About from its mouth, the Cispus River receives Muddy Fork from the left. Muddy Fork starts from this junction, at Mount Adams' Lava Glacier and is named for the glacial debris and silt in the water which give the river a muddy appearance. The Cispus River flows through a heavily forested valley, much of the forest regrown after the Cispus Burn, which occurred the first decade of the 20th century and consumed most of the lower drainage.