Tributary is where streams come together. Currently an iOS app is in a closed beta with generic ReplayKit support for RTMP, and Twitch.
Plans are in place for a cloud based solution for creating multiple input streams (headwaters) and combining them into a single stream (the tributary), from there the tributary can be directly exposed or forwarded to a broadcasting service such as Twitch, Periscope, YouTube, etc (an estuary).
Some apps support live streaming via Apple's ReplayKit. The Tributary iOS app is a ReplayKit provider and does not support streaming on its own. It merely provides streaming capabilities to other applications.
Creating a source looks something like this:
Then a supported application prompts for a choice of providers
The Tributary provider gives a choice of the configured sources