Dealing with salinity in Wheatbelt Valleys - Department of Water

Unlike the valleys of most rivers, which usually broaden downstream, the valley of the Avon is wide near
its source (77 km) and narrows to 5 km or less after Toodyay. These broad shallow valleys of the upper
Avon are characteristic of the wheatbelt rivers.
The eastern part of the Avon catchment is characterised by inter-connected lake systems. Some of
these lake systems are terminal and do not connect to the main drainage channel even in major flood
events.
Following significant rainfall in the Avon River catchment on 21 st and 22 nd January 2000, there were high
river levels experienced from Lake King to Perth. The 2 day rainfall was in excess of 100 mm in a large
area from east of Hyden to Beverley, with the highest reading being 172 mm east of Corrigin.
Much of the mainstream Avon River upstream of Northam, the Mortlock River upstream of Northam and
the Salt River upstream of Yenyening, had flow rates in excess of 150 m³/s. Below Northam the flow
rates were in excess of 300 m³/s with the peak flow in the Swan River at Great Northern Highway (GNH)
of 312 m³/s occurring on Tuesday 25 th January 2000.
On the upper Swan River the flood had an overall average recurrence interval of 1 in 8 years (using all
record since 1970), and an average recurrence interval of 1 in 20 years for summer events.
The volume of water reaching the Swan River during the event was 270 GL which compares to the
approximate estuary volume of 50 GL. Downstream tributaries of the Swan River like Ellen Brook and
Canning River had proportionally minimal contribution to the flow.
Water Quality from the upper Swan River indicated that the river carried an estimated 1,200 kT of Salt,
800 T of Nitrogen and 35 T of Phosphorous from 23 January 2000 to 1 March 2000. For this event, the
flow-weighted salinity on the Swan River at Walyunga averaged 4,500 mg/L TDS, Total Nitrogen
3.0 mg/L TDS and Total Phosphorous 0.12 mg/L. The salinity of the Swan River at the Narrows Bridge
reduced from its normal 24,000 mg/L TDS prior to the event to 4,400 mg/L at peak flow.
The Lockhart River experienced the worst flooding of the event with high river levels inundating large
areas of land adjacent to the mainstream and high flow velocities causing extensive erosion. While a
total of 108 GL of water flowed in the river during the event there was also 307 kt of salt. However just
after the first peak on Sunday 23 January, the water was actually fresh with a salinity of 480 mg/L
measured for a period of three hours. The estimated peak flow on the Lockhart River at Kwolyn Hill (just
upstream of the confluence with the Yilgarn River is 196 m³/s which had an average recurrence interval
of 1 in 25 years (Figure 3).
Flow (m3/s)
200
180
160
140
120
100
80
60
40
20
Flow
Lockhart - January 2000
Salinity
0
0
20/01 21/01 22/01 23/01 24/01 25/01 26/01 27/01 28/01 29/01 30/01 31/01 01/02
Fi 3 Fl d S li it L kh t Ri
40,000
36,000
32,000
28,000
24,000
20,000
16,000
12,000
8,000
4,000
Salinity (mg/L)