The river is the bay's front door. It is where the walleye spawn, where the season starts, and where you go when the bay itself is unfishable.
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The Saginaw River is short but it drains an enormous watershed. The Tittabawassee, the Cass, the Flint, and the Shiawassee all feed it, which is why the flow at Saginaw can be large and why it responds to rain far inland rather than to weather on the bay. For fishing purposes the important tributary is the Tittabawassee, which carries the best known walleye run in the state.
The run generally builds from late March through mid April, with timing set by water temperature and by how the ice went out. Fish stack in the river, then drop back toward the bay after spawning, which is why the mouth stays productive well after the river itself has quieted down.
Where the river meets the bay there is structure worth knowing. The Dredge Island near the mouth has rip rap edges that hold baitfish, and it is the first real structure post spawn fish encounter coming back out. Pitching jigs along that rock is a reliable way to catch fish moving in both directions during the transition, and it is close enough to the ramps to be worth an evening.
The shipping channel runs out from the mouth and is a genuine feature in a bay that is otherwise flat. Fish use it, and on an east or northeast wind local knowledge puts walleye in and along it. It is also a working channel with commercial traffic, so it is not a place to anchor casually.
The USGS gauge on the Saginaw River sometimes reports a negative discharge, meaning water is moving upstream rather than down. That is not an error. On a shallow bay with a long axis, a sustained wind pushes water toward one end and it has to go somewhere. When it piles into the southwest corner, it backs up the river.
This is a genuinely useful signal. A strongly negative reading means the bay is stacking hard toward the river mouth, which tells you the wind has been north or northeast for a while, which tells you the inner bay is rough and the water at the mouth is being pushed rather than flushed. It is one of the few places where a single number describes the whole bay's state.
The most practical reason to know the river is that it is fishable when the bay is not. On a day the buoys read twenty plus and every shore is a bad idea, the river is sheltered, has current, has structure, and holds fish. Plenty of good days on the Saginaw started as cancelled days on the bay.
The tradeoff is that river water and bay water behave differently. The river responds to rain upstream and can be high and dirty when the bay is fine, or low and clear when the bay is churned. Reading them as one system is the mistake; they are two, connected at the mouth.