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Relative sea-level data for Hudson Bay

Why is the sea level in Hudson Bay decreasing so much? Hudson Bay is pretty far up north, much closer to glaciers. Would it make sense for it to recede at this level with sources of fresh water relatively close?

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The area is experiencing post-glacial isostatic rebound.

Much of Canada was covered in an extensive ice sheet in the last glacial period (the 'Ice Age'), from about 110 ka until 12 ka. The ice in the Hudson Bay area was among the last to melt:

Ice retreat in North America

A thick ice sheet depresses the crust (the lithosphere), making a small dent in the uppermost mantle (the asthenosphere) in the process. Well, not that small: p 375 in Gornitz (2009, Encyclopedia of Paleoclimatology and Ancient Environments) says it could be 800 m for a 3000 metre-thick ice sheet!

Since the asthenosphere is highly viscous, it takes a long time time for the depression to 'bounce' back up. This map from Natural Resources Canada shows the current rate:

Isostatic rebound rate in Canada

Since global sea-level is currently rising at about 3 mm/a, a local uplift at this rate will break even. Anything more will result in relative sea-level fall, as we see in Hudson Bay (as well as in Scandinavia, the UK, Alaska, and elsewhere — this map is wonderful).

Interesting, for geologists anyway, is the sedimentological record this leaves. I love this example of raised beaches and a small delta experiencing forced regression on the shores of Hudson Bay:

Forced regression of small delta

Last thing — you asked:

Would it make sense for it to recede at the level that it is receding with sources of freshwater relatively close?

Since Hudson Bay is connected to the world's ocean, mainly through Hudson Strait, the runoff into the Bay has no measurable effect on the water level.

Credit Ice retreat map by TKostolany, licensed CC-BY-SA. Rebound map by NRCan, free of copyright. Google Maps image contains own credit.

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  • $\begingroup$ Great answer - but did you mean "Anything more will result in relative sea-level fall" near the middle? $\endgroup$ – Semidiurnal Simon Apr 23 '15 at 10:48
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Because of post glacial rebound. The asthenosphere was pressed down under Laurentide ice sheet during last ice age and is now finding a new balance, without the weight of the ice.

Note that around the ice sheet, the land is actually sinking today, like when ones partner gets up from a waterbed mattress.

"PGR Paulson2007 Rate of Lithospheric Uplift due to PGR" by Erik Ivins, JPL. -  Licensed under Public Domain via Wikimedia Commons

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enter image description here

The graph shows sea level fall (SLF) at the Churchill tide gauge station.

It is on the west side of Hudson Bay.

These jagged edge ups and downs, and severe drop in a short time, indicate this is not the result of rising land mass.

Greenland ice mass loss is the source of SLF in a wide area.

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  • $\begingroup$ Sea level has been declining in New Jersey for many years now...which says to me global warming is leading to greater evaporation...especially in areas where fresh water sits on top of salt water while the current takes the salt water away (western Hudson Bay, New Jersey to Nova Scotia, the Outer Banks of North Carolina, North Florida) If continuing entire Gulf of Mexico might see a huge drop because of Mississippi "boring" and Eastward Flow. Tide go out...not come back for fifty years... $\endgroup$ – Doctor Zhivago Aug 23 '16 at 3:40
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Gravitational dynamics of the Greenland Ice sheet have an impact, sea level fall, not often mentioned (e.g. on Europe: Proof of Concept - 5).

It causes sea level fall and sea level rise, depending on distance from the ice sheet (The Gravity of Sea Level Change).

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