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.
By T.A. 2941 there had been great floods, rains, and a couple of earthquakes, causing the marshes and bogs to spread widely of either side of the river, erasing the paths (along with many riders and wanderers). The Forest River broke into hundreds of winding courses when it first encountered the Long Marshes, and still flowed strongly through the midst and offered the only safe way.
Gandalf, who didn't know of the recent developments, when he left Thorin and Company at the Forest Gate, told them to stick to the forest-track until they see the Long Marshes lying below them. But when Gandalf learned of the hazards that befell them, he was in great anxiety to finish his other business in Dol Guldur and to find Thorin.