The freshwater macrophytes of - Fisheries Reports

NSW Dept of Primary Industries 7
One secondary subcatchment is also present:
Lane Cove River: joins the Parramatta River at Hunters Hill to become Port Jackson. Much of this
subcatchment is forested, being in the Lane Cove National Park, Pennant Hills Park and Twin
Creeks Reserve.
Classically, for hydrologic purposes such as estimating water yield or determining
erosive/sedimentary characteristics, tributaries within catchments have been categorised on the
basis of the Strahler (1957) or the Shreve (1966) methods. Both techniques begin at the top of a
catchment and use an escalating number system when two small tributaries join to make a larger
one.
We applied neither the Strahler nor the Shreve numbering system in this project, but instead used
the tidal plane as the fundamental reference point and numbered the tributaries proceeding from
downstream to upstream. There were several reasons for doing this:
1. The project did not focus on hydrology per se but on the distributions of freshwater
macrophytes.
2. The distribution of freshwater macrophytes in the lower reaches of the tributaries is
mediated by the presence of saltwater. While variations in penetration of saltwater along
tributaries are brought about by changes in rainfall (and changes in sealevel), the tidal
plane offers a common reference point across subcatchments.
3. Depending on slope, two tributaries given the same reference number based on the Strahler
(1957) or the Shreve (1966) methods might have fundamentally different characteristics.
For example, many tributaries of in the steep portions of a catchment can be ephemeral,
carrying water only during wet weather intervals, whereas equivalently numbered
tributaries of lower relief may be permanently inundated.
To put this in a more specific context, both the Middle Harbour Creek and Lane Cove River
systems are in steep terrain and have many more tributaries than the Upper Parramatta River
system. Because of this, the order numbers of their lower tributaries will be higher than for those of
the Upper Parramatta. This means that direct ecological comparisons of vegetation types are not
possible. By reversing the Strahler and Shreve methods, and using a common datum in the form of
tidal limit, tributaries can be compared from the bottom of the subcatchment progressively
upwards.
The whole of the lower portion of the Parramatta River, from the heads to the most downstream of
the Parramatta weirs, is tidal and therefore considered estuarine. The second main arm of the
estuary is Middle Harbour Creek. Because of their direct contribution to the river system these
elements were considered to be “subcatchments” and assigned the “1 st tributary” category. The
Lane Cove River drains into the Parramatta River, hence it was considered a “sub-subcatchment”
and given “2 nd tributary” status. The other small creeks flowing into directly into Middle Harbour
Creek and the Parramatta River were also given 2 nd tributary status. Upper branches were then
numbered as 3 rd tributary, 4 th tributary, etc.
The first data layer we used to record tributaries was the NSW Land and Property Information
(LPI) streamflow network. There were deficiencies in this data set, and these were of two types.
Not all creeks were marked even though in many cases the drainage lines were obvious. This is not
an uncommon occurrence for NSW waterways (A. Creese, NSW DPI, pers. comm., 2007). In the
second situation creeks were marked but were not named, or the branch to which a name had been
provided was unclear, and this made record keeping difficult.
Freshwater macrophytes of the Parramatta Catchment (Williams and Thiebaud)