The Long Lake was the lake that lay to the south of the Lonely Mountain and Dale. On its western shore near the mouth of the Forest River was the site of Lake-town, home of the trading people of the Lake-men. It was wide enough that the opposite shores looked small and far, while the northern and southern ends could not be seen at all. It was fed by the Forest River and the River Running which then flowed out of the Lake over a high waterfall at its southern end. The two rivers filled what once must have been a deep rocky valley.
Before the exile of the Dwarves from Moria, the realm of Thranduil is said to have extended to the western shores of the Long Lake. Later, after the founding of the Kingdom under the Mountain and the city of Dale, the trade-town of Lake-town grew up near the inflow of the River Running. In T.A. 2941 the lake became the grave site of the dragon Smaug.