It's called a "filter sock"

For the last decade or so I've been seeing these boom-like structures deployed at construction sites and other locations where sediment runoff needs to be controlled.  They are particularly abundant at road construction sites in the vicinity of rivers and waterways, and seem to have replaced the older method of staking straw bales ("wattles") to the ground.

I never gave any thought to what might be inside them until I encountered one (in the subdivision being built next to our neighborhood) that had been installed last year and is now breaking down:

Wood mulch.  Heavy enough to restrain runoff, lighter than sand, biodegradable.  Very clever.  In our fairly-woodsy part of the country, wood mulch is abundant.  Arborists will virtually give it away, and our town has immense piles that are free to any gardener who wants to truck it away.  A quick Google of key words yielded the term "filter sock."

Now I know "the rest of the story."  You learn something every day.