Armando’s class was looking at images of rivers formed by flowing water. Most of the rivers were wide and shallow, but one river was narrow and deep. Armando’s class thinks that this river is narrow and deep because:

the hill that the water flowed down was very steep, or
the sand grains that the water flowed through were very small.
Armando designed the model below to try to answer the question: Why is this river so narrow and deep?

1a. What features of this model will help Armando answer the question?
1b. How could Armando use this model to test the idea that very steep hills lead to narrow, deep rivers?
1c. How could Armando use this model to test the idea that very small sand grains lead to narrow, deep rivers?
2. Armando thinks that it is the very steep hill that makes this river narrow and deep, but his classmate thinks it is the small size of the sand grains that the water flows through. What results from the model would be evidence that Armando’s idea is correct?