Answers may vary. Students will probably say their results demonstrate that erosion is associated with clearcutting. Students will probably recognize that the greater grade, the more serious the problem. Silt and excess gravel can smother the eggs and young fish. It can reduce circulation around them, possibly causing oxygen starvation.
Trees help keep the water temperature low, which helps keep dissolved oxygen in the water high.
Trees help hold the soil, controlling eroson and siltation. Removal of trees reduces the food supply of aquatic insects, which salmon fry eat. You might like to make your students aware that lumber is currently being cut for sale to countries like Japan to which the United States has built up a very large trade deficit. Becaude lumber is used to balance this deficit, buying foriegn-made cars, audio equipment, and game computers also puts pressure on our forests.