About Wilson Inlet and its Catchment
About the inlet
Wilson Inlet covers 48 km² and is approximately 14km long and 4km wide.
The western and northern margins of the Wilson Inlet are mainly rocky shores, with
sandy beaches on the eastern and southern margins.
The inlet has a sandbar at the ocean entrance that is seasonally opened, causing a
tidal exchange with the sea. During this time the water level in the estuary is related to sea level, and the Inlet contains water from both sea and catchment. When the bar is closed, water level and the character of the estuary water change with the rainfall, river flow and evaporation.
The condition of the inlet has changed over time, with an increase in algal blooms, seagrass and an increase in black oozy mud on the once white beaches. The Department of Water (and its predecessors) have been conducting regular monitoring of the inlet. Reports Report
to the community 5 : Water Quality in the Wilson Inlet from 1995 -2002 and Report
to the community 6 : Managing the Bar and the Inlet
are available on the WICC website.
About the catchment
Wilson Inlet catchment covers approximately 2379 km².
Much of the lower catchment has been cleared for agriculture and clearing. Despite the clearing around 43% of the catchment is under some native vegetation, much of this in amazing forests and National parks.
The main land uses in the catchment are grazing (beef and sheep), dairy, vineyards, plantations and cropping.
Where do people fit in?
There are three main local governments in the catchment, these being the Shire of Denmark, the Shire of Plantagenet and the City of Albany. The catchment comprises a mixture of urban areas and agriculture.
The town of Denmark is the largest population centre with just under 5000 people. Other settlements are found at Youngs Siding, Denbarker and parts of Mt Barker.
People have been living in and using the area for thousands of years. The land and waters are part of the Noongar region of the Minang people. The inlet has evidence of fish traps that would have provided a great feed. A midden has also been found on the banks of the Kordabup (Denmark river) in the Denmark townsite. This highlights that the barbeques that tourists and residents enjoy on the banks of the river today have been enjoyed by families for far longer than european settlement. There was a strong Noongar presence around the town and inlet until the early 1900's
There are five rivers that flow into the inlet. These are the Hay River, Denmark River, Cuppup Creek, Sleeman River and Little River.
The largest of these are the Denmark River and the Hay River (which includes the Mitchell River tributary). Together these two rivers serve 89% of the catchment.
The Denmark River was once the town's drinking water source, however it has become too salty due to rising groundwater tables following clearing. "The Denmark River is included in the Kent/Denmark River Recovery Catchment, under the Salinity Action Plan. It is planned to restore salinity levels to potable water standard, and the river is
seen as a very important medium-term water supply for the region."
The smaller Little River enters the inlet on the western foreshore while Cuppup Creek and
the Sleeman River enter on the eastern end.
The Cuppup and lower Sleeman are a part of the Albany Drainage District and as such are managed as drains by the Water Corporation. As a result the hydrology of the Albany Drainage District is no longer natural.
The Lake Saide drain, which serves the annual horticultural area around the lake, is part of the Cuppup system. The Cuppup has floodgates to prevent salty water from the inlet moving into this horticultural area. Much of the Cuppup catchment is in what should naturally be part of the Wilson Inlet floodplain. The drainage system and artificial breaching of Wilson Inlet has reduced the area and length of inundation.
The table below (from the WIC Compendium) highlights various aspects of the sub-catchments.
There are a number of lakes and wetlands in the catchment. Information on these will be provided soon
Studies have shown that the amount of phosphorus and nitrogen leaving each sub-catchment is not the same. In fact the amount of nutrients does not just relate to how big an area is, or how much water it receives. A number of factors influence nutrient export from each sub-catchment. These include:
31% of WIC
58% of WIC
4% of WIC
1.4% of WIC
5% of WIC
|Catchment area||708 square km||1301 square km||94 square km||32 square km||119 square km
|Cleared area||220 square km||909 square km||72 square km||18.5 square km||91.6 square km
|% area cleared||28%||70%||77%||62%||77%
|Length of waterway||60 km||80 km||22 km||10.5 km||8 km
|Tidal distance||2.5 km||5 km||none||?||-
|Annual average runnoff||70 mm||60.3 mm||154 mm||?||-
|Salinity at dischange||Marginal||Brackish||Marginal||Fresh||Fresh
|Depth||Up to 4 metres||3-4 metres||2.5 metres||4 metres||4 metres
|Mean annual flow||37 million m3/year||70 million m3/year||11.3 million m3/year||3.2 million m3/year||-
- soil type;
- number and type of waterways;
- the type of agriculture;
- agricultural practices (eg linking soil testing to fertiliser application);
- amount of urban development;
- ground water and its movement;
- height in the landscape;
- amount and position of native vegetation;
- amount and position of perennials (pasture, crops or agro-forestry);
- amount and type of Landcare work being undertaken.
"The nutrient monitoring results show that the load of nutrients is not necessarily proportional to the flow between catchments. In 1998 the Sleeman River provided only seven percent of the measured water flow. However, the Sleeman River provided 42 percent of the bio-available phosphorus and 29 percent of the total phosphorus. So it can be seen that the loads coming down the Sleeman River and the Cuppup and Sunny Glen Creeks are higher than the Hay River given the disparity in flows."
Nutrient levels in the Denmark, Hay and Sleeman Rivers clearly respond to increasing discharge and storm events deliver most of the nutrients to the Denmark and Hay Rivers. The Cuppup Creek by contrast appears to receive most of its nutrients by groundwater (base) flow. The Sleeman River delivers nutrients from both base flow and high runoff events.
"Groundwater sampled over a year from a number of bores shows that although nutrient concentrations are relatively high in some bores, the flow rates are low. Therefore the load of nutrients delivered to the Inlet from groundwater other than that carried in streams is likely to be low."
The Wilson Inlet Action Nutrient Reduction Plan identifies the lower catchments of the Sleeman, Sunny Glen and Lake Saide-Cuppup Creek as contributing a disproportionably high nutrient load to the inlet, given their size. As such a greater effort is being directed towards this area, though we are still active in the other sub-catchments.
Not every landuse contributes, or has the potential to contribute, the same amount of nutrients to the inlet. Generally urban areas and intensive agriculure (such as dairies, piggeries, or vegetable production) have the potential to release more nutrients than an activity like grazing or plantations if they are the same size. However, due to the amount of land utilised by grazing it is this landuse that contributes the most nitrogen and phosphorus to Wilson Inlet.
(note : much of this text is taken from the Wilson Inlet Catchment Compendium, August 1998, produced by Jack Mercer on behalf of the WRC and WICC. Sections that have been taken directly from this document are indicated in "...")