nriver as silt. There are
enough fine suspended sediment particles in the water of the
metropolitan river to make it drably opaque most of the time, even
during relatively dry spells, when heavy sand and gravel dredging helps
to keep it stirred up. As the current loses force and washes back and
forth with the tides, the particles settle out slowly into smothering,
continually renewed blankets on the bottom, and over two centuries have
accreted into great mudbanks and shoals. Channel dredging to maintain
navigation has been going on since the early 19th century, about 180,000
cubic yards being presently removed each year. The dumping of the
dredged materials on the marshes and long low shores has built up wide,
flat, new flood plain areas around the city over the years, including
the sites of Washington National Airport, Anacostia and Bolling Air
Fields, and East and West Potomac Parks.

Such channel dredging has little effect on the gradual shoaling of this
whole part of the river in general. Miles of formerly navigable water
downstream from Memorial Bridge are now only one to four feet deep and
useless for either pleasure or commercial craft. It has been estimated
that present rates of deposition will within fifty years fill in the
upper estuary completely to a mile or so below Alexandria, except for a
river channel. The same process is at work in the tributary creek-bays
that give onto the estuary, some of which have silted so heavily since
Colonial Days that formerly thriving ports--among them Bladensburg,
Dumfries, and Port Tobacco--are now distant from the water.

The bulk of estuarial silt comes down the main river from the upper
Basin. But a heavy increment is added in the metropolitan area. Modern
mechanized development of the city's hilly environs on a huge scale,
continuing year by year with few thoughtful rules to guide it
heretofore, has brought about erosion that on individual patches of
bared land may reach a temporary rate of 50,000 tons per square mile per
year, and even average rates in this area are far in excess of anything
else in the Basin.

We had to examine the reasons for this rather closely last year in a
study of Rock Creek's ailments, whose findings we published in a report
called _The Creek and the City_. This much-admired metropolitan stream
has been relatively well protected, with the parks along its wooded
valley and an upper watershed that until quite recently remained
essentially rura