A bit more info.
Here's another picture just at the end of the calm bit.
Looking at the charts
there is nothing up wind
or up current
or under the area to create rips or overfalls and prevent wave build up. It is 150m deep here and 45NM to the next bit of land in Tobago
. Any water
should have picked up a good chop over that fetch. Current
here is over 2kt, but if if there was a deep overfall like a continental shelf near Tobago
then I would have though its effect should be perpendicular to the current not in the same direction.
There was a small increase in water
speed as I crossed the calm rivers with the autopilot
squirreling a bit.
They didn't look like eddies as they were several of these and running straight all the way to the horizon.
As well as Internal Waves, I wonder if it might have been fresh water. Fresh water floats and being warmer than the sea will not want to mix I suspect and so may stay in river like bands, much like spume from waves forms into long strings on windy days. I found some pictures showing Orinoco water going right through this area. If it the water were rising as well, it would be like an overfall and the Internal Wave and might produce a calm area.
Science Focus:The Orinoco River Plume as seen by MODIS — GES DISC
Amazon and Orinoco river Plumes evolution tracked by SMOS - Sea Surface Salinity - Remote Sensing