Dhansiri River: Origin, Course and Its Role in the 2026 Assam Floods

The Dhansiri River is one of Northeast India's major rivers, forming one of the principal southern tributaries of the Brahmaputra. It flows through Nagaland and Assam and plays an important role in agriculture, biodiversity and local ecosystems along its course.

The river drew attention during the 2026 Assam floods, when heavy monsoon rainfall in its upper catchment raised water levels and contributed to flooding in the state. Understanding its origin, course and tributaries helps explain how changes in its upper reaches affect areas downstream.

Origin

The Dhansiri originates in the Laisang Peak region of Nagaland, at an elevation of about 800 metres above mean sea level. It is one of the major southern tributaries of the Brahmaputra River.

The river's course runs for approximately 352 km, with a drainage area of about 12,200 square kilometres. It was historically known as Khe-Nam-Ti-Ma, a name associated with the Ahom period.

From its mountainous origin, the Dhansiri flows first through hilly terrain before entering the plains of Assam.

Course

The river's initial stretch flows in a north-western direction for about 40 kilometres through the hills of Nagaland.

It then changes direction to flow northward, forming a natural boundary between the Karbi Anglong area of Assam and the Dimapur and Chümoukedima areas of Nagaland.

In the Assam plains, the river is significant for Golaghat district. It continues downstream until it merges with the Brahmaputra at Dhansirimukh, on the Brahmaputra's southern bank.

In its lower course, the river widens, with a downstream average width of about 150 metres.

Major tributaries

The Dhansiri basin includes several tributaries and seasonal streams that feed the river, particularly during the monsoon.

Left-bank tributaries include:

• Langlong river
• Dhansiri (south)
• Diphu river (Diphupani)
• Seasonal streams of Karbi Anglong

Right-bank tributaries include:

• Dayang river, its largest tributary
• Khora river
• Beopani river
• Kaliani river

These tributaries drain the surrounding hills and contribute significant runoff to the Dhansiri.

Why the river matters

The Dhansiri holds ecological, agricultural and economic significance.

Environmental significance: The river basin lies within a biodiverse terrain and includes forests such as the Dhansiri Reserved Forest. Its catchment is linked to the wider biodiversity of Assam and Nagaland, and the basin is home to more than 34 fish species.

Agriculture and economy: The lower basin, particularly near Golaghat, is a fertile agricultural area supporting paddy cultivation and other farming. The river's waters are also important for industrial activity in the region, including at the Numaligarh Refinery site.

Link to the Kaziranga ecosystem: The lower Dhansiri helps sustain wetlands around Kaziranga National Park. Seasonal flooding maintains wetlands, swamps, oxbow lakes and bils that provide habitat for the greater one-horned rhinoceros, wild water buffaloes and migratory birds. Such seasonal flooding is a natural part of Assam's floodplain ecosystems, even though excessive flooding can be harmful to people.

Role in the 2026 Assam floods

The river became a focus of attention during the July-August 2026 Assam flood events, as heavy rainfall in its upper catchment increased runoff into the river system.

Rainfall over the hills of Nagaland can send large volumes of water downstream in a short time, which can worsen flood risk when water levels and rainfall in Assam are already high.

In early August 2026, rising water levels on the Dhansiri (South) prompted alerts and an emergency advisory in vulnerable areas. Flooding was reported in parts of Golaghat district, mainly within the Sarupathar sub-division.

Transportation routes and farming activity were affected, and a number of people were moved to relief camps as a result.

Why does the Dhansiri flood quickly?

Several geographical and environmental factors contribute to the river's flood risk.

Monsoon rainfall in the upper catchment combines with steep terrain to produce fast-flowing water into the river. Soil erosion and sediment flow can cause siltation of the riverbed, reducing its capacity to discharge water.

In Assam's plains, the river has bends and meanders, and monsoon floods in such stretches can cause substantial erosion along the banks.

Environmental challenges facing the basin

The Dhansiri basin faces several environmental threats.

Sedimentation and soil erosion are among them, with deforestation and unregulated mining in the upper reaches contributing to soil runoff into the river.

Riverbank erosion is another common issue: heavy discharge can wash away large sections of riverbank, causing loss of farmland and displacing people living along the banks.

Pollution and encroachment are additional environmental threats to the river.