The Long Marshes were a region of marshes and pools dotted with isles that flanked both sides of the Forest River. They extended from the eastern edge of Mirkwood nearly to the shores of the Long Lake.
When Gandalf left Thorin and company at the Forest Gate on the western side of Mirkwood, he told them to stick to the forest-track and that with tremendous luck they might come out one day and see the Long Marshes lying below them. At that time he did not expect the Long Marshes to present any great hazard to the party. However, as the story explained, the lands had changed much in recent years since the last news that Gandalf had had of them. There had been great floods, rains, and an earthquake or two, causing the marshes and bogs to spread widely of either side of the river, resulting in the vanishing of paths along with many riders and wanderers. While the Forest River did break into hundreds of winding courses when it first encountered the Long Marshes, the main river still flowed strongly through the midst and offered the only safe way. When Gandalf learned of these conditions he was in great anxiety to finish his other business (dealing with the Necromancer) and to find Thorin's company.
It is not know if the Long Marshes were named for their length along the Forest River or for their proximity to the Long Lake.