Abstract
The Naini Lake is a product of rotational movement on a NW-SE trending Nainital Fault, quite after the establishment of the drainage of a mature stream named Balia Nala. Detailed bathymetric study, permits division of this crescent-shaped lake into two subbasins separated by an approximately 100 m wide transverse underwater ridge, 7 m to 20 m below the lake surface. The southeastern and northwestern parts, 11 to 25 m and 4 to 27 m deep, respectively, are filled with unconsolidated sediments, 15 m thick on average. The semiconsolidated sedimentary layers locally exhibit slumping on slopes and deformation in channel beds. These features, together with the transverse Central Ridge with a deep channel, the small scarps and terraces and its thick filling in parts due to blockade resulting presumably from fault movements, and the existence of deformed and displaced acoustic reflectors are interpreted as indicating quite recent movements along the National Fault. Hitherto unidentified pinnacle-like objects penetrating through or perched atop sediments, are suggestive of either tectonic displacement and squeezing or of the hazard of mass movements which have occurred. A number of small but rapidly growing deltas along shores indicate accelerated pace of sediment input, resulting from quickened erosion in the catchment area.