Sirannon was a river in Eriador.
According to the The Atlas of Middle-earth, the river was about one-hundred miles in length. In the late Third Age, it flowed near a knoll by the Elven-way from a pool close to the Doors of Durin.
History
The Sirannon once flowed from an underground source near the West side of Moria and ran alongside the road that once led to Ost-in-Edhil. It was dammed at an unknown date in the last thousand years of the Third Age, closing off the Doors of Durin from neighboring lands. The foul pool created by the damming of the stream was where the Watcher in the Water lived.
After the War of the Ring and the resettling of Moria by the Dwarves, the stream and the foul pool it ran through was probably cleaned and freed up to flow naturally to its original destination.
Etymology
Sirannon is a Sindarin word for 'stream-gate', from the words sir, meaning 'stream', or 'river', and annon meaning 'gate'.
Portrayal in adaptations
In The Lord of the Rings Online, the waters of Sirannon still flow into the Black Pool before the Door of Durin but not out - instead the water seeps through its bottom into the waterworks of Moria. The dry riverbed of Sirannon's former course can be followed to the point of meeting with Glanduin.