The river rises on the 1,123 m (3,684 ft) high Arnett Peak,[7] in the Cameron Mountains, runs about 23 km (14 mi) to the lake.

Its upper reaches are mainly on undated, undifferentiated Cameron Group rocks, probably of Cambrian age.

They are layers of metasedimentary and metavolcanic rocks, with quartz-rich psammite and some pelite, containing sillimanite, feldspar and migmatites.

[15][16] Medium-grained, massive to weakly foliated, Devonian, Big Pluton rocks, make up the lower part of the Lake Monk (east) arm of the river and down to the sea.

Big is part of the Ridge Suite of plutons, made up of biotite, muscovite, garnet, tonalite, grandiorite, granite and monzogranite.