Formed where the Glanduin (mistakenly labelled as "Swanfleet river" on some maps) joined the Mitheithel (Greyflood), the Swanfleet was effectively an inland delta, with uncertain streams and a very uncertain difference between land and water.
It was about fifty miles in length from west to east and over twenty-five miles from north and south.
These marshes strongly resembled the Gladden Fields on the other side of Misty Mountains, and so many Stoors felt right at home here after their migration into Eriador. These Stoors remained here until the Great Plague nearly wiped them out, and the remainder then went to the Southfarthing of the Shire.
- The Atlas of Middle-earth pg. 81