the Merrimack, Nashua, and Concord rivers, and other streams. The MetroWest region comprises much of the southern portion of the county. Adjacent counties Hillsborough County, New Hampshire (north) Essex County (northeast) Suffolk County (southeast) Norfolk County (south) Worcester County (west) Transportation These routes pass through Middlesex County , From Hopkinton to Newton , From Somerville to Tewksbury , From Newton to Wakefield , In Marlboro , From Hopkinton to Tewksbury , From Cambridge to Malden /Route 3, From Cambridge to Tyngsborough , From Marlborough to Watertown , From Littleton to Cambridge , From Shirley to Cambridge , From Burlington to Tyngsborough , From Lexington-Arlington line to Chelmsford , From Framingham to Newton , In Townsend , From Holliston to Everett , From Sherborn to Chelmsford , From Cambridge to North Reading , From Framingham to Newton , In Ashby , From Somerville to Dracut , From Groton to Chelmsford , From Waltham to Malden , From Hudson to North Reading , From Hopkinton to Hudson , From Everett to Melrose , From Ayer to Dracut , From Concord to Pepperell , From Pepperell to Dracut , In Sherborn , From Stow to Waltham , From Concord to Ashby , From Wilmington to North Reading , From Holliston to Concord , From Newton to Wakefield , From Chelmsford to Wakefield , From Lowell to Tewksbury , From Hopkinton to Natick , From Shirley to Lexington National protected areas Assabet River National Wildlife Refuge Great Meadows National Wildlife Refuge Longfellow House–Washington's Headquarters National Historic Site Lowell National Historical Park Minute Man National Historical Park Oxbow National Wildlife Refuge (part) Demographics , Middlesex County was tenth in the United States on the list of most millionaires per county. As of the 2010 United States Census, there were 1,503,085 people, 580,688 households, and 366,656 families residing in the county. The population density was . There were 612,004 housing units at an average density of . The racial makeup of the county was 80.0% white, 9.3% Asian, 4.7% black or African American, 0.2% American Indian, 3.3% from other races, and 2.5% from two or more races. Those of Hispanic or Latino origin made up 6.5% of the population. The largest ancestry groups were: 23.5% Irish 16.2% Italian 11.2% English 7.1% German 5.6% French 4.0% Polish 3.6% French Canadian 3.2% Chinese 3.1% Portuguese 2.9% American 2.7% Scottish 2.6% Russian 2.5% Indian 2.4% Brazilian 2.0% Scotch-Irish 2.0%