The eastern portion of Waukesha County has a major sub-continental divide as shown by the line in the map. Water falling west of this crest of land goes into the Fox River watershed and eventually into the Gulf of Mexico via the Mississippi River. Water falling on the east side goes into the Menomonee River watershed and enters the St. Lawrence Seaway by flowing through the Great Lakes.
Wikipedia: Sub-continental Divide
PDF Map of Location of Subcontinental Divide in Southeastern Wisconsin