

In India, Teesta is one of the major rivers that originate from Himalayan glaciers along with Brahmaputra, Ganga, Jhelum, and Sutlej (Fig. 1). More than 1.3 billion people living downstream depends upon the major rivers system of the Himalayan glaciers for water. The Teesta River originates at an elevation of 5033-m from the Tso Lhamo Lake in North Sikkim. Khangse glacier, Pahunri glacier and Chho Lhamo Lake are also considered as the source of the Teesta River.

**Fig. 1.** Teesta Basin and its streams with local points. Image Source: Goyal, M. K., & Goswami, U. P. (2018). Teesta river and its ecosystem. In The Indian Rivers (pp. 537- 551). Springer, Singapore.

Geography

Snow and glaciers cover the upper portion of the catchment area of Teesta River and the lower portion is covered with forest. Canyons and narrow valley in Sikkim and highlands of Kalimpong is a result of the Teesta River flow. The vegetation cover changes with elevation, from tropical deciduous vegetation in the lower elevation zone to alpine vegetation in the high elevation zone. The Teesta basin in Sikkim is divided into five geo-eco-climatic zones based upon the geo-morphological ecological and climatic regimes (Table 1).

Table 1.

Sl. No.	Zone	Elevation
1.	Sub-tropic zone	Up to a 1000-m elevation
2.	Warm temperate zone	Between 1000 and 2000 m
3.	Cold temperate zone	Between 2000 and 2500 m
4.	Cold zone	Between 2500 and 4000 m
5.	Frigid zone	Above 4000 m

Hydro-climatology

Heavy rainfall and floods dominate the Teesta River basin in the monsoon season. It can lead to landslides, slope transformation, and erosion which deposits suspended sediment in the river channel. The huge variation in the elevation from 8598 to 213 m within 100 km is the primary reason for an abrupt change in the climate. The Teesta River basin has an average annual rainfall ranging from 2000 to 5000 mm. The rainfall varies throughout the seasons (Table 2).

Table 2.

Season	Rainfall
Winter	0.7%
Summer	13.6%
Monsoon	80.2%
Post-monsoon	5.5%

Slope variation

The Teesta Basin slope profile varies from 8598 m to 213 m (Fig. 2).

Characteristics of soil

The diversity of soil type is large in the Teesta basin (Fig. 3). In the hilly terrain, random construction, extensive deforestation, random use of land, slope cultivation and improper drainage system are the common issues. To save the natural resources in mountainous regions, it is crucial to practice water and soil conservation.

**Fig. 2.** The variation in slope in the Teesta Basin. Image Source: Goyal, M. K., & Goswami, U. P. (2018). Teesta river and its ecosystem. In The Indian Rivers (pp. 537- 551). Springer, Singapore.

Socio-economic and environmental characteristics

Large infrastructure projects like a dam and/or hydro-power projects help in the economic development but affect the environment and the socio-economic values for downstream communities. Few favourable/unfavourable effects are:

i. The construction changes land use and increases soil erosion.

ii. The disposed construction material degrades the water quality.

iii. The hydrologic regime, siltation and sedimentation are disturbed.

iv. Indiscriminate fishing and migrant fish species increase the pressure on aquatic ecology.

v. The power generation increases employment and revenue.

**Fig. 3.** The soil characteristics of Teesta Basin. Image Source: Goyal, M. K., & Goswami, U. P. (2018). Teesta river and its ecosystem. In The Indian Rivers (pp. 537-551). Springer, Singapore.

Natural hazards

Natural hazards like landslides, floods, glacial lake outburst floods, and drought are most likely due to climate change and global warming. Sikkim is prone to earthquakes and landslides and hence is the most vulnerable zone for natural hazards. Numerous landslides in Sikkim Himalaya affect the river morphology and hydrology.

Many waterbodies and glacial lakes in the Sikkim Himalayas are vulnerable to outburst. This can change the hydrology and geomorphology of the river system.

Conclusion

The Teesta River originates with a high altitude of 5033m and forms the right-bank tributary of the Brahmaputra River. Due to the high altitude, it has a high hydroelectric power production potential. Six hydroelectric projects are proposed in the Teesta River with the Teesta stage II hydropower project being the largest power generation project in Sikkim (Table 3). Hence, understanding the hydrological process and water resources management and planning are important.

Table 3.

S. No.	Name of project	Capacity (MW)
1.	Teesta Hydro-electric Project Stage-I	280
2.	Teesta Hydro-electric Project Stage-II	480
3.	Teesta Hydro-electric Project Stage-III	1200
4.	Teesta Hydro-electric Project Stage-IV	495
5.	Teesta Hydro-electric Project Stage-V	510
6.	Teesta Hydro-electric Project Stage-VI	440