Alteration of forests by human manipulation or natural forces can initiate changes, from small to radical, in downstream water characteristics. The most visible changes are likely to be in stream water yields: storm or other peak flows, low flow during drought periods, and production of particulate matter. Less visible but still important changes include increased concentrations of nutrients leached from the land, changes in acidity and temperature, and introduction of heavy metals, pesticides, and pathogenic organisms. All of these possible changes in water quantity and quality can have drastic impacts on the use of the water downstream by humans and natural stream and lake organisms dependent on the water for their survival. Forest land managers need to understand and recognize the relationships of the major processes that are involved in forest ecosystems for both undisturbed and manipulated sites so they may make wise decisions in land use management that will not jeopardize downstream water.