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When Aldarion sailed back to Middle-earth in the first millennium of the Second Age, he sent explorers into the mysterious lands that his people had never seen. One such group travelled up a wide river, darkened at that time by dense overhanging forests, and so they gave it the name Gwathir, from the Sindarin for 'River of Shadow'. As they travelled further, they emerged from the forests into a vast fen. Imagining - wrongly - that they had found the river's source, they changed its name to Gwathló - 'Shadowy River from the Fens'.
The lands through which the river flowed changed immensely as the years passed. The Númenóreans felled the shadowing forests, and drained the wide fen, leaving only a remnant at Swanfleet. Before the end of the Second Age, nothing was left of the landscape that had given the river its name, but nevertheless it was known as the Gwathló into the Third Age and beyond.