On the northern plains the river is the obstacle and the bridge is the route

Flat country makes wide, unstable rivers

The Indo-Gangetic plain is one of the flattest large landscapes anywhere, built up over a very long period out of sediment carried down from the Himalaya. A river crossing that kind of terrain has almost no gradient to work with, so rather than cutting one deep channel it spreads, braids into several strands and moves sideways over time.

That is why the great northern rivers look so unlike what visitors expect a river to look like. In the dry months a channel may be a modest stream inside an enormous bed of sand and silt. In the weeks after the monsoon the same bed is full, and the water is doing exactly what the bed exists to accommodate.

Some of these rivers are notorious for changing course. Channels shift across the floodplain over years and occasionally within a single season, which is a serious matter for the people who live along them and a background fact for anybody trying to plan a route across.

Crossings are scarce and everything routes through them

Bridging a river that is several kilometres wide in flood, on ground that stays soft to a great depth, is expensive and slow work. Consequently there are far fewer crossings on the big northern rivers than a road map suggests at a glance, and the ones that exist carry a disproportionate share of all movement in the region.

The practical effect is that direct distance becomes close to useless as a measure. Two places on opposite banks may be a few kilometres apart in a straight line and eighty kilometres apart by road, because the road has to run along the bank to the nearest bridge and then come back down the other side.

Those crossings are congestion points too. Heavy goods traffic funnels onto the same structures as everything else, and a queue at a bridge approach in the afternoon can absorb an hour with no warning and no alternative. When somebody says a journey on the plains took twice as long as it should have, this is usually why.

Ferries fill the gaps, on the river’s terms

Where no bridge exists, boats still do the work, and river ferries remain part of everyday transport in parts of Bihar, Bengal, Assam and the eastern plains generally. They range from small motorised craft to vessels carrying vehicles, and they connect places the road network has effectively ignored for a century.

They also run entirely at the river’s discretion. Water level, current, silting and weather all determine whether a service operates on a given day, and the landing point itself moves as the channel moves. A crossing that ran from one spot last year may run from somewhere quite different now.

For a traveller this is either an obstacle or one of the better days of a trip, depending on how much slack the schedule has in it. What it is not is a dependable link in a tight itinerary, and it should never be the only way of reaching somewhere you have to be at a fixed time.

The flood season rewrites the map for weeks

Large parts of the eastern plains flood most years, and this is a normal hydrological process rather than an exceptional disaster, though real disasters happen within it. Water spreads across the floodplain, low-lying roads go under, embankments become the only raised ground and rail formations are washed out in places.

During those weeks the route between two towns may change entirely, running long detours over higher ground, with services suspended and restored at short notice. Local knowledge is the only real guide, and the current advisories from the state authorities are worth more than any itinerary drawn up months in advance.

It passes. Once the water recedes, road and rail links are repaired and the network returns to something like normal, though the repairs take time and rough stretches persist for months afterwards. The season is broadly predictable. The specific week and the specific road are not.

Rail crossed the rivers first and the towns followed

The railway builders faced the same problem in the nineteenth century and solved it the same way, with a small number of very substantial bridges at carefully chosen points. Those crossings determined where the main lines ran, and where the main lines ran determined which towns grew into something and which stayed as they were.

That history is still visible in the network. A number of otherwise unremarkable places on the plains are important junctions with disproportionate train services, because a bridge was put there long ago and everything since has organised itself around it. Travellers routinely change trains in towns they have never heard of for exactly this reason.

For planning it means rail is often the better tool on the plains than road, particularly for east-west movement across several river systems in a day. The train has a dedicated crossing and does not queue behind lorries to use it, which on this terrain is a genuine structural advantage rather than a preference.