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Where is the largest natural harbor in the world?

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kaipara harbor, new zealand

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Added by Clay 4.18.10 - I looked for the answer to this question, and found Sydney Harbor, Australia and Kaipara, NZ both listed - each one definitively and each in many different places - as the world's largest natural harbor. Guanabara Bay/Rio de Janeiro, Brazil, Trincomalee, Sri Lanka and Poole Harbor in the southern UK were also listed in various places as largest. First, I will specify that here I am seeking the physically largest natural harbor, not the one which carries the most traffic (whether measured by vessel number or tonnage, or value of cargo, or whatever).

It is amazing to me that, given the many candidates and widespread uncertainty of even the answer to "Which one is the #1 largest?", there seem to be literally dozens of places claiming the title to second- or third-largest - Among them are Halifax, Nova Scotia; Cork, Ireland; Hampton Roads, VA, USA; Falmouth, Cornwall, UK; Cartagena, Colombia; Kingston, Jamaica; Freetown, Sierra Leone, ad infinitum.

One aspect of the difficulty here is the scarcity of information setting out the size of all the various candidates, measured in square miles (or km). Also, even where such measurements are available, comparing them is risky, as the length of any coastline (and thus size of any harbor) depends on how finely one measures the curves and irregularities - coastlines are fractal, after all!

So, I decided to do a bit of homespun research - I went to Google Maps and compared the candidates! I knew to make certain that the maps were scaled alike, but I anticipated real trouble trying to compare the areas of oddly-shaped bays, inlets and harbors. But, as it turned out, the winner seems to be so far ahead of all the other candidates that it's not even close enough for this problem to arise.

Only Kaipara is even close to the winner, and it has ... issues. Most of it is apparently only 'harbor' at high tide, and the rest of the time it's mostly mudflat cut by streambeds. Plus, so many shifting sandbars at the entrance that it's called the graveyard of NZ. So yes, Kaipara is a harbor, and it looks to be the second-largest in area, but I have serious doubts about the 'harbor' nature of a place which is as much a wetland as a bay.

The winner that I can find, judging by satellite imagery, is San Francisco Bay. What numbers exist seem to agree - It's listed as 250 square miles, while Kaipara Harbor is listed as 149 square miles, and that's presumably at high tide. (nope, it's 366sqm at high tide and 158sqm at low tide - matthk) San Francisco Bay wins not merely by a country mile, but by a hundred and one square miles!

http://www.flickr.com/photos/28217412@N00/sets/72157623759734841/

Above is the URL linking to four screen grabs, each with a pair of satellite shots, comparing San Francisco Bay respectively to Kaipara Bay, Sydney Harbour / Port Jackson, Poole Harbour, UK, and Trincomalee Sri Lanka. The scales for each pair of maps are either identical or, where they are not, the discrepancy is in favor of the 'contender' (the non-San Francisco one). (I don't see quite how to make a link here, but I'll try.) Measured at the same scale, most of these other contenders are barely visible, while San Francisco Bay is immense.

Incidentally, a truly astounding historical factoid about San Francisco Bay (until shown otherwise, hereafter I'm calling it the largest natural harbor on Earth). (Non-native) mariners of all descriptions sailed up and down the entire coast of California for more than two hundred years before anyone even found this treasure, this two hundred and fifty square miles of safe anchorage! Sir Francis Drake (in his pre-Sir days) actually spent the entire winter of 1579 on a beach at the foot of Mt. Tamalpais, and none of his party ever bothered to climb up and see the entire bay spread out there on the other side. Nonetheless, no Western ship is known to have sailed through the Golden Gate into the Bay until 1775 - The first group of Bay settlers arrived exactly one week before July 4, 1776. (See Assembling California or Annals of the Former World by John McPhee.) This failure to find the Golden Gate in two hundred years of looking is, admittedly, somewhat easier to believe once you've seen the fog there ... But still, I find it pretty amazing.
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