Explanation:

between two tributaries of a rive

The Doab designates the flat alluvial tract between the Ganges and Yamuna rivers extending from the Sivalik Hills to the two rivers' confluence at Allahabad. It is also called as Ganges-Yamuna Doab or Ganga Doab. The region has an area of about 23,360 square miles (60,500 square km); it is approximately 500 miles (805 km) in length and 60 miles (97 km) in width. The British raj divided the Doab into three administrative districts, viz., Upper Doab (Meerut), Middle Doab (Agra) and Lower Doab (Allahabad)