At the center of the preserve, the freshwater stream that bisects the wild swamp and tangled woodland are impounded by a weir. On the downstream side, it becomes a shallow, bubbling rill, while the upper section widens out into acres of open picturesque swampland, with stunted red maples and carpets of sphagnum moss.
Like many natural areas on Long Island, Sagg Swamp has a long and interesting history of human use. From the 1600s to the 1800s, various gristmills, where grain was pounded into flour, and fulling mills, where cloth makers cleaned and thickened wool, were constructed and operated with limited success near the bridge on Sagaponack Road. There was even a brewery on the banks of the stream. When the last of these attempts failed around 1850, the area returned to swampland.