A new dataset for designating headwater streams in the Port Phillip and Westernport region

The Victorian Water Act (1989) defines waterways primarily according to their channel form and regular flow of water. For Melbourne Water to designate waterways under the act, they primarily require accurate spatial data representing channel path and extent, and secondarily require reliable predictions of the flow regimes along those channels. In this document we compare a new spatial layer of stream channels, the Melbourne stream network (version 1.1, 2020), to the Victorian Government’s Vicmap watercourse layer, and assess their accuracy against channels evident from a LiDAR hillshade model. We also use other lines of evidence such as vegetation change, direct flow observation, and surface and sub-surface water-level sensors to assess how likely the presence of a channel is evidence of regular flow.

The 2020 Melbourne stream network is a more accurate representation of waterways than the VicMap watercourse layer. The 2020 stream network’s extent of headwaters is very accurate in areas covered by 2009 LiDAR, with some headwater’s unmapped in areas without 2009 LiDAR coverage. Revision of these areas using more recent LiDAR data using the same methods would provide a highly accurate representation of stream channels throughout the region.

Channels are strong indicators of water flow. In addition to the geomorphic evidence of past flows required to form channels in a dendritic network, our observations of wetland plants along channels, and direct observation of flows (either visual observation during dry weather or measured water-level data over a year) strengthens the inference that a naturally formed channel at the head of a dendritic network indicates water flow to satisfy the Water Act’s definition of a waterway. Some headwaters with broad, indistinct channels, and no persistent surface or subsurface flow, are nevertheless important retainers of flow (for the protection of downstream aquatic ecosystems) and conduits of flood waters. Such attributes satisfy the Act’s definition of waterways.