Currently being trialled in Amsterdam in Westerdok at one of the outlets from the canals into the IJ. To prevent Amsterdam canal plastic from flowing into the North Sea. Basically a perforated tube is laid diagonally across a river or canal bed, air is pumped through it and the resulting wall of bubbles prevents the passage of plastic as well as pushing it up to the surface where it is trapped and held. You can read a Guardian article about the system here. Or watch a short video here.