Following the exceptional 1955 flood the army corps of engineers constructed 6 dams in northeast Connecticut and adjacent Massachusetts. Huge lakes like West Thompson and Mansfield Hollow were formed behind dams to help regulate the flow of water and ensure a 1955-like flood never happens again.
The dam in West Thompson effectively prevents Putnam from the Quinebaug River. Further downstream, however, in Jewett City there is less protection because of how far the city is from the dam.
The dam in Mansfield near the Willimantic Airport forms Mansfield Hollow Lake at the confluence of the Natchaug, Fenton, and Mt Hope Rivers which gives protection to the city of Willimantic.
What does all this mean?
Areas upstream of the Mansfield Hollow Dam may see serious flooding along the Mt. Hope, Natchaug, and Fenton Rivers. Areas downstream of the Mansfield Hollow Dam (i.e. Willimantic) are protected and will be ok tonight and tomorrow.
The most vulnerable areas will be the ones outside of flood protection, namely the Yantic River in Norwich and southern parts of the Quinebaug River from Jewett City south toward Norwich.
At the river gauge along the Natchaug River in Willimantic downstream of the
dam (in flood protection) you can see the river levels actually falling as the dam is preventing water from leaving the Mansfield Hollow Lake even though the rain is piling up.