Shortly below its source, the river picks up the waters of the stream draining November Lake.

Its other main tributaries, Ring and Mashiter Creeks, enter near the river's mouth.

Skookum Creek: Skookum Creek flows southwest from remote Mamquam Lake within Garibaldi Provincial Park and enters the Mamquam about 12 km above its mouth.

Ring Creek: Beginning at the head of the Diamond Glacier within Garibaldi Park, Ring Creek flows south then west to its confluence with the Mamquam about 4.2 km above its mouth.

Mashiter Creek: Flowing west then east from its source on Columnar Peak, Mashiter Creek is the Mamquams final tributary, entering the Mamquam about 2.7 km above its mouth near Garibaldi Estates.