In Hiroshima the city is a river split into six

Most river cities have one bank and another. Not here. Here you walk twenty minutes in a straight line and cross two, three, four bridges, and each time the water below is wide, tidal and carries the same surname.

These are not urban canals or dead arms: they are the channels by which a single river reaches the sea divided. Understanding that organises the whole city, because it explains where each district sits, why the ground is flat and why the mountains appear so close at the end of almost any street.

01 The name says what it is

The place name means wide island, and it is not a poetic metaphor: it describes the shape of the ground. A delta is a run of flat islands separated by water, and the city was founded in 1589 exactly there, on the deltaic coastline of the Seto Inland Sea.

Choosing that spot was an engineering decision before it was a scenic one. In a territory where almost everything is mountain, the delta was the only flat, wide, well connected surface: the river arrived from above and the sea from below.

The price of that choice is obvious: a delta is soft, low ground in permanent dispute with water. The urban history of the following centuries is, to a large extent, the history of taking land from the bay and of deciding where the river is allowed through.

02 A hundred and three kilometres of descent

The river rises on Mount Kanmuri, at 1,339 metres, and runs 103 kilometres to the sea collecting the water of a 1,710-square-kilometre basin. It is a short course with a great deal of fall, and that changes everything.

A river like this does not flow: it drops. It drags gravel, sand and silt off the slopes, and when the gradient ends abruptly at the plain, it puts the load down. That continuous dumping, repeated over millennia, is exactly the material the city is made of.

It also explains the interior of the region, which is almost all mountain up to the northern watershed. Plains are few, small and always at the mouth of something. Here flat ground is not the natural state of the terrain: it is what a river leaves at the end.

03 Six watercourses in the same city

In the delta the Ōta divides into five arms with names of their own: the Tenma, the Kyūōta or Honkawa, the Motoyasu, the Kyōbashi and the Enkō. To those is added the flood channel opened in the twentieth century, which today serves as the main course. That is six watercourses crossing the same city.

The urban consequence is that the centre is not a continuous plane but a run of long strips between waters. Each strip has its own character, its own building height and its own rhythm, and to get from one to the next you always have to go up onto a bridge.

The other consequence is that the city has an enormous amount of waterfront for its size. The embankments work as promenades, the banks are planted with trees, and the river level rises and falls with the tide, because the sea comes in unobstructed right into the built-up centre.

04 The arm that was dug

The sixth arm was not made by the river. It was built in the late 1960s along the line of a minor watercourse at the western edge of the delta, and it was given a width and a capacity none of the others had.

The logic is that of any inhabited delta: if the water of a flood has to pass through the centre, the centre floods. An evacuation channel diverts the big flow along the edge and leaves the historic arms with a mild, predictable regime.

The result is a change of hierarchy visible on the map. The original course was renamed the Old Ōta, and the channel became the river's main passage. Where the two separate there is a sluice gate: it is, literally, the place where how much water enters the city is decided.

05 A sea with almost no swell

The delta empties into a bay of the Seto Inland Sea, which is not open sea but an enclosed sea among islands. That means little swell, little energy to redistribute sediment and therefore a delta that grows outwards instead of being swept away by currents.

That same enclosure softens the climate of the whole coast and explains the calm of the water you see from any quay. It also explains the shape of the port: with no swell to break, the wharves can run straight out into the channel and reclaimed land can advance in rectangular blocks.

It helps to keep the scales in mind. The prefecture covers 8,479 square kilometres and is mostly mountain; the city, 907. Almost three million people live on that narrow band between the range and the inland sea, and a good share of them do so on ground that used to be water.

06 How to travel it

The useful way to orient yourself is not by district but by arm. If you know which two watercourses you are between, you know where you are, and the bridges stop being an obstacle and become the city's coordinate system.

The plan that works best is to walk the embankments rather than the avenues. They run at water level, they are planted with trees, they cross the whole centre from north to south and they let you watch the tide at work: the same stretch of river changes width and colour several times a day.

And if you have another day, go up the river. Within a few kilometres the flat ground ends, the valley closes in and the gorges appear from which all that gravel comes down. Seeing where the material starts explains where it ends better than any viewpoint: the city you walked through below used to be up there.