Thank you for your question, Kim.
I’m not sure if there was any product underneath the rock of the dry creek bed. If you used plastic or fabric, you may need to poke holes in it to help aerate so the water is able to go through. Also, there may be frost still in the ground of a shady area that may not let the water through.
To help with the algae – we carry two products.
Moss Max – which comes is ready to spray after you hook to your hose. (Safe to use on Patios, grass, siding)
We also carry Algae off Algaecid – used in ponds to help remove string Algae