Saturday, June 15, 2013

Lessons in Buoyancy at the Spiral Jetty


If you have never taken a trip to the northern Great Salt Lake (a.k.a. Gunnison Bay), do it.  It is a spectacular place, far removed from the hustle and bustle of Salt Lake City.  We made a trip to the Spiral Jetty today and had a great day enjoying the swimming and the sites.

The Spiral Jetty was built by Robert Smithson in 1970
Yeah, that's right, the swimming.  Or, better put, floating as we forgot goggles and the last thing you want to do is get that water in your eyes.  Many people have floated around at Saltair and Antelope Island, which is all fine and dandy, but the salinity in that half of the lake (a.k.a. Gilbert Bay) is "only" about 10–12%, whereas in the north half is usually near 28%.  Ocean water is about 3.5%.

Source: USGS
So, if you really want to bob like a cork, the north arm is the place to do it.  In fact, a float in the north arm is a great lesson in buoyancy, as I discovered today.


I can't hold a pose like that in freshwater, but it's a piece of cake in the northern Great Salt Lake.  This is an example of Archimedes's principle.  The upward force exerted on a body, even one as decayed as mine, equals the weight of the fluid displaced.  Salty water is denser than freshwater, so you don't have to displace as much fluid to balance your weight and you bob higher relative to the water line.  You could do an experiment by taking a day trip to float in Utah Lake, the southern Great Salt Lake, and the northern Great Salt Lake and you'd surely find the worst buoyancy in Utah Lake and the best buoyancy in the northern Great Salt Lake.  Use a toy boat and mark the water line at each site and you'll have clear evidence of Achimedes's principle in action.

Why is the northern Great Salt Lake so salty?  An earthen railroad causeway was built across the center of the Great Salt Lake circa 1960.  There are only a couple of small gaps in this causeway, so there is very little mixing of water between the two halves.  Most of the freshwater inflow comes into the southern half, which dilutes the water.  In contrast, the northern half has little freshwater inflow and is heavily enriched with minerals.

The impact of the construction of the causeway can clearly be seen in the USGS salinity data graphed below.  Note how there was a temporary decrease in salinity in the northern half of the Great Salt Lake (Gunnison Bay) during the huge snow and runoff years of the 1980s, but it still remained substantially higher than that found in the southern half (Gilbert Bay).

Source: USGS
If you want to visit Spiral Jetty and have a look and float for yourself, there is a nice description here.  The road is in excellent condition and is now passable by passenger vehicles around Rozell Point all the way to the Jetty.

2 comments:

  1. Is the lake low enough to see the Spiral Jetty? Are those photos current?

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    Replies
    1. Yes. They were taken this weekend.

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