Sometimes associated with organic wastes and sometimes entering the river system otherwise are dangerous bacteria, and also the so-called "nutrients"--dissolved fertilizing agents that can stimulate excessive growth of algae or weeds in the water to the detriment of other forms of life, often to such a degree that these plants' death and decay sets off a whole new cycle of oxygen demand. And there is sediment washed off the land, which clouds the water and settles out into a smothering cloak on the bottom, building up in quiet stretches into ugly and damaging mud banks and shoals. Troubles above the Fall Line Pollution of the Potomac begins at or near its traditional source, the tiny Appalachian spring at the head of the North Branch where in 1746 Thomas Lord Fairfax's surveyors set an inscribed stone to mark the northwestern corner of that possessive nobleman's vast holdings. Abandoned strip coal mines lie within sight of the spot, and it is doubtful that the infant river trickles more than a few yards before receiving its first injection of the acidic mixture of substances that springs and seeps and runoff water extract from bared coal strata and the mines' spoil heaps and carry down to the streamlet they feed. [Illustration] Such additions are frequent for the next 45 miles or so downstream, as the North Branch in its narrow valley swells into a mountain river with the water of brooks and creeks flowing off ridges pocked with coal mines old and new. The river and such tributaries sustain no aquatic life at all among the discolored stones in their channels. This mine-derived pollution has been much studied but is still not well understood. Sulphuric acid is its most damaging component, but may be accompanied by iron salts and other substances also leached from materials in and around the vast coal beds of Appalachia. Some acid entered the streams there naturally, before men ever touched the coal, but it has increased to deadly proportions with widespread mining. It issues from both surface strip mines and the old-fashioned underground sort, though the latter furnish by far the most--an estimated 75 to 90 percent. The overall magnitude of the problem is indicated by the fact that the more than 60,000 square miles of the Appalachian region underlain by coal, including the Potomac fraction, contributes five to ten million tons of sulphuric acid annually to streams and rivers, a rate of production that is expected to continue for at least a thousand years. At Westernport the North Branch enters more populated realms, and receives one of its latter big doses of acid from Georges Creek, which drains a devastated, economically depressed valley mined since very early days. This creek may be the single most unfortunate stream in the Potomac Basin, for the accumulation of raw wastes it brings down from the valley's communities is pickled rather than assimilated by its heavily acid water. [Illustration]