Question

The northern plains are generally described as flat land with no variations in its relief justify that the above statement is not true

Answer 1

These vast plains also have diverse relief feature. Accordingly they are divided into 4 regions.
  The rivers, after decending from the mountains deposit pebbles in a narrow belt of about 8 to 16km in width lying parallel to south of shiwaliks known as bhabar. All streams dissapear in this belt.
   South of this belt, the streams and rivers re-emerged create a wet, swampy, marshy region known as terai 
   The largest part of the northern plain is formed of older alluvium. They lie above the flood plains of rivers and present a terrace like feature known as bhangar.
   The soil in this region contains calcareous deposits locally known as kankar. The newer, younger deposits of flood plains are called khadar.

Answer 2

No, I do not agree that the Northern Indian Plains have no variation in relief features. The river flowing down from the Himalayas deposit sediments in a belt known as bhabar which runs parallel to the slopes of the Siwaliks. On the south of this belt exists the wet, swampy and marshy region known as the terai region. This region was densely covered with forests and was rich in flora and fauna. However, the forests were later brought down for the expansion of agriculture.