Episodic erosion, riparian vegetation colonisation and the late holocene stability of sand-bed, forest streams in southeastern Australia

Natural interactions between riparian vegetation, large woody debris (LWD) and the fluvial geomorphology of forest streams in North America and Europe have been well researched. In southeastern Australia, where rainfall and runoff are highly variable, where riparian vegetation species are unique and where many streams have been altered since European settlement, there is a paucity of research on such biogeomorphic interactions. This thesis aimed to partly address this knowledge gap by undertaking detailed case studies of four undisturbed streams that varied in their degree of lateral confinement by materials of limited erodibility, i.e. differences in the size and nature of the valley floor trough. These included a laterally and vertically bedrock-confined channel flanked by a series of discontinuous inchannel benches but with no floodplain (Mogo Creek); a laterally bedrock-confined channel flanked by a discontinuous high vertically accreted floodplain and a series of discontinuous in-channel benches (Wheeny Creek); a partially bedrock- and terrace-confined channel discontinuously flanked by pockets of floodplain (Bruces Creek); and a slightly terraceconfined channel continuously flanked by floodplain (Tonghi Creek). Extensive radiocarbon dating of floodplain charcoal at each site highlighted the fact that episodic, rainfall-generated cataclysmic floods have occurred during the late Holocene. These floods totally removed alluvial landforms, including the floodplain forest, from within the valley floor trough. Riparian vegetation communities that have colonised the landforms that redeveloped in the erosional void exhibit a distinct lateral and vertical zonation of species that is determined by the degree of resistance of different species to natural flood disturbance. More flood-resistant tree species, such as Tristaniopsis laurina, are able to grow and survive within the channel and on the channel banks and in—channel benches; possess the ability to reshoot from epicormic buds following high-energy flood disturbance; and can rapidly colonise recently formed alluvial landforms. Less flood—resistant species, such as the Eucalyptus species, grow on higher parts of the floodplain or on remnant Pleistocene river terraces where flood flows are of lower energy and occur less frequently. Measured LWD loadings ranged from 47 m3ha‘I at Mogo Creek to 751 m3ha‘l at Bruces Creek and varied in relation to specific stream power, the relaxation period between cataclysmic floods and the age-structure of the riparian vegetation community from which the LWD was recruited. Large woody debris recruitment processes varied in relation to the geomorphic setting and zonation of riparian vegetation. Episodic bank erosion, undercutting and senescence were identified as the dominant LWD recruitment processes from T. laurina trees growing along the banks of Tonghi and Bruces Creeks, while episodic windthrow was identified as the LWD recruitment process from Eucalyptus trees growing on the floodplains and terraces at these sites. At Mogo and Wheeny Creeks, destruction of in—channel benches by catastrophic floods was identified as the dominant LWD recruitment process from T. laurina and Leptospermum polygalifolium shrubs, while episodic windthrow was the dominant LWD recruitment process from the Eucalyptus trees growing on the high floodplains, where present, and adjacent hillslopes. The longitudinal distribution, orientation and hydraulic impacts of LWD pieces, and the development of log-steps were strongly influenced by the energy of flows experienced and the ratio of tree and LWD size to channel size. Due to high timber densities LWD tends to sink and become partially or completely buried by bedload. It is only in streams with high LWD loadings, low to medium energy bankfull flows and high LWD size to channel size ratios, such as Bruces Creek, that interlocking, wedged pieces of LWD form closely spaced debris dams. The maximum residence time for LWD identified by radiocarbon dating was 980 years on Eucalyptus timber in Wheeny Creek and was interpreted as representing the relaxation period following the most recent cataclysmic flood. Biogeomorphic evidence indicates that riparian vegetation and large woody debris can exert a significant influence on the fluvial geomorphology of sand-bed, forest streams in southeastern Australia by contributing to processes such as pool formation, stream energy dissipation, bank strength, sediment storage patterns, bed stability, oblique bank accretion, in—channel bench and mid-channel bar formation. However, rainfall—generated cataclysmic floods, capable of totally obliterating all landforms within the valley floor trough, including the floodplain forest, have occurred during the late Holocene. Cataclysmic floods exceed vegetationcontrolled thresholds of channel and floodplain stability and resultant specific stream powers of at least 2500 Wm‘2 enable the flood to erode the channel and floodplain to bedrock. Such floods occur more frequently in flood variable regions on high energy streams in narrow valley floor troughs. The width and nature of the valley floor trough, specific stream power and the relaxation period between floods are identified as important factors determining the subsequent influence of riparian vegetation and LWD on the stability of the landforms that redevelop in the erosional void