Catchment Runoff can be directed to a river by dragging the Runoff/Infiltration symbol from the legend in the Schematic view, dropping it on a catchment, then dragging to anywhere along the river. If the Catchment Runoff is to be Headflow to the river, the symbol can be placed anywhere above the first node of the river, and the dialog box that appears will ask you if the Catchment Runoff is to represent Headflow (See River Headflow for additional details).
Note that if you select a Catchment as a headflow source for a river, then under the Headflow variable tab for that river, it will be set/locked to "Inflow from Catchment".
Catchments can contribute runoff to one or more river nodes, and zero or more groundwater nodes.