Every day tremendous rivers flow above your head. These rivers contain some water but are mostly air. Like our more familiar liquid rivers, what goes into the air "upstream" affects those living "downstream." Much of the East Coast of the United States is "downstream" of the industrial Midwest. Thus, sulfur dioxide and nitrogen oxides produced in the Midwest flow eastward, and, like a net in a river, mountain ranges tend to catch a large share of these pollutants. They descend as gases, or particles, or are washed out of the air as rain, snow, or fog. Whether dry or wet these deposits are acidic.

Acid deposition is a special problem for the Adirondack Mountains. Precipitation is naturally a little acidic. Soils and lakes usually buffer (or off-set) that acidity with "bases," (also referred to as "alkaline") the opposite of acids. Because many lakes and much of the thin soil in the Adirondacks have relatively few bases; they are very sensitive to increases in acidity.

Every plant and animal can tolerate some range of acidity. Humans skin, for example, is irritated both by bleach (too basic) and battery acid (too acidic). Blueberry plants thrive in relatively acidic soils. Lilac bushes prefer slightly basic soils. Most fish cannot tolerate water with a pH less than 5.0. Many insects, like black flies, can thrive in more acidic water.

In 2001 the precipitation that fell on the Adirondacks had, on average, a pH of 4.5. (A pH of 5.6 is normal for rainfall, and 7.0 is neutral.) For many lakes with little buffering capacity, acid precipitation translates into more acidic water, until eventually all of the fish die. About 27% of the 2,800 lakes and ponds in the Adirondacks have a pH of 5.0 or less all year. Many more dip below 5.0 during certain times of the year. During snowmelt in the spring, many streams which contain young, susceptible trout drop below a pH of 5.0 for several weeks. The Environmental Protection Agency estimates that by 2040, 43% of lakes in the Adirondacks will have pH below 5.0 year-round and therefore be deadly to fish.