andscape as a whole,
but for now it may be worthwhile to note that insofar as urban erosion
and silt stem from decisions of political agencies inclined to subjugate
well-known good land use principles to speculative pressures,
expediency, and other things, their origin is political and economic.

Organic materials are pervasive enough in the upper estuary that during
periods of even normal flow their decay pulls oxygen levels down. Under
usual conditions this B.O.D. grows worse and worse downstream and
reaches a peak in the neighborhood of Mount Vernon, though its effects
continue to be felt below. Fish kills among the rugged resident species
that predominate in these reaches of the river are not uncommon, the
shoreline windrows of deceased carp and perch periodically adding their
essence to what metropolitans have come to accept as the Potomac's
normal summer smell. And along with the organic materials are heavy
concentrations of bacteria.

The organic and bacterial load enters the estuary from many sources,
most of them local, for only a little of this material comes down from
the upper river. A significant amount of it issues from the network of
small urban watercourses like Rock Creek. Many of these were covered
over as storm sewers or troughed in concrete long ago, but they continue
to serve their age-old function of draining the lands they traverse,
even if through cast-iron gratings.

A good bit of the organic load in these tributaries consists of raw
human waste, incongruous and particularly obnoxious around a modern
city. The bulk of it is released in periodic surges when local
rainstorms overload the old-fashioned combined sewer systems of the
District of Columbia and Alexandria. In dry weather these systems send
both collected sanitary wastes and street drainage down to the cities'
respective treatment plants, but during storms when street drainage is
heavy the sewers' capacity is exceeded and overflow gates gush mixed
stormwater and sewage out into the streams, which carry it to the
estuary.

In the suburbs, more modern separate storm and sanitary sewers are the
rule, but they too have some problems of a kind we noted in relation to
the upper river. Investigations on Rock Creek revealed steady dribbles
of raw sewage entering the creek or its tributaries from a large number
of storm-sewer outfalls and other places. Partly these flow from
malfunctioning individual septic systems in outlying areas,
surrept