The East Fork Foss River originates at , from the meltwater of Lynch Glacier on the north slope of Mount Daniel. It gathers tributaries from glaciers on the north slope of Mount Hinman as well as tributaries flowing from the lakes of Necklace Valley such as Lake Ilswoot, Locket Lake, Opal Lake, La Bohn Lakes, Alturas Lake, and others. The East Fork Foss River flows generally north, joining the West Fork to form the main stem Foss River.
