There are a lot of impacts to write about, my advice is to think of all the impacts of human water usage that we explored in class as they relate to the American southwest.
What changes have humans made to the hydrosphere in our study area?
What impacts have the changes in the hydrosphere had on the geosphere? If you already have 10(ish) impacts, ignore this advice.
Marks will be lost if you do not link the impacts to the spheres involved. So, it is best to label your section on the hydrosphere and label your section on the hydrosphere.