The Grand River is a 252-mile-long river in the Lower Peninsula of the U.S. state of Michigan. The longest river in Michigan, the Grand River rises in Hillsdale County, and flows in a generally northwesterly direction to its mouth at Lake Michigan in the city of Grand Haven. The river flows through a number of cities, including Jackson, Lansing, Ionia, and Gr…The Grand River is a 252-mile-long river in the Lower Peninsula of the U.S. state of Michigan. The longest river in Michigan, the Grand River rises in Hillsdale County, and flows in a generally northwesterly direction to its mouth at Lake Michigan in the city of Grand Haven. The river flows through a number of cities, including Jackson, Lansing, Ionia, and Grand Rapids.