Originally, 12,000–13,000 years ago, this was the glacial outwash channel that drained water away from the glacier westward, northward and finally eastward into the Atlantic some distance from where Provincetown now lies.

This upper freshwater marsh dates from the middle 19th century, when, to promote agriculture, the saltwater tides were prevented from entering by means of a dike that traverses the valley where Truro Center Road (a former routing of US Route 6) now passes.

A one-way clapper valve permitted fresh water to leave at low tide.

The result is the meandering, slow-flowing stream that flows from the Atlantic dunes at Ballston Beach west to the bay, with low, flat banks that lie just above the water table.

All plant species growing in this upper portion were brought in as seeds, mostly by birds and mammals.