The discovery of the wall also challenges conventional thinking about the temple's ancient facade, Boraik said.
Previous theories about the facade and courtyard in front of temple were based on depictions found in private tombs dating back to the 18th dynasty (1550-1295 B.C.).
(Read related story: "Egypt's Oldest Known Art Identified, Is 15,000 Years Old" [July 11, 2007].)
One depiction from the tomb of Neferhotep, an official from that period, depicted a large rectangular pool in front of the temple that was linked to the Nile by a canal.
Archaeologists had first uncovered small parts of this wall in the 1970s but assumed it was the back wall of the pool, Boraik said.
That theory held until January, when Egyptian archaeologists found a piece of the same wall several meters away, too far off to be part of the enclosed basin.
Now experts believe that the pool depicted in ancient drawings was backfilled in antiquity and that the temple was expanded on top of it, built to the edge of where the Nile flowed 3,000 years ago.
"It means that the Nile was reaching the foot of Karnak in the time of the pharaohs," said Boraik. "It changes everything."
"Completely Reevaluating" Karnak
This new theory has been backed by tests of the sediment at the base of the embankment wall, which show alternating levels of silt and sand that suggest running water once flowed there.
Based on cartouches and other writings found on the wall, experts believe construction started in the 22nd dynasty (945-715 B.C.) and was completed by the middle of the fourth century B.C.
W. Raymond Johnson, an Egyptologist at the Oriental Institute at the University of Chicago who has visited the site, said the discovery shows the expertise of ancient Egyptian builders.
"Being good engineers and practical, [ancient Egyptians knew] that to build something so big so close to the Nile, you have to have reinforcement in front of it. It stopped any erosion of the Nile River bank."
The find sheds new light on the ancient city of Thebes, of which Karnak was the religious center, he added.
"We've assumed the ancient landscape in Thebes is relatively unchanged, and we have to completely reevaluate that now," Johnson said.
"It really gives us pause when we make certain assumptions and then find out they are completely wrong."
While excavating the embankment, archaeologists also discovered two public baths and a jar holding more than 300 coins dating to the era of Macedonian rule of Egypt, from the first to the fourth centuries B.C.
One of the giant circular baths has been completely excavated, revealing an intricate mosaic tile floor and seating for 16 people.
The other partially excavated bath has been found to have seats flanked by statuettes of dolphins.
The baths were found just outside the wall, and experts believe they were built on the plateau of silt left behind after the Nile moved to the west.
The jar of bronze coins, featuring the likenesses of Macedonian rulers Ptolemy I, II, and III, were discovered near the baths and are currently being cleaned to reveal their inscriptions.
The baths may have served as purification sites where visitors could wash before entering the temple complex.
Other experts suspect they may be the first signs of a much larger residential area that has yet to be explored.
Archaeologists have also excavated a giant ramp leading up to the temple complex that is inscribed with the name of the pharaoh Taharka (or Taharqa), who ruled in the late seventh century B.C.
The ramp probably served as the ruler's personal landing area, extending directly into the Nile to allow the pharaoh to transfer directly from his boat to the temple.
This raises the prospect that parts of ancient boats may also be buried in the former riverbed, including pieces of the gigantic ceremonial barges known to have carried images of the gods during religious processions, the archaeologists said.
"Now that we know the Nile has moved to the west, it means something is waiting for future generations of archaeologists and Egyptologists to possibly recover," Johnson of the University of Chicago said.
"It's a wonderful gift now that you realize there is something down there."
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