Hydrology
of Bendethera Caves
For many visitors to Bendethera Main Cave who walk beside and
cross many times the beautiful clean clear waters of Con Creek whilst trekking
on their journey to the main cave, the thought of where the water originates is
of little concern.
However, to the speleologist, the source of the creek is of
extreme interest and it has puzzled many of us for several years!
The catchment area of Con Creek is bounded on the west by the
escarpment of the Minuma Range to a height of 1000m ASL and to the east by the
Duea River at 300m.
As the
diagram at left shows, the approximate 11 square kilometres of catchment for
Con Creek and the approximate 8 square kilometres for Little Con Creek,
provided an almost 20 square kilometre catchment area that collects water
before emptying into the Duea River near the current Horse yards near the site
of the old homestead.
However, Con creek is only a perennial stream from the efflux at
BD6 and Little Con Creek begins from a spring at the base of the Gin Outcrop.
The rest of the creeks are intermittent.
As the year 2002 heads into a severe drought and Eurobodalla
Council begins to impose water restrictions on its towns the water from the BD6
efflux continues to flow at an unabated rate and the water in Little Con creek
reduces it’s flow. The information from the Bureau of Meteorology supports this
from the automatic weather station information shown in the climatic chapter of
the field guide and as shown below.
Mean
Rainfall record for Bettowynd for 71 years from 1897 to 1976(mm)
|
J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Yr |
|
74 |
76 |
74 |
60 |
64 |
65 |
53 |
44 |
48 |
62 |
57 |
67 |
744 |
Bendethera
over a 3 year period - Bureau of Meteorology remote site details
|
Year |
Jan |
Feb |
Mar |
Apr |
May |
June |
July |
Aug |
Sept |
Oct |
Nov |
Dec |
Year |
|
1999 |
0 |
0 |
0 |
74 |
2 |
32 |
61 |
30 |
17 |
5 |
37 |
58 |
316 |
|
2000 |
2 |
14 |
89 |
27 |
13 |
3 |
4 |
15 |
36 |
17 |
149 |
25 |
394 |
|
2001 |
66 |
31 |
41 |
11 |
0 |
2 |
66 |
17 |
24 |
27 |
55 |
12 |
352 |
|
2002 |
30 |
103 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
133 |
|
avge |
33 |
49 |
65 |
37 |
5 |
12 |
44 |
21 |
26 |
16 |
80 |
32 |
354 |
As Speleologists we are concerned with KARST HYDROLOGY—The
branch of Hydrology that deals with the hydrology of geological
formations having large underground passages or fractures which enable
underground movement of large quantities of water. (From the 2002 North
American Lake Management Society Karst Dictionary.)
From the SEKI Natural
Resources Home page “Karst hydrology is the study of the movement and
properties of groundwater flowing through karst areas. It is the actions of
this water that makes caves, sinkholes and other karst features. Karst
hydrology has become an important area of research in recent years because
groundwater in this setting behaves very differently from groundwater in a
normal geologic region. Usually groundwater moves through the earth very slowly
through tiny pores, spaces and fractures in rock. These small spaces may act as
filters that clean and purify groundwater. In a karst system, groundwater flows
rapidly through open conduits and passages in the rock.”
In an attempt to understand the hydrology of the Bendethera karst
lets follow the water downhill from above Bendethera Main Cave BD1. As the
water infiltrates through the thin soils of the region that cover the
limestone, the water percolates into the limestone and either seeps into the
porous vadose zone forming spelothems or continuing on its journey downwards to
the water table where it becomes apart of the phreatic zone and emerges at the
BD6 Efflux. There is a large underground reservoir in the area of BD6 and BD4
Water Cave and this possibly extends under the Flagpole Flat area as evidenced
by the only doline in the area adjacent to the track at Flagpole Flat. The
reservoir possibly extends further north east and westward as the water that
emerges from the BD2 Efflux fails to flow now and disappears into the stream
bed near this efflux. The water then possibly becomes part of this large
underground reservoir.
The description below of the BD 2 Efflux area is from and ISS
Journal Vol 4 number 3
“About 2 tons
of rock was removed and we cleared about 10 feet of passage”. This large amount of rock has assisted in
the disappearance of what was a small swimable pool just downstream of the BD 2
Efflux as depicted from the following extract:
From Southern Star Vol 2 no 135. December 20th 1972
Bendethera
Vandalised!
The Eurobodalla Shire's most outstanding natural
attraction - the Bendethera Caves - have been subjected to a vandalism which is
steeped in irony.
Ironic because the people who have contributed to the vandalism have done so in the name of conservation.
Organised groups of cave hunters have, in their efforts to seek out new caves, despoiled the natural beauty of the area and have left behind them what can only be described as a mess.
Where once there existed a fascinating and beautiful fresh water deep pool, fed by an underground stream, there now exists only a pile of rubble.
The pool, which previous admirers have described as hypnotic in its depth and beauty has been virtually filled in by the debris.
What water is left is covered by green algae.
The debris is the remains of a futile effort to discover what was thought might have been a chain of caves along the underground stream.
The cave hunters, in their efforts at discovery, dug into the side of the hill in a fashion similar to sinking a horizontal mining shaft.
The rubble from their diggings was deposited at the entrance - located immediately above the former pool - and the debris eventually filled a large portion of the pool.
And the digging also served to cut off the stream water which now seeps through the area to form a sort of marsh.
"The crazy thing about the whole effort is that these people believed that they were doing the right thing", said Bruce Robinson, one of the two persons now living permanently in the Bendethera Valley which once was alive with thousands of people during the gold mining days.
"They spent days in their digging, thinking they were going to open up a new beauty spot for people.
"Instead they destroyed one of the most beautiful deep water pools you could imagine", he said.
People used to come and sit by the pool, watching the depths for hours - it was beautiful to watch.
"Now there is just that pile of rubble", he said sadly.
And the people of the Eurobodalla Shire have lost a rare beauty spot in a most beautiful valley - because a misguided efforts at conservation.
A similar situation exists on the southern extremities of the Gin
outcrop. The water used to exit at the Wet Cave (BD36) Efflux BD37. A
description of BD36 below intimates that there is a great deal of water in the
area. But now the water seeps from a spring on the western side of Little Con
Creek at the base of the Gin Outcrop.
from the Journal
of SSS 1995 volume 39 no 6 page 125/126 by Bruce Stewart
trip
report 2 October 1993
I ... had
been given a good description of what the sump looked like, I decided that it
was a marginal prospect for anything amazing. So, I only packed a 12cu foot
pony bottle, a lightweight 3mm wetsuit, a helmet, shoves and dive reel. The
little sump was the usual clear turquoise blue, with a silty bottom and what
had been described to me as a slight squeeze hole, was the way on. So, I geared
up and put my head through the hole and realised there was definitely a way on,
but water having magnifying properties, this little hole was a bit of a
concern. Anyway, going through feet first and trailing dive line, my positive
buoyancy was a bit of a problem (as carrying a weight belt down here would have
been awful), so I came back through the hole and surfaced with thoughts of what
could I use as a weight belt. Unfortunately there was nothing that I could use
and I didn't want to wear a pack full of rocks, so I went back through the hole
again for a further look. The water now had zero visibility in the squeeze and
it was back to diving by feel.
After
coming out of the silt cloud, I noticed a lead on the left hand side and a
passage heading off in front of me. Diving into the passage, I found my
positive buoyancy made it very difficult, so I stopped and gathered my dive
line and looked at the air remaining and decided to
turn back, with a mudbank rising in front of me.
References
Gillieson, David. (1998)– Processes, Development,
Management, Blackwell UK
Ian Household (1989) Bendethera Karst Area – Resources and
Significance – A field Trip 18-2-1989 for NPWS
Internet
SEKI Natural
Resources Home Page
Cave and Karst Management
http://www.nps.gov/seki/snrm/geology/karst_hydrology.htm
Hydrology,
Hazards, and Geomorphic Development of Gypsum Karst in the Northern Black
Hills, South Dakota and Wyoming
By Jack B. Epstein
U.S. Geological Survey, National Center, MS 926a, Reston, VA 20192
http://water.usgs.gov/ogw/karst/kigconference/jbe_hydrologyhazards.htm
WHAT IS KARST?
http://www.dyetracing.com/karst/ka01000.html
Lake and Water Word Glossary
In-Cave Dye
Tracing and Drainage Basin Divides in the Mammoth Cave Karst Aquifer, Kentucky
http://water.usgs.gov/ogw/karst/kigconference/jm_incavedye.htm
Karst Features
and Hydrogeology in West-central Florida -- A Field Perspective
By Ann B. Tihansky and Lari A. Knochenmus
U.S. Geological Survey, 4710 Eisenhower Blvd. B-5 Tampa, FL 33634
http://water.usgs.gov/ogw/karst/kigconference/abt_karstfeatures.htm