Table 9. Summary of long-term gully erosion rates
During the 1950s and early 1960s, the mudflat
at selected sites.
areas of Racine Island were largely undissected
and had a mostly complete cover of vegetation
Duration
Distance
Rate
(interior sedge meadow; Racine and Brouillette
Gully
(years)
(m)
(m/yr)
1995a). The C, C/D and D areas were mostly
B
195060
10
35.4
3.5
vegetated (halophytic herb meadow; Racine and
196072
12
70.8
5.9
197286
14
330.4
23.6
Brouillette 1995a), except where dissected by nar-
198695
9
35.4
3.9
row, relict drainageways that extended into fresh-
Total
45
472.0
10.5
water marsh (bulrush) in the areas where the C-,
Parachute
195060
10
11.8
1.2
Bread Truck and Pond Beyond ponds now occur
196072
12
82.6
6.9
(Fig. 3, 4 and 5). Open or standing water covered
197286
14
70.8
5.1
198695
9
35.4
3.9
the drainageways that feed Mortar, Bread Truck,
Total
45
200.6
4.5
Parachute and B gullies in the early 1960s. In
In-Between
195060
10
23.6
2.4
Parachute and B gullies, open water covered what
196072
12
99.1
8.3
now are gullies to within about 100 to 150 m of
197286
14
94.4
6.7
the Eagle River. At Mortar and Bread Truck, the
198695
9
66.1
7.3
Total
45
283.2
6.3
narrow relict drainageways were arranged in a
dendritic pattern penetrating into emergent
Mortar
195060
10
21.4
2.1
196072
12
25.2
2.1
sedge marsh (Racine and Brouillette 1995a). They
197286
14
82.0
5.9
formed a tight drainage network along the
198695
9
35.3
3.9
boundary of what is now Bread Truck Pond and
Total
45
164.0
3.6
Pond Beyond, and where there is at present stand-
Bread Truck
195060
10
34.3
3.4
196072
12
34.3
2.9
ing water. Relict drainageways directed water
197286
14
251.5
18.0
flow through an abandoned meander present
198695
9
194.3
21.6
in Bread Truck Pond. Small abandoned pools are
Total
45
514.4
11.4
also apparent in several of these relict gullies in
Coastal 5
195060
10
11.4
1.1
locations where headwall recession is active
196072
12
175.8
14.7
197284
12
225.1
18.8
Open water first became evident on photos in
198495
11
202.6
18.4
1967 in the areas near Clunie Creek and Clunie
Total
45
614.9
13.7
Point (Fig. 5). There were also several small bod-
ies of water in the C/D and D areas, where small
at Parachute Gully (Tables 8) is almost identical
sedge bogs are now located. The major gullies of
to the 3.9-m/year average since 1986; the 1.6-m
Bread Truck and Mortar were also actively erod-
rate observed at Mortar is lower than the 11-year
ing into the mudflats by 1967 (Fig. 7, 32b and c;
average of 3.9 m/year (Fig. 23d, 32b; Table 8b).
Table 9). By 1974, areas in C-Pond, the channel in
Since our hub and line stakes at B-Gully record
Bread Truck Pond, and large relict drainages in
only lateral erosion, the current rates are not com-
Pond Beyond and C/D-Pond had additional open
parable to the 11-year average of 3.9 m/year of
water and, by 1984, ponds had dimensions simi-
headward recession.
lar to the present. Pond expansion and gully
recession rates (Table 9) appear to have been great-
Historical analysis
est during the late 1970s and early 1980s at a time
of the drainage system
when isostatic uplift rates were high (~1.5 cm/
The surface morphology and drainage patterns
year; Brown et al. 1977).
of ERF have undergone significant change over
The Eagle River exhibited major changes only
the last 45 years, with large scale changes particu-
where it enters ERF; these changes are probably
larly evident since the 1964 earthquake. The pho-
related to floods of the river, rather than tidal
tographic coverage from 8 August 1950, 27 June
inundation. In 1950, two well-developed chan-
1953 and 30 August 1960 shows an apparently
nels, which bifurcated about 850 m southeast of
stable environment prior to 1960. The only indi-
the Route Bravo Bridge, entered ERF (Fig. 34a).
cators of on-going change are the incised mean-
The northern channel was straight and confined
ders of the Eagle River where vegetation was ab-
by the uplands, while the south channel appears
sent and some slumping was evident. Small,
slightly subordinate in discharge volume and
low-relief channels drained into tidal gullies,
had a meandering pattern. Between 1957 and
which were eroding slowly into the mudflats
1960, the south channel had eroded and captured
(Table 9).
44
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