According to DEQ and watershed maps, the river originates just south of Potterville in what is now called the Thornapple Drain.  Snow Lake and Taylor Lake, just east of Royston Rd. and Kinsel Hwy. are the closest to origin lakes, perhaps.  These are both very small lakes, and they may not have been the original beginning points. The area around the apparent beginning point of the river is heavily channelized, with no less than 7 drains feeding into it - this area, known as the Thornapple River at Butternut Creek subbasin (not the Butternut Creek subbasin, which is west of this area) ranges from just north of Vermontville Hwy down to Long Hwy, and from Gunnell Rd. to Gidner Rd. in Eaton County.  It appears that this area was marshland before farming began.  According to the MNFI wetland change maps, quite a bit of wetland loss occured around Snow and Taylor lakes, and also from Island Hwy up toward Kinsel in a diagonal line.  This area may have been the actual origin of the river years ago.

From this origin, the Thornapple River flows westerly til just below Needmore Rd. at Stine Rd., then drops southwesterly to just below Vermontville Hwy. at Bradley Rd., then westerly through Nashville to the Thornapple Lake.  The Thornapple Lake is a natural lake in line with the river (it is not created by a dam structure).  As you probably know, the river then flows north-westerly to about Middleville, then north through Caledonia, Cascade and Ada.  Many major creeks flow into the Thornapple River both above and below the lake, contributing to its growing size from the point of origin to its mouth, where it connects to the Grand River in Ada.