Garrett Nelson and Alasdair Rae's recent analysis of American commuter flows, An Economic Geography of the United States: From Commutes to Megaregions, identified the Pittsburgh megaregion as a region encompassing the entirety or significant portions of 54 counties in Western Pennsylvania, Eastern Ohio, Northern West Virginia, and Western Maryland. By this definition, the informal regional boundaries are similar to historical interpretations where the region is defined as the central portion of the Allegheny Plateau to the west and north of the Allegheny Front and south of Lake Erie and Pennsylvania's Northern Tier. The hills and river valleys along the Upper Ohio River and its many eastern tributaries, including the Allegheny and Monongahela rivers have historically been the major centers of population of the region.