It looks like a Guelder Rose (V. Opulus)  to me as well.  Birds seem to like the berries of most varieties of Viburnum, which is most likely how it got where it is.

The problem with a lot of self-sown plants is that they have hybridised, often with a vigorous variety used as a root-stock for a more attractive garden variety.  So often the more tender variety grafted onto it dies, the root stock then takes over and all too often is an unattractive, sprawling plant more akin to its "wild" ancestor. 

That could also be why your plant is leggy and drooping.  If so, even with support, it will just produce more leggy growth.  If that's the case it is trying to propagate itself already, by layering.  Many Viburnums have a tendency to do this in gardens, any twig that touches the ground will root itself.

Easy to do on a wet, muddy bank.  Just find a rock heavy enough to weigh down one of the shoots and put it over a pair of leaves, it helps if you break up the soil a bit first to press the shoot into the soil and give maximum contact.  By next summer you will probably find a new shrub growing out from under the rock.  An exact copy of the one in the pictures and much easier to move.

But if it is a hybrid with the characteristics of its wild ancestors it will likely still grow into a straggly bush, even if planted in a bit of open ground.  But even if it does the birds will thank you for it.