[R] Heirarchical tree data structure

[Janmaat, John]

Hello,

I'm modeling a watershed, and have a file linking individual tributaries
- the name of each tributary and the water body into which it flows
(which can be a tributary of another water body).  I want to use this to
build a tree data structure (a root, and then child nodes, linked to
further child nodes, etc.).  Is there a package that builds and
traverses such data structures?

Thanks,

John.