When it comes to the Amazon River, there’s no such thing as water under the bridge. The idiom simply doesn’t apply there, as no bridges cross the Amazon River despite it being at least 4,000 miles long. This isn’t because the idea has never occurred to anyone — it would just be extremely difficult to build any. The Amazon has both a dry season and a rainy season, and during the latter its waters rise 30 feet, causing three-mile-wide crossings to grow by a factor of 10 as previously dry areas are submerged. The river bank itself is also in a near-constant state of erosion due to how soft the sediment it consists of is, and there’s no shortage of debris floating in the water.
Beyond all those logistical hurdles, there simply isn’t much use for bridges across the massive river. For one thing, there are few roads on either side of the Amazon that need to be connected. The river is, of course, in the middle of a dense rainforest, the vast majority of which is sparsely populated. Other long rivers have numerous crossings, however: The Nile has nine bridges in Cairo alone, for instance, and more than 100 bridges have been built across China’s Yangtze River in the last three decades. For now, boats and ferries are the preferred method of crossing the Amazon, and are likely to remain so for the foreseeable future.
These days, the river flows east and into the Atlantic. That wasn't always the case, as it used to flow west into the Pacific — and even both directions simultaneously. This was during the Cretaceous Period, between 65 million and 145 million years ago, and was the result of a highland (mountainous area) that formed along the east coast of South America when that landmass and Africa broke apart. The Andes eventually formed on the western half of the continent, which forced the river into its current eastward flow.