I think it might be a willow.
Possibly a hybrid with a goat willow, willows very freely hybridize and do tend to crop up wherever there is a wet spot. If you cut a branch and you can peel of the bark relatively easily it is definetly a willow. Lookiing at the way thye leaves are attached to the stem it does not look like a plum. You will know by spring, if it is a willow you will get nice catkins or fury paws.
DD