I can give you the basic protocols for managing Himalayan blackberry on your property, but since the area abuts a stream, you will need to contact our Cowlitz County Noxious Weed Board directly (Phone:<personal data hidden>), because a special permit may be required for control of infestations in riparian/aquatic areas.
Meanwhile, in order to get a real handle on your problem, look through these management guides.
https://www.nwcb.wa.gov/pdfs/blackberries_<personal data hidden>0090494_Cowlitz.pdf
https://www.nwcb.wa.gov/pdfs/blackberry_brochure_PW_7-5-16_Clark.pdf

I just got a reply from our Noxious Weed Board:

If the homeowner is interested in using herbicides, we recommend making application in the late summer or early fall, after the berries have developed. This is the most effective time to treat blackberry with herbicide. To reduce potential for the herbicide to enter the water, a few precautions can be taken. First, I would recommend cutting the blackberries to the ground within 5-10 feet of the water, and then waiting about 2 weeks for them to grow back before treating (they could cut back the whole patch to reduce overall herbicide use if they wanted). Second, I would recommend standing along the edge of the water and spraying towards the bank/away from the water to reduce the likelihood of spraying into the water. As long as the homeowner does not spray into the water, there would not be an issue treating in this zone.

 In terms of herbicide options as a homeowner, there are a few that can be used in an aquatic or riparian environment,

Option 1: An aquatic labelled glyphosate product such as Roundup Custom or Rodeo – these products are non-selective and would impact any plants sprayed on

Option 2: A Triclopyr amine product such as Garlon 3A or Vastlan or Crossbow – these type of products are typically broadleaf selective – and would not harm grasses

 Lastly, I would recommend that they follow up their herbicide treatment with some sort of seed dispersal in the area … desirable forage, native seed mix, or native grass species…this would prevent other herbaceous weeds from taking over in the freshly disturbed and desirable area where the blackberry is no longer.

 If you don’t feel comfortable providing this advice, feel free to provide my contact info to them 

Jennifer Mendoza  | Program Coordinator

Cowlitz County Noxious Weed Control Program

207 Fourth Avenue North | Kelso, WA 98626

Office:<personal data hidden> x1 | Cell:<personal data hidden>

www.co.cowlitz.wa.us/noxiousweeds