Original Lake Edgar and Lake ...?

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Original Lake Edgar and Lake ...?

Postby guyburns » Sun 12 Jul, 2020 2:05 pm

As part of the script for my AV on Lake Pedder, I want to show and name the two small lakes that are now flooded and lie under the new lake. See attachment, taken from a HEC map "Proposed Gordon Power Development (Restricted Issue)".
Lake Edgar.jpg


The photos I have, and the attachment, show two separate lakes. Were they both Lake Edgar?

The lettering next to the southern lake, partly covered, seems to be something like "Sagar"? But I suppose it's "Edgar".

Any guesses how far underwater these lakes are?
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Re: Original Lake Edgar and Lake ...?

Postby north-north-west » Sun 12 Jul, 2020 4:07 pm

There are some maps on the Restore Lake Pedder site, although most don't go far enough to the south east to show Edgar. As far as I can see - and as far as I recall - the name of Edgar was applied only to the larger, more southerly lake. The other was unnamed.

There should be detailed pre-dam geological maps of the area, however, given the location of the Edgar Fault and its relevance to the construction of those dams. Hydro might have some in their archives. Dams added about 20m to Pedder's depth.

This is from Wikipedia, of all places. It uses the name collectively for both lakes and then contradicts that by saying it's only the larger:

Lake Edgar was a natural fault scarp pond on the upper reaches of the Huon River in South West Tasmania.
Description

Lake Edgar was actually two small lakes that were created when the Edgar fault caused the plains below Mount Anne, on the upper reaches of the Huon River, to move upwards by little more than 6 metres (20 ft), causing the river to dam and divert northwards around Scotts Peak. The lake was eventually inundated when the Lake Pedder was flooded as part of a hydroelectric power scheme in 1972. Lake Edgar should not be confused with the Edgar pond which was created when the Edgar Dam intersected one of the smaller tributaries of Lake Edgar.

The Lake Edgar Fault originally formed in the Cambrian as a sinistral wrench fault with a displacement of 12 kilometres (7.5 mi). It was reactivated in the Quaternary. The fault passes through the west part of the Edgar Dam. The fault is about 25–33 kilometres (16–21 mi) long. the fault dips to the west 60–70 degrees. The fault ruptured twice in the Quaternary period, with the early rupture around 25 kiloares (27,000,000 sq ft) and the last rupture since glaciation (less than 10 kiloares (11,000,000 sq ft)). The two Quaternary ruptures each elevated the western side by 2.5 metres (8 ft 2 in). This is the only fault known to have ruptured twice in the Quaternary period. The fault was discovered by Carey and Newstead in 1960. It cracks up Precambrian rocks in the Jubilee Element. The fault dammed up the water to form a lake. This was later split by partial draining. Lake Edgar is the southern of the two small lakes formed. Average rainfall is 2,500 millimetres (98 in) per year. Sediment would have rapidly filled the lake if it was old. The earthquakes associated with the formation of the uplifts would be of magnitude 6.6 to 7.

The fault separated graphitic phyllite in the east from metasiltstone, argillite, quartz sandstone conglomerate on the west. The Hydro Electric Commission dug a trench through the fault and dug pits to determine if water would leak through the fault when the Edgar Dam was built. The fault possibly continues through weak talc containing ultrabasic rocks and into the Gell River Fault about 30 kilometres (19 mi) north, which has an older Late Pleistocene displacement.[1]


But the only reference article from that says that the Edgar name was only applied to the larger.
https://www.earth-prints.org/bitstream/2122/1006/1/1107_1118McCue.pdf
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Re: Original Lake Edgar and Lake ...?

Postby norts » Sun 12 Jul, 2020 4:32 pm

This is a scan of a 1:250000 from 1963
Scan_20200712.jpg
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Re: Original Lake Edgar and Lake ...?

Postby tastrax » Sun 12 Jul, 2020 5:28 pm

Same on my 1960 Mines map - Lake Edgar for the southern lake, nothing on the one to the north

LakeEdgar.JPG
LakeEdgar.JPG (48.46 KiB) Viewed 1958 times
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Re: Original Lake Edgar and Lake ...?

Postby tastrax » Sun 12 Jul, 2020 5:52 pm

Cheers - Phil

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Re: Original Lake Edgar and Lake ...?

Postby bogholesbuckethats » Mon 13 Jul, 2020 4:20 pm

map from ~1935 shows just one body of water called Lake Edgar
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Screen Shot 2020-07-12 at 9.19.22 am.png
That looks like a pad.
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Re: Original Lake Edgar and Lake ...?

Postby guyburns » Mon 13 Jul, 2020 6:11 pm

Thanks for the replies.

So It looks like Lake Edgar was the larger southern lake, and the other was unnamed.

As for the depth of water over them, the photo below shows the dam.

D1064.jpg


Here are some figures and guesses:

Dam Height: 17m
Estimated height of dam above water (in photo): 6m

17-6 = 11m

I'm guessing there is somewhere between 30 and 40 feet of water over the lakes. Anyone disagree with that figure?
In feet because this section of the AV is set in the 1950s.
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Re: Original Lake Edgar and Lake ...?

Postby DaveNoble » Sat 18 Jul, 2020 2:28 pm

I just checked my copy of the Field West - Cox Bight Sketch Map (R N Smith 1952) and it has Lake Edgar for the southern lake, and no name for the smaller northern one.
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