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 wide, and from thirty to forty feet long.  They are strengthened by cross-ties of timber, uniting together the outward walls of the crib.  Piles are usually driven down into the clay, inside of these cribs, and they are covered with a deck or flooring of plank.  As the action of the currents is constantly tending to remove the bed on which the cribs rest, and thus cause them to tilt over, their bottoms are constructed in a sort of open lattice-work, with openings large enough to allow the stones with which they are loaded to drop through and supply the place of the earth which is washed away.

The effect of these piers is to concentrate and deepen the river-channel, and to retard the formation of bars, though they do not wholly prevent it.  In the spring it is often necessary to employ the services of a steam-dredge-boat to cut through the bar, before vessels can pass out.

The portion of these cribs above water is found not to last more than ten or fifteen years; so that it is now recommended to replace them with piers of stone masonry, wherever the material is easy of access.

As to the cause of the shore-currents which produce this mischief, Col.  Graham says, in one of his Reports,—­

“The great power which operates to produce the littoral or shore currents of the lake is the prevailing winds; just as the great ocean current called the Gulf Stream is produced by the trade-winds.  The first-mentioned phenomenon is but a miniature demonstration of the same principle which is more boldly shown in the other.  The wind, acting in its most prevalent lakeward direction, combined with this littoral current, produces the great power which is constantly forming sand-bars and shoals at all the harbor-entrances on our extensive lake-coasts.  To counteract the effect of this great power, upon a given point, is what we have chiefly to contend for in planning the harbor-piers for all the lake-ports intended to be improved.  The point which an engineer first aims at, in undertaking to plan any of these harbor-works, is to ascertain as nearly as possible the direction and force of the prevailing winds.”