Indigirka – a river in Yakutia, which flows into the East Siberian Sea. It is 1,726 km long. The area of its basin is 360,000 km2. The area of its delta covers about 5,500 km2. It is formed by confluence of the Tuora-Yuryakh and the Taryn-Yuryakh rivers that flow down from the northern slopes of the Khalkan Range. Its basin lies in-between permafrost, northward of 70° N in tundra zone; hence, immense icing is characteristic for its rivers (in the basin of the Moma, a tributary of the Indigirka, Ulakhan-Taryn is placed – the largest icing in Russia with the area of 80–90 km2 and maximum ice thickness of 5–6 m). In relation to the structure of the valley, bed, and flow rate, the Indigirka can be broken into two reaches: upper mountain (640 km) and lower plain (1,086 km). In its headwater the Indigirka flows along Yana-Oymyakon Highland and then in crevasse cuts a number of mountain ridges of the Chersky Range system leaving riffles behind, lower reach – at Yana-Indigirka Lowland. The mouth of the river is deltaic and composed of estuarine stretch, about 200 km in length, and open estuarine flat coastline. The top of the estuarine area lies 200 km away from the sea – higher than Chokurdakh settlement where large surges can come. The head of the delta is located 130 km from the sea. Its width along the seaside is about 150 km. There are three main arms (Protoka) in the delta, Russko-Ustinskaya, Srednyaya – the greatest one – and Kolymskaya, as well as many minor branches building up around hundreds of islands. The Srednyaya Protoka is the natural riverbed extension that gets more than 50 % of liquid runoff and, correspondingly, the bulk of suspended load with the total amount of 19 million tons. The surface of the delta is low and muskeg with many lakes of both residual (Ilmen type) and thermokarst origin. Substantial part of deltaic surface is submerged at the time of surges. Waves advance 60 km upstream the Srednyaya Arm with surge as high as 1 m. Deposits of longshore bars make up the most deltaic area between arms, alluvial deposits being distributed in narrow stripes along the arms.
Down the source Bosyakov, Kolyma, and Maydan branches go off that arm and leftward – Kitinskiy and Malenkiy branches. Through low islands (Nemkov, Smerti, Derevyanniy, and others), the Srednyaya Arm in a number of branches meets the estuarine coastline; the leftmost one (Nemkov) forms an extensive bar 20 km long. Water is 1.5–2 m deep at the bar crest. In 1974, a short navigation passage was cut through this bar – 7 km long, 40 m wide, and 2.1–2.7 m deep. In subsequent years the passage was drifted, especially during seasonal floods. The surface of the delta is low and swamped; multitude of lakes is present, thermokarst origin predominately. Deltaic islands are composed of sand-shale sediments with ice lens.
The Indigirka is nourished by rain and melt (snow, ice, and aufeis) waters. Main tributaries of the low reach are Selennyakh, Uyandina, Allaikha, and Byoryolyokh (on the left) and Nera, Moma, and Badyarikha (on the right). Annual runoff into the mouth is 54.2 km3/year. The runoff in spring is 32 %, in summer 52 % (seasonal flood in June–early July), and in autumn 16 %. The average water discharge downstream is 1,750 m3/s and maximum 11,500 m3/s. The flow of solid matter equals to 13.7 million tons per annum. River breathing is up to 11 m. Ice covers the Indigirka from October to early May–early June. In winter it freezes through in some stretches. It is navigable from the embouchure of the Moma (1,154 km). Main harbors are Khonuu, Druzhina, Chokurdakh, and Tabor. The Indigirka is abounding in fish, in the mouth – fishery of vendace, broad whitefish, muksun, nelma, omul, and cisco.
The Indigirka basin has gold mines.