Just an addition to all the suggestions that have been posted he

If you aren't growing anything from seed (i.e. it's all being planted) there
is a product available called Ronstar. It's in granulated form and the idea
is you spread it over an area you have planted, and no seeds will germinate
there.
Found it to be particularly good for ground covers, as spraying weeds that
pop up through groundcovers isn't the easiest thing.
As I mentioned, only good if you are not growing anything there from seed.
Oh, and not too sure on the situation with being near a riverbank. If it's
likely to wash into the bank don't use it.

Good luck

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