In its most basic form, a drainage network can consist of a single conduit, with or without a retarding basin at its head. This situation is defined as a single link. An input hydrograph is derived from a single sub-catchment immediately above each link.
The drainage system is usually made up of many such links combined to form a complete network where the individual catchments (links) become sub-catchments of the overall watershed. The figure below illustrates a link and the way in which individual links are joined to form a network.