"Although the great depth of Lake Michigan prevents the surface from freezing, yet the ice accumulates in large bodies in the shallow water near the shores, and is driven by the wind into the mouths of the rivers. A barrier being thus formed to the force of the lake-waves, the sudden check of velocity causes them to deposit a portion of the silt 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 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.