I haven't had to deal with butterbur myself, Helen, but it is notorious as being difficult to get rid of. I'd try spraying it with Roundup. End of August and into September is a good time to apply it as it will go into the roots, the plants are getting ready for winter.

I'd spray it on as soon as you like, wait a month or so and spray any remaining growth. You'll need to be prepared to retreat it next year as well, once it's started growing well.

If you've got a large area to deal with, don't mess about with those little bottles of prediluted ready-to-use spray. Get a knapsack sprayer to use just for weedkiller and get the concentrate that you dilute yourself. You don't say where you are, but if it's anywhere at all rural, go to your local farmers suppliers and buy it. That's what I do and it's cheaper than at garden centres or DIY sheds.

Just a point though, you need to be very careful using weedkiller near streams and other watercourses. You must avoid run-off, so you should be even more careful that the weedkiller doesn't drift, drip or run into the water. Roundup should be applied on a dry windless day so that a light mist lands on the leaves of the plant you want to kill, and no-where else.

What a beautiful site! I was going to say what Beattie has- make sure you keep any spray from the watercourse. Even a very light breeze will spread spray a lot further than you think (as I discovered when I damaged some of my plants at some distance from where i sprayed)
Alternatively if it defeats you,you could add natural plants such as valerian, iris, marsh marigold etc and have a wild garden.

Thank you to Beattie and Steragram. Sorry I'm a little late in replying, small accident involving 2m long butterbur, a full stream, some slippy stones a pair of secateurs and a frog. No bones broken, but I looked like I'd been 3 rounds with Mike Tyson. Butterbur is proving as dificult as ever, it seems to like Roundup. SO.... drastic measures are called for. I'm not sure what drastic measures yet, but as soon as I get a new pair of wellies, I'll work it out. Butterbur....here I come.

Oh Helen, so sorry to hear you had such an unpleasant accident. Roundup takes a week or two before any signs of dying back occur and persistent weeds need several applications.

Will wellies be long enough, or are waist length waders caled for?

Poor Helen- I am trying to visualise what happened. Poor you, hope you are feeling less bruised and ready to wage war against the enemy!