GCSE Geography Rivers Question

Hi, I'm currently doing the controlled assessment for the AQA A Geography exam board, and my controlled assessment is based on the changes of a river in its long profile. The key terms we're investigating are vertical erosion, velocity and lateral erosion. In the introduction we're supposed to come up with a hypothesis as to how these variable change downstream. My current hypothesis is:
Vertical erosion and lateral erosion will both increase downstream but lateral erosion will increase at a higher rate and be more prominent downstream as the river velocity increases due to tributaries, increasing discharge thus velocity, which increases lateral erosion due to the increased kinetic energy. Velocity will increase downstream due to tributaries, increasing discharge.

I'm not sure exactly why lateral erosion increases due to an increase in velocity, but is that hypothesis good enough? Also, for the evaluation, do we need to just list the limitations of the method, or do we actually need to explain how we can overcome those limitations?

Thanks.