River Questions

1. Why aren't rivers straight? What's going on with them?
   
   
   
2. What are mountains streams like (given what you know about the upper reaches of rivers)?
   
   
   
   
   
3. What are valley streams like (given what you know about the lower reaches of rivers)?
   
   
   
   
   
4. What causes rivers of the Colorado Plateau (like the Colorado River in the Grand Canyon) to be so deep and dramatic?