try setting a humane trap to check what mammal you actually have. I suspect that next season they wont be a problem. annoying for this year. how big are the tunnels?
welcome to GoY too

They won't be be likely to be coming from the stream -there are three kinds of British vole, bank, field and water. Water voles are quite rare nowadays. They eat riverside geen plants not roots. Its more likely to be field voles as they do eat roots among other things. Voles can be a right pest in the garden. I tried all sorts to discourage them from stripping bark from shrubs and had to resort to chilli powder in the end though I hated doing it. They had ring barked and killed a 10 foot cotoneaster and had started on something else. I tried protecting the base of the trunk,I even tried used cat litter hoping the smell would put them off but it didn't. Our cat now catches quite a few of them and there have been no recent problems. Shame as they are lovely little things.

As you say Stera, they are delightful little creatures. Having set mouse traps in the shed (caught over 40!), three of those were voles - why are they in the shed? Have also found them drowned in the water butts. Don't know how to put them off though, mice/voles eat all our Pyracantha berries before the birds need them and there are holes everywhere! We had a hardy mole, we are on solid clay, which only tunneled along just under the surface, slight raised earth, but no mole hills. Great for suddenly cricking your ankle.