The Indigirka forms a large delta, consisting of a number of streams (each one being labeled on Russian maps as a "photo ka" (river arm)) and islands. About before reaching the East Siberian Sea (), the river splits into two major northeast-flowing streams. The left (westernmost) arm is known as the "Russko-Ustyinskaya Protoka"; the right arm, the "Srednyaya Protoka" (Russian for the "Middle Arm"). Further downstream, the third major arm, the "Kolymskaya Protoka" splits off the "Srednyaya Protoka" as its right (eastern) distributary, thus justifying the "middle" moniker for the Srednyaya Protoka.