One look at the map to the right should be all that is necessary to see there is a big barrier to any immediate migration down the West Coast due to the rugged mountains.  After crossing the Bering Land Bridge, the best path is Eastward across Alaska's Northern Coast past the Brooks Mountain Range. The travelers will then be in the MacKenzie Water Basin where it is possible to head South without encountering any further obsticles.

Place your mouse over the map to see the route described above. The Yellow arrow's tip points to Portage LaLoche.