Lake Ulzhay, Salt lake and protected natural area in Cherlak District, Omsk Oblast, Russia

Lake Ulzhay is a relic drainless salt lake in the Cherlak District of Omsk Oblast, 25 km northeast of the settlement of Cherlak, in the northwest of the Kurumbel steppe. It is an oval 4.3 km long and 4 km wide, with a water surface of 14.5 km² and depths of 0.3 to 1.3 m. Its northern shore rises in three terraces 6–8 m above the water; the eastern shore is cut by ravines.

Mineralized lakes of the West Siberian plain formed in the Quaternary period, when a glacier advancing from the north dammed the rivers of the Ob-Irtysh basin and created a vast freshwater sea that later evaporated into separate basins. Ulzhay lies in the Kurumbel steppe of Omsk Oblast's Cherlak District, 25 km northeast of Cherlak. In 1978 it was declared a water monument of nature, and its therapeutic mud reserves, 876,000 m³ as of 1 January 2011, are recorded in the oblast's mineral balance.

The lake's name also appears as Ul'dzhay. Its black, highly plastic mud is used therapeutically: clinical studies reported activity against diseases of internal organs, the nervous system and the musculoskeletal system, and the mud is bactericidal against Staphylococcus aureus and Pseudomonas aeruginosa. Bathers and mud users come for these properties.

The lake lies 25 km northeast of Cherlak in the northwest part of the Kurumbel steppe, reached by road from Omsk via Cherlak. The northern shore rises in three terraces 6–8 m above the water, while the other shores are low. Mud lies 1–2 m down, in a layer up to 0.6 m thick, so the bottom is soft underfoot.

The mud layer covers about 9.27 km² of the lake floor, lying 1 to 2 m deep and up to 0.6 m thick. Its clay skeleton, 74–88% of dry mud, consists mostly of particles under 0.01 mm, which explains the mud's high plasticity and viscosity.