Paleo Kathmandu Lake
Paleo Kathmandu Lake is the former lake (or lakes) which lied where Kathmandu Valley is today. Similar to the situation of Mexico City and Pokhara Valley, the valley where the lake once stood is densely populated, and highly vulnerable to both nearby and even distant earthquakes due to liquefaction and amplification of waves because of unsettled clay soil, specifically here called kalimata. The valley has the world's worst quake risk.
The valley is filled by thick lacustrine and fluvial deposits, more than 550 m thick, which is Pliocene to Pleistocene in age. The valley has a maximum 500 – 550 meter thickness of unconsolidated sediment in the central part. The quaternary sedimentary history is consistent with two major sedimentary stages. The lower fluviatile stage consisting of 200–250 m thick granular sediment is overlain by lacustrine sedimentary stage of 200-300m section of clay sediment. This sedimentary scenario existing in central part of the basin transitions into fluviatile facies in the northern part of the valley. Southern part is characterized by fluviatile and proximal lacustrine deposit.
Lake changes over time
The lake was formed over one million years ago with the tectonic uplift of the southern edge of the Valley and the damming up of the proto-Bagmati River, probably where Katuwal Daha is today. As the southern rim of the Valley rose at a geologically rapid 3 cm a year, the lake shifted northwards. The level of the lake ebbed and rose between 1,400-1,440m, depending on the prevailing climate, making it about 200m deep in places.
Geologists have found the fossil remains of the extinct ancestors of mammoths, hippos and crocodiles in the Kathmandu Valley. They have pollen-dated the sediment, indicating that the shoreline of the Kathmandu Lake was heavily forested with pine and hemlock and teeming with wildlife.
The last Ice Age cooled and dried the Himalaya and the lake started to shrink starting 30,000 years ago. The main water body carved its way through the soft limestone at Chobhar, leaving behind a shrinking lake and three smaller ones at Gokarna, Pashupati and Kitni. These lakes later carved 'mini-Chobhars' and emptied into the main lake.
As the lake receded, flats emerged above the water that were later separated by the tributaries of the Bagmati, which slicedÂ channels through the soft clay. The airport is one such flat from the Gokarna Period 30,000 years ago, and geologists think Thimi and Patan surfaced 25,000 and 18,000 years ago respectively.
The Kathmandu Valley civilisation with Lichhavi and then Newar settlements grew and prospered because of the fertility of the soil, and later because of the Valley's location along the ancient trade route between India and Tibet. In their wisdom the early settlers set up towns along ridgelines, leaving the fertile slopes for farms.
The deposits are interpreted as “Tsunami deposits” in the paleo-Kathmandu Lake that appeared at around 37-38 ka.
This lake is said to have been drained by Manjushree Bodhisattva, a Buddhist saint, by cutting open an outlet in the southern rim of the valley. As a result the valley that was created was fertile and people started cultivating here and building their homes here. As the valley grew, Manjushree is said to have worshipped Swayambhu on the hillock where the present Swayambhu temple is located.