I believe they have cycles (of population peak and crash - I don't mean they use wee bicycles
) and like all the chaps at the bottom of the food chain, they're capable of increasing the ranks fast if they get the opportunity. That was probably what was happening in your stream, Meta Menardi.
I think that's why we feel we're in with a chance of saving them from extinction, because although numbers of water voles in the UK are critically low, these animals do recover fairly quickly if you can provide and sustain adequate protection for them. Hence our obsession with mink!