We are going to walk the river, south to north, across seven crossings. This walk is different from the others: the things you are looking at are far away, so you will be standing still more and walking less. Bridges are windy, so bring a layer. And if you can choose the time, come after four in the afternoon — the light comes down the river from behind you. About an hour. Start on the blue bridge, five minutes south of the big gate.

Start on the blue bridge. It is about five minutes south of Kaminarimon if you follow the water. Walk out to the middle of it, stop, put your back to the traffic and face north — upstream, towards the temple you have just come from. Here is what is in front of you. A wide, flat, grey-green river moving slowly. A concrete wall running down both banks. And a line of bridges going away from you into the distance. Count them. On an ordinary day you will see three, and on a clear one, four. Every single one is a different shape, and every single one is a different colour. None of that is an accident, and working out why is what this walk is for. Now look at the thing holding you up. This is Komagata Bridge, opened in 1927, and it was the first full-scale steel half-through arch bridge built in Japan. That sounds like jargon right up until you look along the bridge, and then it becomes the most interesting thing here. Lean over the rail and look sideways, towards the middle of the span. The arch comes up through the roadway. The deck runs through the middle of it, so at mid-river the structure is beside you and above the level of your feet rather than under them. Now look towards either end of the bridge. Out there the arch drops away below the road and vanishes, and the deck simply sits on top of it. One bridge, two arch types, in a single crossing — and that is what was new about it. Before this, nobody in Japan had built a full-scale steel bridge that ran the deck through the arch like that. Look at the piers as well: they carry semicircular stone balconies, which is a lot of care for a structure thrown up in a hurry after a catastrophe. Now the part you would have walked straight past. Look back at the bank you came from, at the western end of this bridge. Set back from the road there is a small hall in dark wood, easy to mistake for an office. That is Komagatado, and the bridge is named after the hall, not the hall after the bridge. By the temple's own account, this is where the whole thing began. In the seventh century, two fishermen brothers pulled a small golden figure of Kannon out of this river in their net. They put it back in the water. It came up again. I am telling you that as a story, because that is what it is — the temple's account of its own origin, not a documented event. But hold on to the shape of it. Two working men, a net, and a river that fed a village. The pilgrims, the shopping street, the theatres, the crowds and the giant lantern all arrived afterwards, and they arrived because of the water first and the temple second. Asakusa faces the temple today. For most of its life it faced this. And then the bridges over this water were destroyed twice in twenty-two years, and the question of how to cross it had to be argued out from the beginning. Walk north along the bank, about five minutes, to the big red one. That is where the argument happened.

Walk out to the middle of the red bridge and stop. Face south, back the way you came. This is Azumabashi, finished in 1931 — the bridge in almost every photograph of Asakusa ever taken. Before you look at the view, look up, and notice there is nothing there: no arch, no truss, no cables. This bridge carries all three of its steel arches entirely beneath the roadway. That took work. To get the deck high enough to sit on top of the arches, the engineers raised the ground at both ends by up to five metres — they rebuilt the banks so nothing would stand between people on the bridge and the river. Five minutes ago you were on a bridge whose arch came up through the road beside you; this one buries the structure completely. Same river, same decade, same government department, opposite answers. Hold on to that. Now look south, down the water. You can see three bridges at once from here, and no two of them are the same colour. On the first of September 1923, an earthquake struck at lunchtime, and the fires that followed burned for two days. Most of eastern Tokyo was wooden and packed tight, and it went. The wooden bridge on this spot went too. That is the detail worth stopping on: people ran to the bridges. A bridge is the obvious place to go when the ground behind you is burning — flat, open, and it leads out. Very large numbers had that idea at the same moment, on timber structures, over water they could not reach. The bridges did not save them. Along this river, in several places, the bridges were the trap. What followed was one of the largest reconstruction projects of the twentieth century, and the brief given to the engineers was specific. The new crossings would be permanent. They would be steel and concrete, and they would not burn. And then the unusual part: they should not look like one another. That last requirement has nothing to do with engineering. The reason usually given is that people had lost the city they knew — the landmarks, the street pattern, the buildings they navigated by, all of it gone inside two days. Someone standing on a riverbank in the rebuilt city should be able to look at a bridge and know immediately where they were. So the river got a half-through arch at Komagata, a deck arch here, a suspension bridge downstream, and so on. Nine permanent bridges, no two alike, each in its own colour. It is a set of navigation aids disguised as civil engineering. A crossing has existed on this line far longer. The first went up in 1774 and — unusually — it charged a toll: two mon from everybody except samurai. People paid, because the alternative was waiting for a ferry. One last thing. The red is not a heritage colour — these bridges have been repainted many times since the thirties. But the rule that each must stay distinct has been kept at every repainting. Nobody has ever quietly made them match. Now stay where you are and look east, across to the far bank. There is a black building over there, and something gold lying down beside it. That is the next stop, and you can see it from here.