t, concentrating at
various points in the natural food chain and often in man himself. It is
said that an average adult Californian's tissues today contain more DDT
than is allowed in beef for interstate shipment. But no one is yet
certain what this means in relation to that average Californian's
physical wellbeing, and in terms of fish and wildlife, though the link
between these materials and certain destructive changes can be seen,
evidence in other cases--the declining fertility and numbers of bald
eagles, for instance, which some investigators believe to derive from
pesticide residues--only points toward such a link. Until all the facts
are in and the impact of such poisons has been clearly restricted to the
pest species at which they are aimed, they are going to continue to be
a heavy concern for conservationists and others alarmed about
environmental pollution, along the Potomac and elsewhere.

[Illustration]

[Illustration]

One of the principal Potomac pollutants, silt, not only comes from the
land but is the land, most often good topsoil, washing away toward the
sea. Even under pristine conditions streams are likely to run somewhat
muddy after storms; it is a natural phenomenon, a by-effect of the way
climate carves landscapes. On the evidence, however, the Potomac
landscape since its colonization by white men has been undergoing a much
more rapid carving than anyone could consider to be natural. Most of its
streams, particularly in their lower reaches, are thickly opaque for
long periods after rain, and gross erosion in the Basin--the amount of
soil washed away from where it usefully belongs to somewhere
else--averages about 50 million tons per year, a major depletion of the
soil resource and a degrading influence on the landscape through
erosion. The part of this silt that gets into streams cuts down on the
usefulness of the water, creates ugly turbidity, chokes quiet pools and
reservoirs, suffocates bottom-dwelling creatures and plants on which the
streams' wholeness may depend, and rides down the current to add heavily
to the problems of the estuary, into which some 2.5 million tons of it
are annually discharged.

Sediment is dislodged from the land by the pounding action of raindrops
and the flow of runoff, and sometimes is washed from streambanks during
high flows--which may themselves be higher and more frequent because of
silt-clogged channels. The bulk of it can be blamed on unsound land use.
This