Complex sentence: The topography of Arunachal Pradesh is characterized by mountainous ranges and sub-mountainous terrains along the northern parts: abrasion by the mighty rivers that flows through the mountains have created broad valley, which is a major feature of the geography in Arunachal Pradesh.
Split into simpler sentences: The topography of Arunachal Pradesh is characterized by mountainous ranges and sub-mountainous terrains along the northern parts. Abrasion by the mighty rivers that flows through the mountains has created a broad valley, which is a major feature of the geography in Arunachal Pradesh.