The Narmada and the Tapi are the only long rivers, which flow through trough, which flow west and make estuaries. The drainage basins of the peninsular rivers are small in size in comparison to other basins. Estuaries are the wide part of a river where it nears the sea; fresh and salt water mix and trough means a narrow depression (as in the earth or between ocean waves or in the ocean bed), also known as a channel along the eaves or on the roof; collects and carries away rainwater.