Umm… you lower your expectations.

There are herbicides that are considered safe and even recommended for use in aquatic settings. Crossbow and Rodeo are two brand names but RoundUp also has an aquatic approved herbicide available.

I agree that you can use herbicides carefully.

I was kind of responding to “permanently”. An herbicide can kill existing weeds - if they’re tall , though , you’ll have to think about cutting down dead foliage, or trimming back first) — but more weeds will come back. At least that’s what I expect to happen if riprap is older installation and has had time to collect some surface dirt/ plant debris and a seed bank. Or if it’s a thin layer. If you’re talking about a large lake perimeter of riprap, that could be quite a job. If you’ve got a few smaller areas of your own waterfront you’re able to “ police”, then that’s more do- able.