During 2002, significant progress has been
made on the collection and analysis of geomorphologic data from
two river catchments along the NSW coast, namely the Macdonald
and Tuross Rivers. Both catchments are of about 2000 km2 in size,
have highly confined floodplains, similar landuses, and climatic
regimes, yet appear to have undergone different geomorphic evolution
throughout the twentieth century despite similar climatic histories
(though with different flood histories).
Along the Macdonald River, topographic surveying
of the channel and associated flood plains has allowed clarification
of the relationship between discharge events of various return
periods and the various flood plain units evident in the valley.
The sediments of the various morphologic units have been studied
and described, and samples collected for OSL dating, which is
underway. In addition, hydraulic modelling exploring boundary
shear stress and mean stream power rates under various channel
morphology and flow scenarios has begun, leading to estimates
of the magnitude of these variables associated with different
geomorphic processes such as bench accretion, minor channel scour
and major bank erosion.
On the Tuross River, similar topographic surveying
has been conducted on the channel and associated flood plains.
The present day thalweg of the Tuross River from the lowest bedrock
bar to mean sea level displays a concave upwards profile, which
differs from the generally straight profile found along the Macdonald
River. In addition, the Tuross River has a cobble bed along the
upper lengths of the study reach (a feature absent along the
lower Macdonald River). The relationship of this cobble bed to
the flood plain units is under investigation. In a similar fashion
to the Macdonald River, a number of morphologic flood plain units
are evident along the Tuross River, each with a characteristic
sedimentary profile. Sediment samples have been collected for
OSL dating and being processed.
Striking similarities in flood plain stratigraphy
and morphology occur between these two river systems. They share
a high level alluvial bar feature with flood basins on their
distal sides, and a lower alluvial bench unit apparently inset
within this high level bar. Along both rivers, the high alluvial
bars have similar sedimentology, consisting of fine to medium
sands, dark grey to brown in colour, whilst the inset benches
are commonly yellow sands with clearly defined dark horizons
of finer sediments separating sand layers. The OSL dating of
these features may indicate whether or not the formation of these
morphologically and sedimentologically similar alluvial deposits
was synchronous in time.