The boundary line or high ground that separates one drainage basin from another is called a:

A watershed, also called a water divide or drainage divide, is an elevated area, such as a mountain ridge or upland, that separates neighbouring drainage basins. Rain falling on one side drains into one river system, while rain on the other side drains into another.

Two uses of the word "watershed":

• In British usage, "watershed" means the dividing line between two basins.
• In American usage, "watershed" often means the whole area drained by a river (the drainage basin itself).
• In Indian exams, "watershed" or "water divide" usually refers to the dividing line, while the area drained is called the drainage basin.

Drainage basin:

• A drainage basin is the total area drained by a river and its tributaries.
• Each basin is separated from others by water divides.

Examples of water divides in India:

• The Western Ghats form the main water divide of peninsular India, separating west-flowing rivers from the larger east-flowing rivers.
• The Vindhya Range acts as a divide between the Ganga basin to the north and the peninsular rivers to the south.

Watershed management:

• Watershed management is the planned use of land and water resources within a watershed area to conserve soil and water, recharge groundwater and improve agriculture.
• It includes building check dams, contour bunds, percolation tanks, afforestation and water harvesting structures.
• Successful examples in India include Ralegan Siddhi (Maharashtra), developed by Anna Hazare, and Sukhomajri (Haryana).
• The Pradhan Mantri Krishi Sinchayee Yojana (PMKSY) includes a watershed development component.

Why not the other options?

• ✗A floodplain is the flat land beside a river that is flooded during high water.
• ✗A tributary is a river that joins a larger river.
• ✗A confluence is the place where two rivers meet.