It is formed in northern Tolland County, near Stafford Springs by the confluence of Middle River and Furnace Brook.

It flows south to the city of Willimantic, where it joins the Natchaug River to form the Shetucket.

It is commonly translated as "land of the swift running water", but the word more likely originally meant "place near the evergreen swamp".

[5] The word was first attested in English writing as Waramanticut in 1684,[6] and later as Wallamanticuk, Wewemantic and Weammantuck before being standardized as Willimantic.

[7] Shortly upstream from its confluence with the Natchaug, the Willimantic experiences a drop of ninety feet in one mile.