For Geomorphological Study:

Drainage density (Dd) expresses total stream length per unit drainage area. It depends on climate (mainly rainfall), geology, vegetation cover, erosivity, infiltration capacity and permeability of underlying rock and soil, relief and slope aspect of the basin (Horton, 1932; Strahler, 1964; Morisawa, 1968; Verstappen, 1983). Drainage density are quietly depends on the stage of evolution. With the progress of youth stage towards maturity, Dd increases rapidly; while towards old stage it decreases.

There are lots of possible answers,

• A drainage basin
• A watershed
• A drainage system
• Dendritic drainage pattern
• Trellis drainage pattern
• Parallel drainage pattern
• Rectangular drainage pattern
• Radial drainage pattern
• Annular drainage pattern
• Discordant drainage pattern
• A spring head
• Tributaries
• Confluences
• A river system