To have one river flow into another (a tributary), move the endpoint from the first river onto a previously unused place along the second river. A tributary node will appear, connecting the two rivers. To disconnect a tributary, right click on it and choose Disconnect Endpoint.
To have one river flow out of another (a diversion), move the headflow point from the first river onto a previously unused place along the second river. A diversion node will appear, connecting the two rivers. To disconnect a diversion, right click on it and choose Disconnect Headflow.
To have a river flow into a groundwater node, move the endpoint from river onto the groundwater node. This will cause all water flowing out of the last reach on the river to flow into the groundwater node.