Flower looks similar to the sedges but I don't think the leaves are right. Old botanical saying: Sedges have edges, Reeds are round but grasses are groovy - refers to the leaf shape of those water loving plants. This one appears to have rounded leaves so I'd be looking at the family known as reeds.
Over here I would say Juncus sp but I just searched for reeds for ponds in Ca. and came up with this one -Scirpus validus, Check this link and see if it matches the plant you have :
Prairie Moon nursery