Lake Poyang is located in the north of Jiangxi Province near the border with Hubei and Anhui Provinces - more precisely, the lake is situated on an imaginary line that stretches southward into Jiangxi Province directly below the common, roughly north-south border that separates Hubei and Anhui Provinces at this point. It is China's largest freshwater lake, spanning some 3500 square kilometers, although it is rather shallow as lakes go, being only 8 meters in depth, on average. An abberation of nature (the lake formed due to a non-manmade shift in the course of the mighty Yangtze River, around CE 400), the lake is nonetheless an important habitat for a number of migratory birds, and, as part of the Yangtze River system, is home to the freshwater finless porpoise (jiangzhu, or "river pig"), a variant of the saltwater finless porpoise (Neophocaena phocaenoides) found in the coastal waters off Japan, China, Indonesia, and India.*
Lake Poyang's Military History
The largest naval battle during medieval times - if not the largest naval battle, ever - was fought on Lake Poyang from the 30th of August to the 4th of October, 1361, during the final throes of the weakened Yuan (CE 1279-1368) Dynasty. Forces belonging to two of the three rebel states that were competing amongst themselves to replace the last Yuan Dynasty emperor, Emperor Huizong, namely, the forces of the Ming State under the command of Zhu Yuanzhang and the forces of the Han State under the command of Cheng Youliang, fought a ferocious and decisive battle on Lake Poyang for the control of southern China - in effect, wresting southern China from the grasp of the Yuan Dynasty emperor.
A contingent of Han naval forces had beseiged the Ming-controlled city of Nanchang, which lies on the banks of the Gan River about 50 kilometers southwest of Lake Poyang, though without much luck, the Han naval forces having arrived on Lake Poyang via the Yangtze River. The leader of the Ming, Zhu Yuanzhang, was elsewhere with his naval forces, doing battle against a Wu State navy (in this breakdown of central state power, it became a free-for-all, where anyone with a modicum of power was making a bid for more of the same), but when he got word that his capital, Nanchang, was under seige by the Han navy of Cheng Youliang, Zhu hurried to Lake Poyang with whatever naval forces he could spare, leaving a lieutenant to battle on against the Wu navy.
Many of the Han watercraft were little more than floating towers of the kind used on land to besiege cities during the Middle Ages, i.e., launching towers for hurling large stones and burning buckets of tar, etc. Their maneuverability as watercraft was very limited. In the end, the naval craft - and naval cunning - of Zhu Yuanzhang prevailed, Nanchang was saved, and most of southern China was thereafter in the hands of Zhu Yuanzhang, who, having won the Battle of Lake Poyang, proceeded to challenge Emperor Huizong directly. He would succeed 7 years later, in 1368, founding the Ming (CE 1368-1644) Dynasty, as Emperor Taizu.
The Origin of Lake Poyang
Around CE 400, the Yangtze River had a slightly more northerly course in this part of China. It flowed through the area of what is today Lake Longgan, which lies just north of the present-day course of the Yangtze, and straddles Hubei (on the left) and Anhui (on the right) Provinces. The usual explanation for the creation of Lake Poyang is that the shift in the Yangtze caused the Gan River to back up, creating the lake. This may be an oversimplification of how the lake was created. A more likely scenario (granted, this is pure speculation) is as follows...
The Gan River originates near the southern border of Jiangxi Province and flows northward into present-day Lake Poyang. But before the freak creation of Lake Poyang, the Gan continued northward in the direction of Lake Longgan (note the name: Long Gan), where it emptied into the Yangtze (it still "empties" into the Yangtze, albeit, via Lake Poyang). The area where present-day Lake Poyang exists was formerly low-lying marshland, which is probably what aided the Yangtze in shifting course southward, since water seeks the lowest point in the terrain.
If one imagines an atypical flood season for the Yangtze River in this part of China at the time (not hard to imagine at any time!), then the floodwaters might have spread out into the low-lying marshland south of the Yangtze's normal course, and if these floodwaters managed to carve a trough in the soft marsh (not hard to imagine, given that this is the mighty Yangtze!), then the effect would have been to siphon the water from the Yangtze in this direction, filling up the low-lying marsh, before coursing in a northerly direction where it re-connected with the rest of the Yangtze River.
Over time (this could occur over the course of a decade or less), deposits of silt would plug up the "abandoned" piece of former river, leaving a canal-like stretch of more or less dead water, perhaps fed by smaller streams, some of which might eventually find their way to the Yangtze, while the floodwaters of the Yangtze, having once entered the low-lying marshland on its new southerly route, would remain there, creating Lake Poyang. In fact, Lake Longgan may itself be the remains of the former northerly course of the Yangtze River in this part of China (my knowledge is limited here). With no major river to drain this low-lying area, water would accumulate, creating another lake, though, in time, small exit streams might well find their way to the Yangtze, or to a smaller local river which eventually empties into the Yangtze.
An interesting conjecture, in any case, but not unrealistic - anyone who has witnessed how a seasonally high, roiling river can sever a tight loop, eliminating the loop completely while plowing straight ahead, will know that this is not only possible, it occurs regularly; rivers are not indelibly "etched in earth".
With the creation of Lake Poyang, the counties of Poyang and Haihun were flooded, causing the inhabitants to flee en masse to higher land, namely, to Wucheng Town (aka Wuzhen Township) on the northern edge of present-day Lake Poyang. This impromptu migration is captured in the local saying "Drowning Haihun County gives rise to Wuzhen Township".
Present-Day Lake Poyang
Lake Poyang is a birdwatcher's paradise, since it is the seasonal home of countless migratory birds from late October to April each year. Ornithologists have recorded over 200 species of migratory birds in and around Lake Poyang, including one very special species, the Siberian white crane (Grus leucogeranus), 95% of whose entire population spends the winter at Lake Poyang.**
As indicated above, Lake Poyang is also home to the freshwater finless porpoise of the Yangtze River system. This may be the only population of strictly freshwater porpoises in the world; while there are river dolphins in major river systems elsewhere (eg., the Amazon), and while there are so-called harbor porpoises in many places around the world, the former are dolphins, not porpoises, and the latter live in salt water. The Yangtze River system's population of "river pigs" number only 1400, distributed in the Yangtze proper, in Lake Poyang, and in Lake Dongting, about 300 kilometers due west of Lake Poyang, in Hunan Province. Though not endangered, they are a threatened species.
And speaking of danger, Lake Poyang has earned an unenviable reputation as China's equivalent of the Bermuda Triangle, that body of ocean water where ships and airplanes mysteriously disappear without trace, and which lies in the triangle created by the following three points: the island of Bermuda (to the north), the island of Puerto Rico (to the south), and the city of Miami, in Florida (to the west). Lake Poyang, as indicated above, is connected to the sea via the Yangtze River, which empties into the East China Sea near Shanghai. On the 16th of April, 1945, while WWII was still raging in the Pacific theater, a Japanese troop ship, the Kobe Pill, with 200 sailors on board, vanished while traversing Lake Poyang. No trace of the ship, or its passengers, save one (who, it is said, was already mad when he emerged from the water... what had the man witnessed, one wonders?!) has since been seen, even though there were later Japanese attempts to locate the missing ship.
* The difference between a porpoise and a dolphin (they are more or less cousins) is that porpoises have spade-shaped teeth, no "beak", and - where present - the dorsal fin is an upright triangle, while dolphins have cone-shaped teeth, a long "beak", and - where present - the dorsal fin is backward-curved, or "hooked".
** Why wouldn't all of these birds just choose to spend the winter in the same locality, you might well ask? Nature, or the Creator - or whatever - has rigged it such that, for the sake of the survival of the species, a percentage of them do not "follow the flock". The same phenomenon is seen in anadromous fish belonging to the salmonidae (salmon, trout and grayling) family (and surely similar behavior in many other species of animals, birds, insects, etc.): only a certain percentage of adult specimens mount the rivers each year to spawn.
It is as if nature - or whatever - is holding a percentage of the population in reserve, in case a catastrophe occurs on land. For example, if something were to happen to Lake Poyang (God forbid!), causing the lake's entire population of Siberian white cranes to perish, the remaining 5% that overwinter elsewhere would eventually replenish the population in Siberia.