Answer: What are those lines in the bay?

This really WAS a hard search problem!  Congrats to Hans and Fred for figuring it out.

Let's look at what Hans wrote in his comment... (edits are mine).  

For me, the key takeaway lessons are that one should explore other terms than the ones that first come to mind.  This particular problem took me a long time because I was stuck on the idea of the marshland lines as channels or canals.  Turns out that I really needed to use a different word:  ditch.  

Let me first tell you that I live in the Netherlands, so I'm familiar with dikes, ditches, canals and tidal marshes. Further, I'm a professional information researcher (I work in a corporate information center) so I know a lot about how to conduct a search on the internet.

Here are my search steps:
* I found in Google Maps the name of a bay in the neighborhood "Brosewere Bay"

A good move to start with on a geographic search like this.  You want to figure out what the local geographic features are named:  those terms will be useful when you try to set up your search.  

* A search in Google with ["long island" "brosewere bay" +canals] brought me to a website of the Long Island South Shore Estuary Reserve: 

http://www.estuary.cog.ny.us/dredging/1_FINAL_Dredging_Needsweb.pdf

This is exactly how I started.  I searched on place names ("Long Island" and "Brosewere" and "Crooked Creek") were the ones I used.  And they weren't great.  

I eventually found took a very close look at the lines in the satellite photo.  I'd been thinking that they were 15 - 30 feet wide.  I looked at a bunch, and by using the measuring grid on Maps (or the one on Google Earth, shown here), I finally figured out that they were only from 3 - 6 feet wide.  In this screen shot, you can see that it's about 2 meters wide... 6 feet... That's obviously NOT a waterway (except for canoes, perhaps).  

And that was the insight that led me to start searching for the term "ditch," as in my query:
[ long island wetland ditches ] 

This search quickly led me to a PDF from Connecticut College "Human Impacts on Tidal Wetlands: History and Regulations" [pdf] ... and after doing a Control-F find in that report, I came across the following picture and caption (p. 46 of the book "Tidal Marshes of Long Island Sound" by Glenn Dreyer and William Niering, 1995):  

This book chapter told me that there had been extensive mosquito control ditching in the 1930's.

But back to Hans' story.    

* A search on that site using Google [site:www.estuary.cog.ny.us ditches] brought me to a document mentioning grid ditching for mosquito control http://www.estuary.cog.ny.us/ISR2005/ISR%20Outcome%204.pdf

Here, Hans did a smart thing by using the search term "ditches."  I had been searching with terms like "canal" "channel" "waterway" and "dredge"... but those didn't work very well. It wasn't until I realized that those long lines were pretty narrow that I switched terms.  

* A search in Google on ["long island" grid mosquito control] came up with the final document: http://www.geo.sunysb.edu/lig/Conferences/abstracts07/abstracts/potente.pdf 

with the explanation of the grid pattern: 

"Much of the tidal wetlands of Suffolk County, as well as neighboring coastal wetlands of eastern North America, has been previously affected by hydrological manipulations by mosquito control agencies which began early in the twentieth century. In an effort to depress mosquito populations emanating from salt marshes, parallel linear ditches were dug from the high marsh zones through the low marsh and out to estuarine bays, creeks and rivers. In many cases, supplementary ditches were also dug at right angles cross-connecting the linear ditches thus creating a grid pattern of ditches on the marsh surface. The intent was to remove the standing water of the marsh surface in which the marsh mosquitoes develop through their larval stages." 

Absolutely right.  

After searching around a bit more, I discovered that the mosquito control effort wasn't quite as crazy as it sounds.  From the same book (p 45):  

"Mosquito control practices began after the Civil War as homeward bound soldiers brought malaria to Connecticut. The disease soon reached epidemic proportions, and wetlands of all types were filled or drained to prevent malaria transmission by Anopheles mosquitoes. With the elimination of malaria as a health threat, control efforts targeted the large broods of nuisance mosquitoes that originated on tidal wetlands, especially salt marshes.

Hundreds of kilometers of mosquito ditches were hand dug to drain marsh surface waters, especially the intermittent pools or pannes which are the preferred breeding habitat for salt marsh mosquitoes..." 

And finally, after searching around (because I got really interested in Long Island mosquito ditches) I found an interesting technical article on the "The development of a tidal marsh: Upland and oceanic influences" written by James Clark and William Patterson of U. Mass. Amherst.  In their article they have the following diagram, which pretty much convinced me that we had the right name for these right-angled ditches that appear on the jet landing path into JFK.