Valleys and Interfluves
All terrain can be classified in one of two groups. These groups are valleys and interfluves. Valleys are areas of lower terrain, while interfluves are areas of high terrain located between valleys.
Often the difference between valleys and interfluves is obvious. This is especially the case where high mountains surround a low valley. In other cases, the difference is difficult to detect. The interfluve may be just a few feet higher than the valley floor.
Why are valleys and interfluves important? Water always flows from high terrain downward, towards lower terrain. Thus, water is not found in abundance in interfluves, while it gathers in the form of streams and lakes in valleys.