Dealing with salinity in Wheatbelt Valleys - Department of Water

Hatton and Ruprecht
water from one to another when and if they
overflow. Grades along the Lockhart and Pingrup
rivers (the southern tributaries of Salt River) are very
low (0.04 m/km and 0.24 m/km respectively), and
from this Beard (1999) concluded that significant
discharges are unlikely (however, see Flooding
section below).
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 flatness of the bulk of the wheatbelt river
systems leads to historic, and amusing, arguments
regarding catchment boundaries, the putative
connections between systems, and even which way
water flows. It is essential to appreciate that these
river systems do not all flow as one linked system
except in the most extreme events. In the
Blackwood catchment, the Coblinine River and
Dongolocking Creek are two headwater streams
draining to Lake Dumbleyung. This section of the
river has very low grades, approximately 0.17 m/km.
The combined drainage enters Lake Dumbleyung,
which is a permanent salt lake that is said to have
been dry before land clearing (Beard 1999). Since
clearing, Lake Dumbleyung is thought to have
overflowed into the Lower Blackwood only three
times since the 1870s. The chains of (mostly dry)
lakes form a series of local storages that in most
years are not overtopped by the surface flows from
upstream.
Commander et al. (2001, this proceedings) gives the
geological history and background to the evolution of
the wheatbelt systems, and George & Coleman
(2001, this proceedings) provides a description of the
hydrogeology with special reference to regolith
Station
No
River
salinity. The key feature of the latter issue is that the
flat, inland portions of these catchments hold massive
amounts of salt, accumulated as a result of
atmospheric deposition of marine salts, low rainfall,
the natural patterns of native vegetation water use,
highly weathered regolith, and the low gradients
described above.
Vegetation
The wheatbelt catchments were essentially
completely vegetated by a diverse range of woody
plant communities whose distribution was controlled
by climate and soil type (Beard 1981). About 65% of
the Avon catchment has been cleared for agriculture
with most clearing taking place between 1940 and
1970. However, many catchments in the upper
Avon and Blackwood rivers have cleared proportions
ranging from 85 to 95%.
HYDROLOGY
In describing the hydrology of the wheatbelt
catchments, it is important to realise that their
hydrology is characterised by high variability and
nonstationarity. The rivers of this region have higher
variability of streamflow than others worldwide
(McMahon et al. 1992), indicating a usually
unpredictable climate from year to year and season
to season (Table 1).
The smaller rivers may not flow for many years, then
either a major summer event or a wet winter will
lead to flow events. The larger rivers cease flowing
in the wheatbelt regions during summer, except
when extreme tropical cyclonic events or severe
thunderstorm activities lead to heavy, intense
summer rainfall. In “normal” years, winter rainfall is
not enough to move water continuously through the
catchment from the extreme eastern and southern
boundaries to the coast.
Table 1: Comparison of hydrology statistics for Avon River tributaries.
Area
(km 2 )
Mean annual
rainfall (mm)
Clearing
(%)
Mean annual
flow (ML)
Median
annual flow
(ML)
Mean
annual
runoff
(mm)
615012 Lockhart River 32,000 350 85 7,900 1,960 0.24 1.7
615015 Yilgarn River 56,000 300 50 6,500 890 0.12 2.1
615022 Yenyenning 92,000 340 70 12,980 87 0.1 1.4
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