Streams can now be children of other streams.
This enables you to organize related streams.
You can either create a child stream from the parent stream's page (the "Create new child stream" button) or you can put an existing stream under another stream by going to the stream you want to be the child, clicking on "Make child of another stream" and selecting the parent (or creating a new stream to be the parent).
Note that streams can have as many parents as you like.
At some point in the near future, it will be possible to elect, per-stream, whether a stream that has parents is still to be shown at the top-level.