I assume you are asking about the El Dorado, "City of Gold" which was a legend among the Conquistadores? And not the movie or the town in Kansas?
The Muisca or Chibcha people (in today's Colombia) apparently had a ritual in which their leader was covered in powdered gold and then washed it off annually in Lake Guatavita, in the highlands near present day Bogota. El Dorado, or the Gilded Man became confused with other legends, and the actual ritual was apparently abandoned long before the Spanish ever heard the rumors. Spanish explorers assumed the Chibcha had mines like the Aztecs. In 1541 Fransisco de Orellana led the first expedition to find the "City of Gold," although it was as a sidetrip from Gonzalo Pizarros' expedition in the Andes. Orellana's group were the first to descend the Amazon River, which he named.
In 1595 Sir Walter Raleigh traveled the opposite direction, starting at the mouth of the Orinoco River in search of the "golden city" of Manoa and it's king, El Dorado. After expeditions in 1584, 1585, 1590 and '93, the Governor of Guyana and Trinidad, Antonio de Berrio came to the conclusion that the city of Manoa was beyond the mountains of Guyana. A Juan Martinez, turning up in Margarita circa 1586, claimed to have been taken to the city by the Indians he said he lived ten years with.
The first link below deals with whether or not Orellana's observations are acceptable, the second with the legend and the third with the legend and attempts to find the gold in Lake Guatavita.