ime. It is frequently the
case, however, that a succession of waves of high turbidity will
appear so close together that the effect of one has not disappeared
before that of another is felt.

The clarification of the water supply begins at the dam at Great
Falls. Here it is a clarification by exclusion, for when an
excessive quantity of mud appears in the river water, the gates are
closed, and the muddy water is allowed to flow over the dam and form
mud-bars in the Lower Potomac, while the city is supplied from the
water stored in the three settling reservoirs. Until a comparatively
recent date, the excessively muddy water was never excluded, having
been taken, like other decrees of Providence, as it came.

During the summer of 1907, the practice of shutting out water with a
turbidity of 500 or more was established for the warm months. This
practice was discontinued during the cold months, as it was feared
that a very high consumption of water might occur at the time of low
water in the reservoirs, and so cause a partial famine. During the
winter of 1909-10, however, the gates were closed, as was the
practice throughout the summer months.

When the reservoirs are well filled, and the consumption of water
is less than 70,000,000 gal. per day, it is safe to close the gates
at Great Falls for a period of about 4 days.

[Illustration: ~Figure 1--Plan and Profile of Washington Aqueduct.~]

While a considerable reduction in turbidity is effected in each of
the reservoirs, the bulk of the mud is deposited at the upper end of
Dalecarlia Reservoir. This reservoir had become so completely
filled, that, in 1905, it was necessary to dredge a channel through
the deposit, in order to allow the water to pass it. During the
summers of 1907 and 1908, a 10-in. hydraulic dredge removed more
than 100,000 cu. yd. of mud which had been deposited in this
reservoir. The mud deposited in Georgetown and McMillan Park
Reservoirs is so fine that the accumulation of many years is not
very noticeable in its effect on the depth of water.

The particles of clay which remain in the water after its passage
through the three reservoirs, are so exceedingly small that they do
not settle out in any reasonable length of time. Even the filtration
of the water through one or more slow sand filters occasionally
fails to remove the last trace of turbidity. This is especially true
in the colder months, and not a winter has passed when the water
supply has