Select the INCORRECTLY spelt word in the given sentence.
Every season the river overflows its banks and when it goes down it leaves silt for crops to be gruwn.

To find the incorrectly spelt word, let us examine the sentence and the given options:
"Every season the river overflows its banks and when it goes down it leaves silt for crops to be gruwn."

Let us analyze each option:
1. Season: Spelt correctly. It refers to a period of the year characterized by a particular climate or activity.
2. Overflows: Spelt correctly. It means to flow over the brim of a container or riverbank.
3. Silt: Spelt correctly. It refers to fine sand, clay, or other material carried by running water and deposited as a sediment.
4. Gruwn: Spelt incorrectly. The correct spelling in this context is Grown, which is the past participle of the verb "grow" (meaning to undergo natural development by increasing in size and changing physically).

Therefore, the incorrectly spelt word is Gruwn.