they hold in suspension upon the upper surface of this stratum of ice.
By repeated accumulations in this way, the weight becomes sufficient to
sink the whole mass to the bottom. There it rests, together with other
strata, which are sunk in the same way, until the channel is obstructed
by the combined masses of ice and silt. In the spring, when the ice
melts, the silt is dropped to the bottom, which, combined with that
constantly deposited by the lakeshore currents, causes a greater
accumulation in winter than at any other season."

These bars at the natural river-mouths have frequently not more than two
or three feet of water; and some of them have entirely closed up the
entrance, although at a short distance inside there may be a depth of
from twelve to fifteen or even twenty feet of water.

The channels of these rivers have also a tendency to be deflected from
their courses, on entering the lake, by the shore-currents, which,
driven before the prevailing winds, bend the channel off at right
angles, and, carrying it parallel with the lake-shore, form a long spit
of sand between the river and the lake.

Thus, in constructing an artificial harbor at one of these river-mouths,
the first object to be aimed at is to prevent the further formation of a
bar; and the second, to deepen and improve the river-channel. The former
is attained by running out piers into the lake from the mouth of the
river; and the latter, by the use of a dredge-boat, to cut through the
obstructions.

These piers are formed of a line of cribs, built of timber, and loaded
with stone to keep them in place, and enable them to resist the action
of the waves. They are usually built about twenty or twenty-five feet