Each river is unique. The river we see depends on the interplay between the geology and morphology of the watershed, the rainfall regime, and the animals and plants and people present in the area. Because they are unique there is no one preconception of what the word "pollution" means that can be made to serve for all rivers.
Consider the Boquet River as an example. The Boquet is in a watershed that is largely forested or agricultural. No large towns dump sewage directly into the water. Because of this there is usually very little oxygen demand (BOD) in the water and the levels of dissolved oxygen never fall very far below the level of saturation. In addition, large industries no longer dump wastes into the river. We have no evidence that there is any significant contamination by the heavy metals and persistent organic chemicals that plague so many rivers.
The acid rain that affects some areas in the Adirondacks is not yet a major concern in the Boquet watershed and in most parts of the valley the acidity of the water remains within bounds tolerable to most organisms.
Whatever oxygen demand, bacterial, and trace material contamination the river receives probably comes mostly from agricultural runoff. Phosphorous loading to the river, and ultimately to Lake Champlain, is an area of concern.
However, the Boquet faces a major problem. It receives large amounts of sediments. The Boquet River valley is geologically young, probably only seven or eight thousand years old. The river has not yet had time to smooth out the bumps in the valley and it falls rapidly from its origin down to Lake Champlain. In addition, the valley is composed largely of glacial moraine which is poorly consolidated and therefore susceptible to erosion. Taken together, the erosive power of the young river and the poorly consolidated nature of the glacial moraine mean that the Boquet River receives large amounts of sediments. These naturally occurring sediments are augmented by disturbances to the land caused by development, forestry and agricultural practices.
Studies of bank erosion have shown many areas that produce disproportionate amounts of sediments. Usually the areas which load the most sediment to the river are floodplains from which all of the trees and shrubs have been removed for agricultural purposes. This makes the banks especially susceptible to erosive loses.
Although much of this sediment is a natural substance, the amount of it entering the river nonetheless causes problems for the organisms that live in the river. In some cases these problems are so severe that the organisms cannot survive. Studies of embeddedness have shown that many areas, which because of velocity and other characteristics ought to be good spawning areas for trout and salmon, are not. Many of these have embeddedness percentages of from 30% to above 50%. With this much fine sediment clogging the bottom gravels the trout and salmon may not spawn at all, and if they do so the eggs are likely to be smothered. In these sections of the river the existence of a trout population depends on continual restocking by hatchery raised fish.
Studies of stream insects in these same areas show that they are also affected by the sediments. Although stream insects are present, there less diversity than in streams with less sediment.
Pollution can be defined as "any impairment of the suitability of water for any of its beneficial uses, actual or potential, by man caused changes in the quality of the water."
Although we usually think of pollution as the addition of something, like sewage, to the water, it is also possible to damage a river by increasing the amounts of naturally occuring substances like sediments. This also fits the definition above.
In the Boquet, problem sediments are of a natural origin but are augmented by those from disturbances. To what extent is this "pollution" and to what extent is it just the "nature" of our river?