hello abread111 and https://canberra.naturemapr.org/users/25635, I haven't seen Creeping Knotweed with flowers of this colour; I only know CK with white greenish flowers. Could it be that photos depict 2 different plant species? leaves in first 2 photos seem to belong to Creeping Knotweed; I'm not sure about the flower spikes in last 3 photos; the flowers look more like Lythrum hyssopifolia, Small Loosestrife which we also recorded around the dam; see https://canberra.naturemapr.org/sightings/4642064. See also id-features of Small Loosestrife at https://vicflora.rbg.vic.gov.au/flora/taxon/23fb671c-b693-4b6c-9edf-f240374c10fb
Not every frond has the spores on the back of it . You have to keep looking until you find one with spores. they occur in little clumps of 2, 3, or 4 etc. spore casings.