Barrier Lakes
Context
Nepal, which is still dealing with the consequences of severe flooding, is facing another potential flood hazard following the formation of a barrier lake near the Nepal Tibet border.
The lake is believed to have formed after a glacier related collapse produced a large accumulation of ice and rock debris. This debris blocked the drainage channel at the meeting point of the Chhocken and Purepu Tsangpo rivers, leading to the formation of the lake.
What is a Barrier Lake?
A barrier lake, also called a landslide lake, develops when material such as rocks, soil and debris from a landslide blocks the course of a river or stream.
Such landslides can be triggered by events including:
- Earthquakes
- Intense rainfall
- Glacier-related slope failures
- Other forms of mass movement in mountainous terrain
Once the river is obstructed, water begins to accumulate behind the natural debris dam. Since these lakes may not have a proper natural outlet, their water levels can rise rapidly.
If the debris barrier becomes unstable or is breached, a large quantity of stored water can suddenly move downstream. Such an event is known as a Landslide Lake Outburst Flood (LLOF).
LLOF and GLOF
An LLOF can have effects similar to a Glacial Lake Outburst Flood (GLOF), but the origin of the water barrier is different.
- LLOF: Occurs when a lake formed behind a landslide or debris dam suddenly drains after the barrier fails.
- GLOF: Occurs when water stored behind a glacier, moraine or other glacial barrier is suddenly released.
Both events can generate powerful flood waves and cause extensive damage downstream.
Why is an LLOF Dangerous?
1. Sudden Release of Large Volumes of Water
Failure of a natural debris dam can release millions of cubic metres of water within a short period. The resulting flood can transport rocks, sediments and other debris, increasing its destructive capacity. It may also trigger additional landslides along the downstream valley.
2. Difficult to Predict
The timing of a landslide dam failure is often uncertain. Some natural barriers may collapse within a few days, while others can remain stable for many years or even decades.
This unpredictability makes continuous monitoring and early-warning systems particularly important in vulnerable mountain regions.
3. Potential for Large-Scale Destruction
The combination of a sudden water release, steep mountain slopes and large quantities of loose debris can produce a highly destructive flood. Downstream settlements, roads, bridges, agricultural areas and other infrastructure may be exposed to significant risk.
Global Example
One of the most severe recorded Landslide Lake Outburst Floods occurred in 1786 in Sichuan Province, China. An earthquake triggered a massive landslide that blocked the Dadu River. The subsequent failure of the natural barrier resulted in a catastrophic flood downstream.
Significance for the Himalayan Region
The formation of barrier lakes in the Himalayas is particularly significant because of the region’s steep terrain, active tectonics, glaciers and intense rainfall events. Climate change may further increase the instability of glaciers and mountain slopes, potentially contributing to cascading hazards.
Therefore, vulnerable valleys require:
- Regular monitoring of newly formed lakes and natural dams
- Remote sensing and field based assessments
- Early warning and emergency response systems
- Hazard mapping and risk sensitive land use planning
- Careful planning of settlements and infrastructure in downstream areas
The emergence of a barrier lake near the Nepal-Tibet border highlights the interconnected nature of Himalayan hazards, where landslides, glaciers, rivers and floods can combine to create sudden and potentially devastating disasters.