In the 40 or 50 miles below that point, the North Branch accumulates great quantities of more usual kinds of pollution as it runs down a broadening valley past towns and industries that have grown up because of the conjunction of coal, timber, water, and railways--and in the old days water transport, for flatboats used to shoot the river at high water, and later the C. & O. Canal operated upriver as far as Cumberland. Treatment of wastes in this reach is spotty and mainly inadequate. Some industries and towns sluice them raw into the dark, sad water, and others give only perfunctory primary treatment; the city of Cumberland releases the equivalent of about 18,000 persons' body wastes each day as effluent, besides extra raw wastes whenever storm runoff overloads its combined storm and sanitary sewer system and causes it to overflow. Where major efforts have been made, as at the Upper Potomac River Commission's Westernport plant below the big Luke, Maryland, pulp and paper mill, the wastes are so voluminous and complex that some of them still have to be dumped, and the effluent from even highly efficient treatment further degrades the river. Fortunately, the North Branch, acid above, deprived of oxygen and overenriched and septic below, is not typical of the flowing parts of the Potomac river system, but it stands as a good grim example of what pollution can mean, and as a foretaste of what may be expected to happen elsewhere in the Basin if it is not stopped soon. Mine acid is not a significant problem in any streams outside of that region, but untreated or inadequately treated wastes are badly blighting many streams and rivers or stretches of them. Some smaller watercourses, like historic Antietam Creek below Hagerstown, Maryland, have deteriorated under the influence of discharges from single or limited sources, while larger ones suffer from a cumulative waste buildup in areas of concentrated population or industry. Some twenty miles of both industrial and municipal pollution in the South River Branch of the Shenandoah's South Fork below Waynesboro, Virginia, have done much damage to that legended river for a good distance downstream, a situation that is worsening with the area's growth. On the North Fork of the Shenandoah similar effects have been wrought by heavy organic loads from poultry processing and other things. The list could be extended: aside from a few happy exceptions like the prized Cacapon, draining rugged, forested, thinly peopled hill country, nearly all the Basin's flowing streams of any size receive damaging loads of waste from towns and industries. [Illustration: WATER TREATMENT STEPS (1) River water enters here (2) Water chlorinated (3) Water settles. Heavy particles sink (4) Water pumped to Pretreatment Building (5) Various chemicals (chlorine, alum, lime, carbon) added. Chemicals and water stirred in rapid mixing basins (6) Slow mixing to form "floc" (see Alum below)