Complex sentence: Gradually most of the east side's white population migrated either to the west side or to adjacent townships thus making the Saginaw River the virtual dividing line for segregated racial groups in Saginaw.
Split into simpler sentences: Gradually most of the east side's white population migrated either to the west side or to adjacent townships. The Saginaw River became a dividing line for segregated racial groups in Saginaw.