Keyboard short-cut: Alt, E, E.
Mouse short-cut: 
Changes the function of the mouse so that it can connect two objects. With the map window in Input Mode, click the left mouse button down on an upstream catchment, river segment, or reservoir segment. Drag the mouse so that it is over the catchment, river or reservoir segment immediately downstream, then let the mouse button back up. All tributary connections will then be displayed. To remove a connection, delete its tributary connection arrow. To stop displaying tributary connections, select Tributary Connections in the View menu.