just read that silting up will stop the streamer weed from growing/regrowing,so if the stretches on the goyt have had large amounts of sand/silt washed down in floods then this could be the cause of the weed not recovering in those areas.i know round pm after flooding there's always lots of new sandy silt everywhere,so replanting it might be futile unfortunately,but it may be worth finding a hardier weed that will grow in silty beds

cheers

kev.