The gorge lies between Romania to the north and Serbia to the south. At this point, the river separates the southern Carpathian Mountains from the northwestern foothills of the Balkan Mountains. The Romanian side of the gorge constitutes the Iron Gates natural park, whereas the Serbian part constitutes the Đerdap national park.
In English, the gorge is known as Iron Gates or Iron Gate. An 1853 article about the Danube in The Times of London referred to it as "the Iron Gate, or the Gate of Trajan."
In languages of the region including Romanian, Hungarian, Polish, Slovak, Czech, German and Bulgarian, names literally meaning "Iron Gates" are used to name the entire range of gorges. These names are Romanian: Porțile de Fier ([ˈport͡sile de ˈfjer], Hungarian: Vaskapu, Slovak: Železné vráta, Polish: Żelazne Wrota, German: Eisernes Tor, and Bulgarian: Железни врата Železni vrata"). An alternative Romanian name for the last part of the route is Defileul Dunării, literally "Danube Gorge".
In Serbian, the gorge is known as Đerdap (Ђердап; [d͡ʑě̞rdaːp]), with the last part named Đerdapska klisura (Ђердапска клисура; [d͡ʑě̞rdaːpskaː klǐsura]) from the Byzantine Greek Κλεισούρα (kleisoura), "enclosure" or "pass."
The first narrowing of the Danube lies beyond the Romanian isle of Moldova Veche and is known as the Golubac gorge. It is 14.5 km long and 230 m (755 ft) wide at the narrowest point. At its head, there is a medieval fort at Golubac, on the Serbian bank. Through the valley of Ljupovska lies the second gorge, Gospodjin Vir, which is 15 km long and narrows to 220 m (722 ft). The cliffs scale to 500 m and are the most difficult to reach here from land. The broader Donji Milanovac forms the connection with the Great and the Small Kazan gorge, which have a combined length of 19 km (12 mi). The Orșova valley is the last broad section before the river reaches the plains of Wallachia at the last gorge, the Sip gorge.
The Great Kazan (kazan meaning "cauldron" or "reservoir") is the most famous and the most narrow gorge of the whole route: the river here narrows to 150 m and reaches a depth of up to 53 m (174 ft). East of this site the Roman emperor Trajan had built the legendary bridge erected by Apollodorus of Damascus. Construction of the bridge ran from 103 through 105, preceding Trajan's final conquest of Dacia. On the right (Serbian) bank a Roman plaque commemorates him. On the Romanian bank, at the Small Kazan, the likeness of Trajan's Dacian opponent Decebalus was carved in rock from 1994 through 2004.
Significantly older discoveries have been found in the geographically less spectacular gorge of Gospodjin Vir: in the 1960s the archaeological site Lepenski Vir was unearthed, the most important in all of southeastern Europe. The sandstone statues dated to the early neolithic era are particularly splendid. Together with many other findings in the Iron Gates gorges area, they indicate that the region has been inhabited for a very long time.
The riverbed rocks and the associated rapids made the gorge valley an infamous passage for shipping. In German, the passage is still known as the Kataraktenstrecke, even though the cataracts are gone. Near the actual "Iron Gates" strait the Prigrada rock was the most important obstacle until 1896: the river widened considerably here and the water level was consequently low. Upstream, the Greben rock near the "Kazan" gorge was notorious.
In 1831 a plan had already been drafted to make the passage navigable, at the initiative of the Hungarian politician István Széchenyi. Finally Gábor Baross, Hungary's "Iron Minister", succeeded in financing this project.
Not being satisfied with the solutions for the navigation through the gorge compiled by the Austrio-Hungarian government and the Austro-Turkish commission, Hungary independently formed its commission for the organization of the navigation through the Iron Gates. The project was finished in 1883 and the works on the gorge section were done by the Hungarian Technical Administration, from 1889 to 1900. The works were divided in two sectors, the upper and the lower Iron Gates. The channels in the upper section, at the town of Orșova (the tripoint between Austria-Hungary, Romania and Serbia at the time) were up to 60 m (197 ft) wide and 2 m (7 ft) deep, at the zero water level in Orșova. In the southern section, the channels were 60 m (197 ft) wide and 3 m (10 ft) deep, except for the Sip Channel, which was 73 m (240 ft) wide.
In 1890, near Orșova, the last border town of Hungary, rocks were cleared by explosion over a 2 km (1.2 mi) stretch in order to create channels. A spur of the Greben Mountains was removed across a length of over 2 km (1.2 mi). Here, a depth of 2 m (7 ft) sufficed. On 17 September 1896, the Sip Channel thus created (named after the Serbian Sip village on the right bank) was inaugurated by the Austro-Hungarian emperor Franz Joseph, the Romanian king Carol I, and the Serbian king Alexander Obrenovich.
Some of the channels created included:
In total, 15,465 m (50,738 ft) of navigable channels was created. They were flooded when the artificial Lake Đerdap was created in the early 1970's.
The results of these efforts were slightly disappointing. The currents in the Sip Channel were so strong at 15kts (8m/s) that, until 1973, ships had to be dragged upstream along the canal by locomotive. The Iron Gates thus remained an obstacle of note.
The construction of the joint Romanian-Yugoslavian mega project commenced in 1964. In 1972 the Iron Gate I Dam was opened, followed by Iron Gate II Dam, in 1984, along with two hydroelectric power stations,two sluices and navigation locks for shipping.
The construction of these dams gave the valley of the Danube below Belgrade the nature of a reservoir, and additionally caused a 35 m rise in the water level of the river near the dam. The old Orșova, the Danube island of Ada Kaleh (below) and at least five other villages, totaling a population of 17,000, had to make way. People were relocated and the settlements have been lost forever to the Danube.
The dam's construction had a major impact on the local fauna and flora as well—for example, the spawning routes of several species of sturgeon were permanently interrupted.
The flora and fauna, as well as the geomorphological, archaeological and cultural historical artifacts of the Iron Gates have been under protection of both nations since the construction of the dam. In Serbia this was done with the Đerdap National Park (since 1974, 636.08 km2 (245.59 sq mi)) and in Romania by the Porțile de Fier National Park (since 2001, 1,156.55 km2 (446.55 sq mi)).
The isle of Ada Kaleh is probably the most evocative victim of the Đerdap dam's construction. A Turkish exclave, it had a mosque and a thousand twisting alleys, and was known as a free port and smuggler's nest. Many other ethnic groups lived there beside Turks.
The island was about 3 km (1.9 mi) downstream from Orșova and measured 1.7 by 0.4-0.5 km. It was walled; the Austrians built a fort there in 1669 to defend it from the Turks, and that fort would remain a bone of contention for the two empires. In 1699 the island came under Turkish control, from 1716 to 1718 it was Austrian, after a four-month siege in 1738 it was Turkish again, followed by the Austrians reconquering it in 1789, only to have to yield it to the Turks in the following peace treaty.
Thereafter, the island lost its military importance. The 1878 Congress of Berlin forced the Ottoman Empire to retreat far into the south, but the island remained the property of the Turkish sultan, allegedly because it was forgotten to be mentioned in the treaty. The inhabitants enjoyed exemption from taxes and customs and were not conscripted. In 1923, when the Ottoman monarchy had disappeared, the island was given to Romania in the Treaty of Lausanne
The Ada Kaleh mosque dated from 1903 and was built on the site of an earlier Franciscan monastery. The mosque's carpet, a gift from the Turkish sultan Abdülhamid II, has been located in the Constanța mosque since 1965.
Most Ada Kaleh inhabitants emigrated to Turkey after the evacuation of the island. A smaller part went to Northern Dobruja, another Romanian territory with a Turkish minority.