question: In the 15th and 16th Centuries and before, how did they build bridges over rivers?
comment: Building temporary cofferdams to have a dry environment to build the foundation for a pillar of a bridge is a old idea. Temporary earth dams to diverted a river was used in 539 BC when King Cyprus of Persia captured Bablyon, they diverged the Eupjrates river. There is known example of temporary wooden coffer dams that the roman used for a bridge over the Danumbe in 102 AD . They down down a wooden pilings into the riverbed around where the bridge pillars should be perhaps in two rows with clay in between and the removed the water from the inside. It is done the same way today but with u shaped metal pilings. A illustration of a roman coffer dam [ URL_0 ]( URL_0 ) I suspect the the distance between the outer in inner wooden wall might have been larger Depending of how deep or shallow the river is it might be the case that there was a temporary earthen dam so half of it was dry when the build the bridge. Just google for cofferdam and you find a lot of modern coffer dams made of metal and earth. You could make wooden one just lik the the metal one but is required more work. Diverting rivers and making small dams is not that had and humans have done the for millennias. Is it just a loft of work and material you need to move.
passages: In India, the "Arthashastra" treatise by Kautilya mentions the construction of dams and bridges. A Mauryan bridge near Girnar was surveyed by James Princep. The bridge was swept away during a flood, and later repaired by Puspagupta, the chief architect of emperor Chandragupta I. The use of stronger bridges using plaited bamboo and iron chain was visible in India by about the 4th century. A number of bridges, both for military and commercial purposes, were constructed by the Mughal administration in India.
   The earliest reference to suspension bridges appear in Han dynasty records on the travels of Chinese diplomatic missions to the countries on the western and southern fringe of the Himalaya, namely the Hindukush range in Afghanistan, and the lands of Gandhara and Gilgit. These were simple suspension bridges of three or more cables made from vines, where people walked directly on the ropes to cross. Later, they also used decking made from planks resting on two cables.
   In ancient China, the Zhou Dynasty Chinese text of the "Shi Jing" ("Book of Odes") records that King Wen of Zhou was the first to create a pontoon bridge in the 11th century BC. However, the historian Joseph Needham has pointed out that in all likely scenarios, the temporary pontoon bridge was invented during the 9th or 8th century BC in China, as this part was perhaps a later addition to the book (considering how the book had been edited up until the Han Dynasty, 202 BC – 220 AD). Although earlier temporary pontoon bridges had been made in China, the first secure and permanent ones (and linked with iron chains) in China came first during the Qin Dynasty (221–207 BC). The later Song Dynasty (960–1279 AD) Chinese statesman Cao Cheng once wrote of early pontoon bridges in China (spelling of Chinese in Wade-Giles format):
   Simple suspension bridges using iron chains are also documented in Tibet and China. One bridge on the upper Yangtze dates back to the 7th century. Several are attributed to Tibetan monk Thang Tong Gyalpo, who reportedly built several in Tibet and Bhutan in the 15th century, including Chakzam Bridge and one at Chuka. Another example, the Luding Bridge, dates from 1703, spanning 100 m using 11 iron chains.
   [The road had] "no less than 48 timber bridges of such width that two carts can drive over them side by side". It had been built by Chang Jung [Zhang Rong] and the other engineers of the Chagatai some years before. The wooden trestles of Chinese bridges from the −3rd century [BC] onwards were no doubt similar to those supposed to have been employed in Julius Caesar's bridge of −55 [BC] across the Rhine, or drawn by Leonardo, or found in use in Africa. But where in +13th century [AD] Europe could a two-lane highway like Chang Jung's have been found?
   Section::::Engineering and invention.:Bridges and Hydraulics.
  
  Leonardo's study of the motion of water led him to design machinery that utilized its force. Much of his work on hydraulics was for Ludovico il Moro. Leonardo wrote to Ludovico describing his skills and what he could build:
  
  …very light and strong bridges that can easily be carried, with which to pursue, and sometimes flee from, the enemy; and others safe and indestructible by fire or assault, easy and convenient to transport and place into position.
   The simplest type of a bridge is stepping stones, so this may have been one of the earliest types. Neolithic people also built a form of boardwalk across marshes, of which the Sweet Track and the Post Track, are examples from England that are around 6000 years old. Undoubtedly ancient peoples would also have used log bridges; that is a timber bridge that fall naturally or are intentionally felled or placed across streams. Some of the first man-made bridges with significant span were probably intentionally felled trees.
   There were multiple types of bridges used throughout the road system and they were sometimes built in pairs. Some bridges were made of parallel logs tied together with ropes and covered with earth and vegetal fibers supported by stone abutments, while others were built of stone slabs resting on piled stones. One of the difficulties of creating wooden bridges was obtaining logs. Sometimes, the laborers who were making the bridges had to bring the lumber from very far away. Wooden bridges would be replaced about every eight years.
   In South America, Inca rope bridges predate the arrival of the Spanish in the Andes in the 16th century. The oldest known suspension bridge, reported from ruins, dates from the 7th century in Central America (see Maya Bridge at Yaxchilan).
   Section::::Work.:Proposed suspension bridge at Kiev, 1847.
  
  Late 1840s Bourne started working for Charles Blacker Vignoles, who was employed to construct the Nicholas Chain Bridge in Kiev over the Dnieper River. Bourne made the following sketch, which was presented to the Tsar in 1847. The construction of the bridge started in 1848 and was completed in 1855. Bourne made drawings and later weekly photographs of every stage of its construction.
   During the Second World War, there was a factory making the components for the Bailey bridge in the neighbouring town of Christchurch, where a section of bridge still remains, at a retail park in Barrack Road. The components were shipped to training grounds in Cumbria, where men learned the difficult technique of assembling them in rivers at night, to simulate combat conditions.
  
  Field Marshal Bernard Montgomery wrote in 1947:
  
  Section::::Honours and awards.
   The simple suspension bridge is the oldest known type of suspension bridge and, ignoring the possibility of pre-Columbian trans-oceanic contact, there were at least two independent inventions of the simple suspension bridge, in the wider Himalaya region and South America.
   For simple log bridges, the construction was done by placing a series of logs over projecting canes. 
  
  Stone bridges could span shorter lengths and needed shallower rivers to be built . Some slabs were placed over the abutments and intermediate stone pillars when necessary. A very special stone bridge was recently discovered in Bolivia consisting of a relatively small opening to allow the stream to flow and a quite imposing stone embankment filling the valley sides in order to allow the road to pass on top of it.
   The simplest type of a bridge is stepping stones, so this may have been one of the earliest types of footbridge. Neolithic people also built a form of a boardwalk across marshes, of which the Sweet Track, and the Post Track are examples from England, that are around 6000 years old. Undoubtedly ancient peoples would also have used log bridges; that is a timber bridge that fall naturally or are intentionally felled or placed across streams. Some of the first man-made bridges with significant span were probably intentionally felled trees. 
   Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 16th century.
  
  During the 18th century there were many innovations in the design of timber bridges by Hans Ulrich Grubenmann, Johannes Grubenmann, and others. The first book on bridge engineering was written by Hubert Gautier in 1716.
  
  A major breakthrough in bridge technology came with the erection of the Iron Bridge in Shropshire, England in 1779. It used cast iron for the first time as arches to cross the river Severn.
   On October 22, 1420, Ghiyasu'd-Din Naqqah, the official diarist of the embassy sent by the Timurid ruler of Persia, Mirza Shahrukh (r. 1404–1447), to the Ming Dynasty of China during the reign of the Yongle Emperor (r. 1402–1424), recorded his sight and travel over a large floating pontoon bridge at Lanzhou (constructed earlier in 1372) as he crossed the Yellow River on this day. He wrote that it was:
  
  Section::::Historic uses.:Greco-Roman era.
   BULLET::::- Pontoon bridge: The Zhou Dynasty Chinese text of the "Shi Jing" ("Book of Odes") records that King Wen of Zhou was the first to create a pontoon bridge in the 11th century BC. However, the British scientist, sinologist and historian Joseph Needham has pointed out that in all likely scenarios, the temporary pontoon bridge was invented during the 9th or 8th century BC in China, as this part was perhaps a later addition to the book (considering how the book had been edited up until the Han Dynasty, 202 BC–220 AD). Although earlier temporary pontoon bridges had been made in China, the first secure and permanent ones (and linked with iron chains) in China came first during the Qin Dynasty (221 BC–207 BC). The later Song Dynasty (960–1279 AD) Chinese statesman Cao Cheng once wrote a description of the early pontoon bridges in China. During the Eastern Han Dynasty (25–220 AD), the Chinese created a very large pontoon bridge that spanned across the width of the Yellow River. There was also the rebellion of Gongsun Shu in 33 AD, where a large pontoon bridge with fortified posts was constructed across the Yangtze River, eventually broken through with ramming ships by official Han troops under Commander Cen Peng. During the late Eastern Han into the Three Kingdoms period, during the Battle of Chibi in 208 AD, the Prime Minister Cao Cao once linked the majority of his fleet together with iron chains, which proved to be a fatal mistake once he was thwarted with a fire attack by Sun Quan's fleet. The armies of Emperor Taizu of Song had a large pontoon bridge built across the Yangtze River in 974 in order to secure supply lines during the Song Dynasty's conquest of the Southern Tang.
   Floating bridges were historically constructed using wood. Pontoons were formed by simply lashing several barrels together, by rafts of timbers, or by using boats. Each bridge section consisted of one or more pontoons, which were maneuvered into position and then anchored underwater or on land. The pontoons were linked together using wooden stringers called "balks". The balks were covered by a series of cross planks called "chesses" to form the road surface, and the chesses were secured with side guard rails.
   Section::::Historic uses.:Early modern period.
  
  The Spanish Army constructed a pontoon bridge at the Battle of Garigliano (1503). The Spanish Army of Flanders under Alexander Farnese, Duke of Parma built an unprecedented 730-meter, fortified pontoon bridge across the Scheldt river at the Siege of Antwerp in 1584 to close off water access to the city. In the battle of Saraighat in 1671, the Ahoms spanned the river Brahmaputra at Saraighat over an improviso bridge of boats.
   BULLET::::- Shovel services: construction of buildings, construction of roads, ditches
  
  Hand and tension services in history often have been done by the public. This was, for example, the setting of field bridges in the fields, the preservation of dams or even the construction of agricultural roads and local roads. The necessary materials (stones, wood, etc.) usually has been provided by the authorities.
  
  Section::::Hand and tension services today.
   One of the most common bridge-types built during the Han was the wooden-trestle beam bridge, described by literary sources and seen in reliefs carved on tomb bricks. Evidence for arch bridges is elusive: one outside of Chengdu's south gate is claimed to date to the Han period, while that built by Ma Xian (馬賢) (fl. 135 CE) was certainly a beam bridge. In artwork, a relief sculpture from a Han tomb in Sichuan province shows an arch bridge with a gradual curve, suggesting that it is segmental, although the use of such bridges are not entirely confirmed. Although there are rare references to simple suspension bridges in Han sources, these are only mentioned in connection with travels to foreign countries in the Himalaya, Hindukush and Afghanistan, demonstrating the antiquity of the invention there. Floating pontoon bridges made of boats secured by iron chains were built during the Han (some even spanning the Yellow River and Yangzi River) and were most often employed for military purposes since they could be easily assembled and then disassembled.
   Xenophon describes his eyewitness account of a massive military bridge created by joining 37 Persian ships across the Tigris. The Persians utilized each boat's buoyancy, in order to support a connected bridge above which supply could be transferred. Herodotus also gives many accounts of Persians utilizing ships to build bridges.
   The Maravi people built bridges ("uraro") of bamboo because of changing river depths. Bamboo was placed parallel to each other and tied together by bark ("maruze"). One end of the bridge would be tied to a tree. The bridge would curve downward.
  
  Section::::Medieval Architecture.:East Africa.
  
  Section::::Medieval Architecture.:East Africa.:Tanzania.
   In the Iya Valley of Japan, bridges have been constructed using wisteria vines. To build such a bridge, these vines were planted on opposite sides of a river and woven together when they grew long enough to span the gap. The addition of planks produced a serviceable bridge.
  
  Section::::Design.
  
  The arc of the bridge deck varies between a catenary and a parabola, depending on the weight of the bridge itself versus the load it carries.
   Herodotus relates in his "Histories" that the Second Persian invasion of Greece already intended by Darius I was carefully prepared by his son Xerxes I. A canal crossing the Athos peninsula was constructed. Among many other things he also ordered ships of war to be furnished and ropes of papyrus and of white flax to be prepared for the bridges.
  