Answer: A river delta is a low-lying plain or landform that occurs at the mouth of a river near where it flows into an ocean or another larger body of water. Deltas' greatest importance to human activities, fish and wildlife lay in their characteristic highly fertile soil and dense, diverse vegetation. 

Himalayan rivers form large deltas as compared to peninsular rivers because: 

• Large size and Speed: Himalayan rivers are much larger, faster, and carry more sediment than peninsular rivers. The increased amount of sediment carried by Himalayan rivers creates large deltas at the mouth of the rivers. 
• Examples of large Himalayan deltas include the Ganges and Brahmaputra, which form the Sunderbans delta in India and Bangladesh.
• More sediments: Himalayan rivers are fed by several tributaries that add to the sediment load of the rivers. This, coupled with the higher velocity of the rivers, creates a powerful force that carries sediment further downstream and helps to form large deltas. 
• For example, the Ganga has many tributaries such as the Yamuna, Ghaghara, and Gandak that all add to the sediment load of the river.
• More erosion: Himalayan rivers erode the land as they flow, which adds sediment to the river. While the peninsular rivers flow over hard rocks which are eroded slowly. 
• For example, The Yamuna River has eroded the land as it flows, creating the Yamuna Valley, which has a rich sediment deposit that flows into the Ganges Delta. 
• Steep slope: The steep gradient of the riverbed of Himalayan rivers as compared to the peninsular rivers causes the river to flow faster, which carries more sediment downstream.
• Examples include the Ganges Delta and the Brahmaputra Delta.
• Perennial flow: Himalayan rivers also experience higher levels of monsoon rainfall which increases the amount of runoff and sediment that is carried downstream. Also, the Himalayan rivers have perennial flow of water as compared to peninsular rivers which are seasonal in nature.
• For example, Ganga flows throughout the year by receiving rainfall in monsoon and melting of water in summer season.  

West-flowing rivers do not form deltas, unlike east-flowing ones: 

• Steeper gradient: West-flowing rivers generally have steeper gradients, which prevents them from depositing sediment when they reach the ocean. This is because the force of the river is too strong for sediment to be deposited. While those east flowing river flow over gentle or plain area.
• Shorter distance: West-flowing rivers tend to have a shorter distance to travel from their source to the ocean, which does not give them enough time to build up the large amounts of sediment needed to form a delta.
• Tectonically active area: West-flowing rivers usually run through areas of high tectonic activity, such as mountains and active plate boundaries. This can cause tectonic uplift, which prevents sediment from being deposited.
• Ocean Currents: West-flowing rivers often encounter strong ocean currents before they reach their destination as compared to east flowing rivers. These ocean currents can erode sediment away, preventing the formation of a delta.
• Small Size of rivers: East-flowing rivers tend to be larger and have more water than west-flowing rivers. This gives them more power and the ability to transport larger amounts of sediment, which is necessary for forming deltas. 
• Examples of east-flowing rivers that form deltas are the Krishna, the Ganges River, and the Godavari River.

River deltas boast some of the most biodiverse systems on the planet. As such, it is essential that these unique and beautiful havens of biodiversity remain as healthy habitat for the many species of plants, animals, insects, and fish.