Enchanted River

The Enchanted River rose in the Mountains of Mirkwood, and flowed northwards in a great loop to meet the Forest River under the canopy of the trees of Mirkwood. Its water was black, and carried a curse that gave the river its name - any person coming into contact with its enchanted water immediately fell into a long, deep sleep.
About halfway along its length, the Enchanted River flowed across the Elf-path that led west to east through Mirkwood to the Elvenking's Halls, and it seems that a ferry was maintained there, presumably for the use of the Wood-elves. At least, Bilbo Baggins and the Dwarves found a boat kept on the eastern bank of the River, which they used to cross it on their journey to Erebor. However, Bombur fell in the river and lost consciousness, and his companions had to pull him out and carry him for several days with ropes.