How does a stream valley form and how deep will it be downcut?
they don't have earth science but this is that subject

Here is the answer to the given question above. The stream valley form and how deep it will be downcut is that, new stream is v-shaped downcut until it reaches sea level. V-shaped valleys is one of the most common form. A valley is a result of downcutting. Hope this is the answer that you are looking for. 

Holding up a hand because you are not speaking but you are still trying to indicate something tough you do not use words to express it.