通る and 渡る: passing along a route and crossing to another side
Compare using a riverside path with crossing a river, then examine the different focus of both verbs for a bridge.
A place or route passed through
通る, read とおる, describes passing through or along a place or route. 川沿いの道を通りました。 says that someone used the riverside path. It does not state a river crossing.
門を通って庭に入りました。 says that someone passed through the gate and entered the garden. 門を marks the passage point, while 庭に marks the place entered.
Crossing from one side to another
渡る, read わたる, describes crossing something such as a river or road. 川を渡りました。 says that someone crossed the river without specifying how.
歩道橋を渡って反対側へ行きました。 says that someone crossed the pedestrian overpass and went to the opposite side. Read the crossing route 歩道橋 with the side reached, 反対側.
Different focus on the same bridge
橋を通る and 橋を渡る can describe the same journey. 通る focuses on using the bridge as a route, while 渡る focuses on crossing from one end to the other.
川沿いを通ってから橋を渡りました。 says that someone went along the river and then crossed the bridge. Separate the riverside segment from the crossing segment in that order.
Does 川沿いの道を通りました。 establish that the person reached the opposite side of the river?
Show the answer and why
No. It says that the person passed along the riverside path. Whether the river was crossed needs separate information.