The Nihonbashi River is a Tokyo waterway whose historical importance is set against its near-erasure from modern view: an elevated expressway runs above it, leaving the river dark and hard to notice. A boat journey with Kyoichiro Kaga and Shuhei Matsumiya makes that concealment physical, turning the river into a place where the city’s older waterways and modern infrastructure meet in stark contrast.

Course and Setting

After passing through Suidobashi Bridge, the boat travels to a fork in the river. The left branch turns almost at a right angle and leads into the Nihonbashi River. The route is not familiar to either detective; Kaga tells Matsumiya that he will soon understand why the river is so little known.

Once the boat turns left, the light abruptly dims. An elevated expressway runs directly overhead, and thick support pillars stand in a line down the river’s middle. The boat continues beneath this structure, approaching Tokiwabashi Bridge. From the water, the river seems less like an open channel than a passage enclosed by the road above it.

The Expressway and the Hidden River

Kaga attributes the river’s obscurity to the expressway. Built for the 1964 Tokyo Olympics, the road took this route after the necessary land could not be secured elsewhere. Its course above the Nihonbashi River makes the waterway nearly invisible on a map and, in Kaga’s account, leaves people barely aware that it exists. Crossing its bridges can feel like passing through a tunnel beneath a road rather than over a river.

The boat journey gives this explanation a concrete shape: overhead infrastructure blocks the sky, while the pillars divide the watercourse. The contrast between the river’s physical presence and its lack of visibility shows how a major urban route can be concealed without being removed.

Historical and Narrative Significance

Looking at the river’s sunless surface, Kaga reflects that its waterways must have played a major role in the economy and culture during the Edo period. His observation places the obscured channel within a longer history: a waterway once central to urban life now lies beneath a structure associated with the preparations for the 1964 Olympics.

The Nihonbashi River also anchors the detectives’ passage through the city. Their boat follows it toward Tokiwabashi Bridge, while Matsumiya consults notes listing bridges by month. The setting thus links the present investigation to the named bridges and waterways that Kaga sees as traces of an older urban landscape.