Explanation:

The limit drawn by L.D. Stamp and later adopted by O.H.K. Spate, roughly corresponding with 100 cm isohyet seems to be far from being practical. Physiographically, the 100 m contour (line joining places of equal height) has been accepted by the geographers as the most effective line of demarcation.

•  Comprising the upper part of the Ganga Plain, this plain is delimited by the 300 m contour in Shiwaliks in the north, the Peninsular boundary in the south and the course of the Yamuna river in the west. Its eastern boundary is rather obscure and has become a controversial topic among geographers.
• This plain is about 550 km long in the east-west direction and nearly 380 km wide in north-south direction, covering an approximate area of 1.49 lakh sq km. Its elevation varies from 100 to 300 m above mean sea level.
• The plain is drained by the Ganga and its tributaries like the Yamuna, the Ram Ganga, the Sarda, the Gomati and the Ghaghara rivers. Almost all the rivers follow NW-SE course concomitant with the lie of the land. The average gradient of the land is about 25 cm per km. The gradient is comparatively steep in the northern part.
• The rivers flow sluggishly in the plain as the gradient decreases. The monotony of this flat and featureless plain is broken by the Tarai-Bhabar submontane belt and on micro level by the river bluffs, river meanders and oxbow lakes, levees, abandoned river courses, sandy stretches (Bhurs) and the river channels themselves.
• The western part of this plain consists of comparatively higher Ganga-Yamuna Doab. East of this doab are the low lying Rohilkhand plains which merge into the Avadh plains further east. The Ghaghara is the main stream of the Avadh Plains.
• The khadar of this river is very wide because the river meanders through this area. Moreover it often changes its course. At places this khadar is 55 km wide. Consequently, there is a consistent fear of devastating floods.