Marchoise, Marcoise, Morcoise, Mortaise, Martorse, Mortayse, Mortoise
A river running through the Waste Forest, dividing it in half.
Gareth slew Sir Gerard and Sir Arnold near the river during his adventures. During the Grail Quest, Lancelot visited the river, was defeated by a black knight, and found a magic barge that gave him news of Galahadís adventures.
In Malory VII, Mortaise is put in the vicinity of Dame Lyonors' Castle Dangerous, which Gareth and Lynette reach from Caerleon. Here it seems to be a natural body of water, its passage guarded by two rascally knights whom Gareth slew. Later in the Grail Adventures, Mortaise appears as the water beside which Lancelot was stranded for a time before boarding the ship with Galahad and the body of Percivale's sister, the water near which the Temptress told Percivale she had seen the Red Knight with the white shield (Perimones), and possibly the water into which Percivale rode the demon horse before reaching the island (of Turnance?).
Vulgate VI gives a fuller description, identifying Mortaise as a lake ("laigue"). After eating bread and water with a recluse, Lancelot passed the night on a high rock. The next day he came to a deep, very beautiful valley between two high rocks, before which lay Mortaise, divided in two parts by a wooded tounge of land. The wood was dense, the water very deep.
To make the Bristol Channel and Severn River the water of Mortaise, and Dundy the Isle of Turnance, should give us room to accommodate all the events Malory describes. The horseshoe bend in the Severn near Newnham might fit in the Vulgate's description.