The primary features of Franklin-Butternut Lakes are a number of small undeveloped lakes and several old-growth hemlock-hardwood stands in a hummocky pitted outwash terrain. The lakes are mostly of the soft water seepage type and range from Sunfish Lake, with its sandy bottom and sterile rosette flora, to a string of bog lakes on the north side of Franklin Lake. Included in the complex is the Bose Lake Hemlock-Hardwoods Research Natural Area, a fine old-growth stand with no evidence of cutting. Mature hemlock, sugar maple, yellow birch, and basswood dominate the forest with paper birch, balsam fir, and red maple are among the other trees present. The forest floor has an accumulation of fallen trees, tip-up mounds, and many cradle knolls. A relatively recent blowdown occurred on the south end of the stand with many large trees on the ground, all of which fell in an ENE direction. The resulting canopy gaps are filled with sugar maple seedlings and saplings. Characteristic groundlayer species are intermediate wood fern, wild sarsaparilla, large leaved aster, Canada mayflower, and club-mosses. Two rare lichens that require old-growth conditions have been found here. Several interconnected stands of hemlock-dominated old growth are on the isthmus between Franklin and Butternut Lakes. The Franklin Nature Trail winds through a portion of them. Smaller stands of old growth hemlock are found on the east side of Sunfish Lake, on the landward end of the sand spit on the north side of Franklin Lake, and on a long esker-like ridge in the vicinity of Two Dutchmen Lake. A few small stands of mature red and white pine occur on the dryer sandy loam soils, while most of the better hardwood and hemlock stands are on silt loams. The hardwood stands are generally sugar maple-dominated with yellow birch, basswood, and white ash as common associates. Bogs, black spruce-tamarack swamps, and small areas of bluejoint grass occupy the lowlands. Ecological gradients, for the most part, are intact. North of Franklin Lake, separated from the main body of the complex, is an area of bogs, bog lakes, and associated conifer swamps set in sandy, cut-over uplands with occasional intact groves of hemlock and red and white pine. Many of the wetland types here are of very good quality. A sand spit anchored by alder and red maple extends far out into Franklin Lake. The lakeshore to the north of the sand spit consists of a long beach ridge and a narrow damp swale supporting mature white pine with royal fern beneath. The lakeward side of the beach ridge is bordered by alder grading to a large floating sedge mat with a good mix of species. The numerous large old trees and snags situated near lakeshores provide important nesting sites for bald eagles and the lakes see much use by common loons. Other characteristic birds include blackburnian and black throated green warblers, red breasted nuthatch, winter wren, and pileated woodpecker. Originally designated as the 25-acre Bose Lake State Natural Area in 1974, Franklin and Butternut Lakes is owned by the U.S. Forest Service and was expanded in 2007. This site is also recognized by the Forest Service as an established Research Natural Area.