The Saw Kill grew from one of the many tributaries of Lake Albany, a proglacial lake that filled most of the Hudson Valley north of the Hudson Highlands centered on the present-day location of Albany during the Wisconsinan glaciation, the last Ice Age, 12-15,000 years ago. At the time of its existence the lake filled the valley up to what is now the contour line, roughly where the Saw Kill today drops through a series of cascades to Camp Rising Sun in Red Hook. The retreat of the lake as it flowed southward to become today's river left the glacial outwash plain the stream flows through from there to the river's current escarpment at Annandale and eventually into the Hudson.