An excerpt from
Department of Translation Studies, University of Tampre
http://www.uta.fi/FAST/US1/REF/us1refs.html
Languages in contact (as opposed to dialects in contact) rarely interact except in extremes and never in phonology. Rather, these may be borrowings, which are eventually integrated into the target language (corduroy, diapers, etc.) or else one language obliterates the other, as in Latin taking over Gaulish, or English taking over Irish.
The principles are the same for any language which "emigrates," as with English, French, Spanish, and Portuguese into North America. First, the emigrant languages start to evolve from a specific homeland dialect. Then, they evolve differently from the homeland because of lack of contact. The less contact, the more different the evolution because the home dialect continues to evolve.
The US kept less of a "British" accent because it threw off British dominance at a rather early stage, which Australia and New Zealand did not. The educational system in those countries continued to import teachers and administrators from "home," as the British linguistic model was considered "superior."
Many of the distinctive phonetic features of American English are in fact from the British Isles. The feature that most Americans recognize as distinctively English is the nearly silent final 'r', making "water" sound like "watuh." This pattern is characteristic of a dialect triangle formed by Cambridge, Oxford, and London... The same pattern is found in areas of the US and Canada which were settled by emigrants from eastern England, such as Boston and Plymouth.
The final 'r' that we associate with American English is quite distinct as a kind of growl produced near the back of the mouth. This sound is standard in the west of England (Shakespeare would have growled his final 'r' sounds), in the north (Wordsworth and the Brontes), and in Ireland. Most of the British settlement in North America in the nineteenth century came from the north and west of England and from Ireland. This emigration provided the dominant settlement and the dominant dialect features of Ontario and of the mid-western US.
