Unquity Brook has headwaters near the Milton Police Station and flows to Gulliver Creek, which flows directly into the Neponset River. 

The brook is an ecologically and economically important breeding ground for Rainbow Smelt, a small fish that is fished commercially and for sport. The smelt is also an important food source for larger Atlantic fish, including striped bass. Smelt was historically a high-population fish in New England fisheries, but smelt populations have long been declining. In 2004, the US National Marine Fisheries Service listed the rainbow smelt as a Species of Concern due to its waning population and the need for conservation efforts.