Seti River

The Seti River is a tributory river of the Sapta Gandaki River (called Narayani after Devghat Dham). This river is located in Western Nepal, now Gandaki Province. This river rises from the base of the Annapurna massif, and flows south and south-east past Pokhara and Damauli to join the Trishuli River near Devghat. The geological shapes and forms are made over various pre-historical time periods. The moraine from Annapurna Himalayas after the burst of glacial lakes at various prehistorical time periods, were deposited on the routes of the river causing the natural lakes.

Near around Pokhara and Tanahun, the river has made a lot of historical erosion causing the formation of gorges. The river has carved a deep, narrow chasm through the middle of pokhara turning the watermilky white in the processes. A gorge is a narrow valley with steep, rocky walls located between hills or mountains. Also the canyon is used for larger than gorge although both words are used to describe deep, narrow valleys with a stream or river running along their bottom. The main reason of gorge formation is erosion due to the flow of river water over a long periods of time. The streams carved through hard layers of rock, breaking down or eroding it. Over time, this erosion will form the steep walls of a gorge. Gorges are formed also from geologic uplifts, movement and melting of glaciers, etc.

The Seti River basin belongs to a high mountain and a humid subtropical climatical zone. In the water shade of the Seti River 80% of rainfall (Precipitation) occurs during Summer season (June-Sept.) due to the influence of the southwest monsoon. The Seti River brings down heavy sediment of limestone and has formed several gorges in an around the Pokhara to the Bimad making it even more vulnerable. The geological features of the Seti River gorges are narrow and deep channel consisting of mostly gravel mixed with lime and soil (to act as a sediment strata) with very vertical drop slope. Such type of slope is very vulnerable when heavy mansoon occurs. In such condition, due to the water infiltrated or percolated into the GMS, there occurs the saturation of soil and ground water pressure causes losen of the interlocked each grain particle and instability to the slope.

Read about the Seti River Flood disaster in 2012

Seti River has the historical events of disasters causing and forming different types of geological significances. In 2012, there was a big devastation causing loses and deaths. According to Jeffrey Kargel, a University of Arizona hydrologist, this happened due to series of events. The slurry of sediments, rocks and water suddenly surged through the valley on May 5th, 2012. It began weeks before the flood with a series of rockfalls that sent debris tumbling into the Seti River, backing water up in the extremely deep and narrow gorge. The situation grew dire, when an unusually powerful ice avalanche and rockfall tumbled down a vertical cliff on a ridge just south of Annapurna IV. The total drop from the Annapurna IV ridge line to the bed of the Seti River is about 6,100 meters (20,00 feet) spread over a distance of only 40 km - (more relief than anywhere in the continental US). There was enormous amount of force accompanied the avalanche, so powerful that the huge amount of snow melted due to the friction, and it produced a blast of hurricane-force winds that flattened old-growth forest near the Seti River gorge. As the force of the avalanche and winds poured into the gorge, it overwhelmed the natural dam created by the earlier rockslides. The dam burst and sent a surge of pent up water and avalanche debris rushing downstream.

References

• Wikipedia: Seti Gandaki River
• nationalgeographic.org
• Upgrading feasibility study on the Upper Seti Storage Hydroelectric Project in Nepal
• earthobservatory.nasa.gov, making sense of Nepal's Seti River disaster.