Southward of the Lonely Mountain, the River Running joined the Forest River to form the Long Lake, then flowed on within the eastern margins of Mirkwood before it emerged from the trees at the eastern end of the Old Forest Road. This entire belt of land along the eastern fringes of the Forest was given to flooding, forming a waterlogged region running many miles north to south, and separating Erebor from northern Mirkwood.

In the later years of the Third Age great rains swept this area, and the flooding of the River Running became worse. Upheaval from earthquakes (caused, it was said, by the Dragon of Erebor) also expanded the Long Marshes, making the eastern end of the Old Forest Road impassable. By III 2941, when Bilbo Baggins passed through Mirkwood, even the northern Elf-path through the Forest trailed off to become lost in the trackless swampland of the Marshes.