The major impact that immigration has had over the entire course of US history is to make our society the melting pot that it is today. Of course, all Americans (outside of Native Americans) are immigrants in some way. We have had many major waves of immigration that have brought in large numbers of people from (in rough order that they came) Ireland, Germany, Southern and Eastern Europe, Asia, and Latin America. These people have all tended to assimilate into our society.
In addition, immigrants have helped to push our economy's development. Immigrants provided the work force that drove the Industrial Revolution. They were much of the work force that built the transcontinental railroads. They helped to settle the Great Plains.
Immigration, then, has shaped our society and our economy over the years.
I think the positive consenquences of immigration far outweighs the negative, but one negtive consenquence of immigration is racial conflict and seperation. When new social groups enter a country, exsisting social groups may feel uncomfortable or even threatened because these new groups hold values and beliefs different to theirs.
Take the construction of America's transcontinental railroads for example. Many Asain immigrants recieved less pay than whites Americans for the same amount of work in constructing railroads. This is an example of racial seperation. Whites saw themselves as superior, distancing themselves and making fun of migrants from Asia
Other examples of racial conflict and seperation include racial stereotyping, slavery, racial hate groups and segregation