Why did early civilization arise in the valleys of the tigris and euphrates rivers ?

Because those areas are very fertile for crop growth, and water is easily avaolable

Answermost of the time it would be ground into fine sediment,and sometimes they are carried away from the original location;

The first immigrants to settle in what is now the united states were the ancient ancestors of modern native americans. the precise routes of those first north american immigrants are disputed, but there is no uncertainty about the fact that every human being on the continent is either a recent immigrant or a descendant of earlier immigrants. since the first immigrants came here from asia more than thirteen thousand years ago, there have been four large and easily recognizable modern waves of immigrants into the united states.