Aren't those kettle lakes created by glaciers? (EARTH SCIENCE FOR THE WIN!) 
... i thing some of those are due to the marsh area character of the river surroundings. I know northern Siberian planes have such wast lands in deltas - there its due to near surface permafrost and ground above changing every year after winter-summer cycles till at some places the ground submerges at certain places forming a little lake, but in summer is basically all the area a no-animal zone, cause even green areas have unsupportive ground characteristics - so much to their wide use of hovercraft, screw-floater vehicles and hi-volume wheel vehicles. In other words - ice termo-mechanics at their best
