Alluvial systems within the dissected uplands of the WCMA region include Mount Cole Creek, Mount William Creek and the Wimmera River. Mount Cole Creek, the upper Wimmera River and other minor tributaries are supplied surface and groundwaters by Palaeozoic bedrock hilly terrain and Neogene cappings that line many lower hillslopes. As a tributary that supplies the Wimmera River, Mount William Creek rises on the slopes of Mount. William in the Grampians Ranges and flows slowly to the north-west through Dadswells Bridge. Extensive alluvial plains of this drainage system represent a low pass (geocol) that separates the dissected uplands (Midlands) to the east from the Grampians Ranges to the west.
These fluviatile alluvial plains are characterised by their effluents and anabranches which leave the main channels (Sibley 1967). The tributaries are called subsequent streams where streams have evolved along areas of rock belt weakness leaving folded or titled strata of differing resistance as prominent strike ridges (Hills 1975). The drainage networks of Mount William Creek, Salt Creek, upper Wimmera River/Mount Cole Creek and Wattle Creek are aligned in a north-north-westerly alignment with strike ridges on a regular spacing of 10-15 km. Valley flats, terraces and flood out plains are common with Mount William Creek the broadest in cross tributary dimension (often in excess of 5 km) while others are typically less than 2 km. Slopes are gently to very gently inclined with terrace slopes of greatest inclination (up to 10%) found along Mount Cole Creek and the upper Wimmera River.
The entrenched valleys of Quaternary sediments including the Shepparton Formation (a thick sequence of unconsolidated fluvial and lacustrine sediments – coarse sand, silt and clay) occur largely as terraces, plains and drainage lines. Sediments have been derived through differential weathering of granite masses that are surrounded by ridges and peaks of resistant metamorphosed sediments. The Coonambidgal Formation (reworked Shepparton Formation of slightly micaceous silty clay, sand and gravel) occur within drainage lines subject to flooding and inset streams (Butler 1958; Krokowski de Vickerod, Moore & Cayle 1997). | |