I lived for 18 years in a house which was on the bank of the Tittabawassee, in Homer Township, right at the corner of its confluence with Carl Creek. Caught a lot of carp and white sucker in the spring where the creek entered the river. Never did any fly fishing on the river, but did a lot of spoon and worm-and-bobber bank fishing ...
In all of that time, I never heard it called the "Lower Saginaw River".
Heck, my elementary school was named "Chippawassee" because of the two river confluence of the Chippewa and Tittabawassee near downtown Midland.
Pine/Chip/Titt/Flint are all part of the lower Saginaw river basin. MI-DNR has moved toward using that nomenclature recently as the whole area moves toward environmental health and they become more involved (think drinking water debacle). Locals just call the river they are standing next to by its own name.
I went to a small liberal arts private college on the Pine River and once worked for the largest dual-energy public utility in Michigan (they have blue and white trucks). Now, I work for a large multi-national chemical company. My office building over looks the Tittabawassee, upstream of a “dam” that bears the name of that large multi-national chemical company.
In the warmer months, I walk out the front door for about 30 yards and go fishing at lunch. Very few times have I gone without a catch.
I am very blessed to be where I am.