Tributary–trunk stream relations in a cut-and-fill landscape: a case study from Wolumla catchment, New South Wales, Australia

At the time of European settlement of the south coast of New South Wales, Australia, in the mid-nineteenth century, river courses were discontinuous throughout Wolumla catchment. Within a few decades of settlement, intact valley fills had been transformed into incised channels along trunk streams. This induced associated patterns of secondary incision into tributary valley fills. Three of the primary subcatchments in Wolumla catchment became fully incised, while an upland swamp and a floodout (i.e., sand sheet deposits atop an intact valley floor) remain in mid-catchment of the fourth subcatchment. Of the 70 lower order tributaries which join the trunk streams in the four subcatchments, 27 have incised (12 of which are discontinuously incised). Incision of tributary streams in Wolumla catchment cannot be explained by subcatchment area and slope relations. The position of the trunk stream channel within the valley floor trough is the key determinant of whether or not tributary streams have incised. Changes to river morphology since European settlement have altered the linkage of tributary streams to the trunk stream, impacting directly on the within-catchment transfer of water and sediment.