Why Are There No Bridges Over the Amazon River?

LiveScience reports:
Despite being around 4,300 miles in length, the Amazon River surprisingly has no bridges.

The Amazon River is the world's second-longest river and one of the planet's most significant waterways... Given the Amazon flows through three countries (Peru, Colombia and Brazil) and more than 30 million people live in the river's basin, according to the World Wildlife Fund, it seems somewhat improbable that no bridges span the river. So why is this the case? Are there fundamental difficulties with building such structures in a rainforest containing swaps, extensive wetlands and deep, thick undergrowth? Are there financial barriers? Or is it simply not worth the effort?

The site got answers from Walter Kaufmann, the chair of Structural Engineering at the Swiss Federal Institute of Technology in Zurich. Long-time Slashdot reader fahrbot-bot shared this summary of the explanations (condensing quotes from the article):

There are technical and logistical difficulties.
The Amazon is far from an ideal location for bridge builders. It has an array of natural stumbling blocks that would need to be conquered by engineers and construction workers. its extensive marshes and soft soils would necessitate 'very long access viaducts and very deep foundations,' and this would require hefty financial investment. Additionally, the changing positions of the river's course across the seasons, with 'pronounced differences' in water depth, would make construction 'extremely demanding.'

The environment at the Amazon is certainly among the most difficult in the world. Pontoons, or floating structures, are not a solution that would work in most parts of the Amazon, Kaufmann said, because the river is hugely impacted by seasonal variances, which adds an additional layer of complexity. For instance, during the dry season — between June and November — the Amazon averages a width of between 2 and 6 miles (3.2 and 9.7 km), while in the wet season — December through April — the river can be as wide as 30 miles (48 km), and the water level can be 50 feet (15 meters) higher than it is during the dry season, according to Britannica.

There is no sufficiently pressing need for a bridge across the Amazon.
The Amazon, for much of its 4,300-mile (6,920 kilometers) length, meanders through areas that are sparsely populated, meaning there are very few major roads for any bridge to connect to. And in the cities and towns that border the river, boats and ferries are an established means of moving goods and people from bank to bank, meaning there is no real need for bridges to be built, other than to make trips slightly quicker.

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