Kerwin Abinoja - ENVE341 Research Poster November 2015 (Assessing River Recovery in the Fish River after Sand and Gravel Extraction)

1. Assessing River Recovery in the Fish River After Sand and Gravel Extraction
By: Kerwin Abinoja – Macquarie University 2015 (Supervisor: Dr Kerrie Tomkins)
Introduction:
The Fish River is a partly-confined low sinuosity laterally
meandering river channel1. It is situated in the Macquarie
catchment within the Murray-Darling basin and located
in the Central Western Region of NSW, Australia2 (Figure
1). It starts at an elevation of 1,160m below Charlies Hill2.
Figure 1 showing the location of the Fish River (black) located
within the Central Western Region (yellow) of NSW (green)
Background:
Sand and gravel extraction in the Fish River began as a
Rivercare ‘stream clearance’ project which aimed to
extract some of the sand and remove the willows which
had been causing channel blockages within the
channel2. However, it over-extracted sand beyond its
proposed 1m below the channel bed, extracting down
into its gravel bed armour and removing all riparian
vegetation, including important native species2. This
problem is significant as previous studies have
highlighted the negative impacts of unsustainable sand
and gravel mining on rivers, which include the
disruption of the river bed armour leading to bed
degradation, localised channel incision and widening,
severe bank damage and erosion, degradation of native
riparian vegetation, fauna, and habitats, and
deterioration of instream/groundwater quality3,4,5,6,7,8,9,10.
This issue is particularly significant to the Fish River
because it is a ‘sediment-starved’ river system, in which
sediment stores on the valley floors are the primary
component of post-disturbance sediment3. This means
that sediments in the Fish River have a higher likelihood
of being removed or re-worked after river mining without
being replenished readily, making it more prone to river
degradation due to its lower rate of recovery in terms of
geomorphic replenishment. The natural rate of recovery
in the Fish River is therefore worth investigating.
Research Objective:
To collect and comparatively analyse geomorphic and
vegetation data from degraded and intact reaches of the
Fish River to determine whether the degraded reaches
have substantially recovered 10 years after sand and
gravel extraction in terms of geomorphic landform
development and/or riparian vegetation characteristics.
This research contributes to the study of river recovery.
Methods:
1) An aerial map of the Fish River was used to identify degraded & intact reaches.
2) Reaches with signs of degradation were selected to represent degraded
reaches while reaches further upstream near Evans Crown Nature Reserve
were selected to represent intact reaches due to it being a nature reserve and
therefore no human disturbance would have preserved these intact reaches.
3) Channel size surveys were conducted on two degraded reaches and compared
to earlier channel size measurements taken in 2001 from similar locations.
4) Visual observations were made on the geomorphic landforms and riparian
vegetation characteristics of degraded and intact reaches. Manning’s n
coefficient for riparian vegetation was also used to assess vegetation density.
Fish River
Study Sites:
Figure 1
Upstream
Downstream
Flow Direction
Upstream
Figure 2: Locations
of selected
representative
reaches in the Fish
River.
Degraded reaches
(left) and intact
reaches (below)
Fish River
D 2
D 1
D 3
D 4
D 5
D 6
U 1
U 2
Figure 6 (top left) showing a mid-
channel bar forming in D4.
Figure 7 (top right) showing a
longitudinal bar forming in D5.
Figure 8 showing a dense cover
of casuarinas and the common
reed with negligible willow growth
in D6 indicating significant
vegetation recovery in this reach.
Figure 9 showing a dense cover
of wattles and common reeds
with no willow growth in U2.
Results and Discussion:
Channel Size and Geomorphic Units:
Channel survey results show that D4 had experienced a
2m channel contraction and D5 experienced a 1m channel
contraction (Figure 5). This indicates that to some extent,
geomorphic recovery after sand and gravel extraction has
occurred, manifested in sediment build up and resulted in
channel contraction over 10 years after river mining
activities stopped in 2005. Sedimentation in these
degraded reaches is also evident from the gradual build
up of a mid-channel bar in D4 (Figure 6) and a longitudinal
bar in D5 (Figure 7). In other degraded reaches there were
instream geomorphic units such as point bars, pools &
riffles, eddies, and small bars, which are all indicative of
geomorphic recovery. However, a 2m and 1m channel
contraction over 10 years, the absence of bench
formations, and the lack of channels with suspended or
mixed-load substrate and cohesive fine-grained banks
and floodplains in degraded reaches also indicates that
geomorphic recovery is a very slow process with limited
short-term impact. This is particularly the case when the
sediment budget is modified11, such as in the Fish River.
Figure 5 showing channel widths of D4 and
D5 reaches for 20012 and 2015.
Riparian Vegetation: Density
Manning’s n coefficient (vegetation) results in Figure 9
(below) show that the degraded reaches (D1 – D6) and the
intact reaches (U1 & U2) did not have a significant
difference in total vegetation density, recording
vegetation density scores of between 0.068 – 0.095. This
indicates that riparian vegetation in degraded reaches has
recovered to a significant extent after river mining had
ceased. This is because the initial recovery of riparian
vegetation can easily trigger a positive feedback – it
increases surface roughness which increases
sedimentation by more readily trapping transported
sediments which then creates more suitable conditions
for further plant colonisation and establishment12. The
establishment riparian vegetation is a major factor in river
recovery as riparian vegetation and woody debris
increases the narrowing of channels, stabilises banks and
floodplains, and reinforces the geomorphological units
within the channel13,14,15. Therefore, geomorphic recovery
is intrinsically linked to riparian vegetation recovery.
Downstream
Figure 5
Figure 2
Figure 6: D4 Figure 7: D5
Conclusion: (1) Geomorphic recovery is a very slow process. It would require hundreds to thousands of
years for sediments in the Fish River to be completely replenished and be utilised for long-term geomorphic
recovery. (2) Riparian vegetation recovery occurs at a much faster rate – over decadal timescales. However, the
type of riparian plant species and the rate of disturbance at different locations will influence recovery rates.
(3) Vegetation recovery and geomorphic recovery are interlinked through increasing roughness in river systems.
Figure 9
D 5
D 6
Figure 8: D6
Bosworth Falls
Figure 9: U2
Riparian Vegetation: Composition
Figures 8 – 12 show that reaches in
different locations have varying riparian
vegetation composition. Casuarinas
decrease while the common reed,
wattle, and grass increase further
upstream. This is because different
riparian species16 and different
disturbance rates17 at different
locations influence vegetation recovery.
Mid-channel bar
Longitudinal bar
Common reed
Casuarina
Wattle
Common reed
Figure 11
Figure 10
Figure 12

2. References
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(2) Tomkins, K. (2015). ‘GSE808 Lecture – Week 5: Overview of the Fish River sand and gravel extraction site’. Department of Environmental Sciences.
Macquarie University.
(3) Fryirs, K. and Brierley, G. J. (2000). ‘Variability in sediment delivery and storage along river courses in Bega catchment, NSW, Australia: implications
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