Beginning in the 19th century, Berwick had a symbiotic economic relationship with Somersworth, New Hampshire, the mill town to which it is connected by bridge. According to the United States Census Bureau, the town has a total area of , of which, of it is land and is water. Berwick is drained by the Little River and Salmon Falls River. Diamond Hill, elevation 490 feet (149.4 m) above sea level, is the town's highest point. The lowest elevation, which is approximately 70 feet (21.3 m) above sea level, is on the Salmon River as it crosses the southern most town border with South Berwick. The town is served by state routes 9 and 236. This climatic region is typified by large seasonal temperature differences, with warm to hot (and often humid) summers and cold (sometimes severely cold) winters. According to the Köppen Climate Classification system, Berwick has a humid continental climate, abbreviated "Dfb" on climate maps. As of the census of 2010, there were 7,246 people, 2,749 households, and 2,029 families residing in the town. The population density was . There were 2,934 housing units at an average density of . The racial makeup of the town was 96.2% White, 0.5% African American, 0.2% Native American, 1.3% Asian, 0.2% from other races, and 1.6% from two or more races. Hispanic or Latino of any race were 1.3% of the population. There were 2,749 households, of which 36.9% had children under the age of 18 living with them, 58.1% were married couples living together, 10.7% had a female householder with no husband present, 5.0% had a male householder with no wife present, and 26.2% were non-families. 20.6% of all households were made up of individuals, and 6.6% had someone living alone who was 65 years of age or older. The average household size was 2.62 and the average family size was 3.01. The median age in the town was 39.1 years. 25.2% of residents were under the age of 18; 6.7% were between the ages of 18 and 24; 26.9% were from 25 to 44; 30.1% were from 45 to 64; and 11% were 65 years of age or older.