MESSRS. EDITORS:—This is a subject of great Importance not only to the people of Louisiana, but to all those who are interested in the trade of the Mississippi. We have in this State over two millions of acres of as fertile land as the sun ever shines upon, which is annually overflowed by water from an outlet of the Mississippi during freshets. This renders it unfit for agricultural purposes, and makes it a grand laboratory of poisonous malaria destructive to health and life for miles beyond its borders. For several years I have advocated closing this outlet CBayou Plaquemine), but have been uniformly opposed by the representatives of those parishes situated below it, who believe it would cause an increased hight of the freshets or floods in the river, and prove injurious to the plantations below the bayou, which is 210 miles above the mouth of the river. After a careful examination of the Mississippi river, I came to the conclusion that, by closing the bayou, we would not only reclaim two millions of acres of land from overflowwhich would add directly to the wealth of the State over one hundred millions of dollars, and give homes to one hundred thousand inhabitantsbut that we would reduce the extreme hight of the floods in the river. I will give some of the most important facts relative to the Mississippi river tributaries, pouring their floods into the main stream for a distance of over 1,200 miles from the ocean. These do not increase the width of the river ; on the contrary, it becomes narrower below each addition to its volume ; and the diffrence between high and low water is less after each tributary is added. This holds good without exception. The rise of the freshets are diminished without much increase of velocity in the current. The Mississippi attains its greatest volume about 500 miles from the Gulf of Mexico, and has there a flow, in extreme high tides, of 95 millions of cubic feet of water per minute. Each five hundred feet of water contains one foot of solid earth, which, of course, is precipitated wherever the water becomes still for a considerable time. The velocity of the current is within a fraction of five feet per second, which carries this sediment onward with the water towards the ocean. At the mouth of the Ohio, the elevation of the surface of the Mississippi is nearly 275 feet above the ocean, and its bottom on the shallow bars about 200 feet above the ocean level. The river has an average descent of four inches per mile, but for the last 250 miles its descent is a fraction less than one inch and a half per mile. Opposite the outlet which I propose to have closed, the river is 2,500 feet wide and 103 feet deep, and has a velocity of current of four feet per second. The surface, in high water, is 30 feet above the ocean, consequently, the bottom is 73 feet below the ocean level. The velocity of the whole volume is two feet greater than the surface current, and the velocity within five feet of the bottom is 10 feet greater than at the surface ; consequently, it has the greater carrying force, and can move particles which the current cannot sustain on the surface!. These facts are contrary to popular belief. And although the water at the bottom of the river over 200 miles from the oceanis 75 feet below the ocean, and has actually to run up hill over three inches per mile, it flows onward with a velocity far greater than the surface current, which flows down a plane of one inch and a half par mile. This proves that there is another cause for velocity of currents in rivers besides the slope of the plane over which they flow, and that the weights of the volume above and behind have a great forcing power. My conclusion is that the larger the volume, or rather the greater the depthprovided there is a volume of considerable weight behindthe greater will be the discharge, and that the dredging power in the bottom is increased in exact ratio with the depth and velocity of the carrent, and that the permanent effect of an addition to the volume will be to deepen the channel where there are no rocks, and oonsequently, will really, in time1., diminish the hight of the surface. The outlet which I wish to have closed in high water takes from the river 28,000 cubic feet per second; consequently, if it were closed in high Water, and no change took place in velocity of current and depth of channel, the surface would ho raised about 10 inches, which would be a serious addition to the hight of the floods. But to sustain my opinion that that would not be the practical effect, we have the experience of 140 years ou thisriver. The first levee was built in front of New Orleans in 1717. These structures have been gradually extended until nearly the whole extent has been leveed for 1,000 miles on each side of the river, and we have reclaimed 30 millions of acres of land from inundation that was formerly overflowed annually to an average depth of three feet. The water for the last 50 years has not been one inch higher at New Orleans than it had been before a levee was built. This is a well authenticated fact, and besides this, the average hight of the high water mark has really decreased as levees have been extended. All the early maps show the river to have been much wider than it is now. As to its depth in early times we have no reliable proof, but the first French commander that entered the river reported to his government that he found 13 feet in the deepest parton the bar. This is about 10 feet less than we now have. I have omitted to mention one fact that should have caused the river to rise higher at New Orleans than it did when the city was first laid out, all other circumstances being the same, viz., that it extends its delta into the ocean about one mile in 25 years. Undoubtedly the Mississippi river is governed by the same immutable laws that govern all livers, when due allowance is made for the variety of circumstances that attend different streams. The Father of Waters has not any rocks or scarcely a pebble an ounce in weight for 1,000 miles from the ocean, and it flows through a basin which has probably been an extension of the Gulf of Mexico. In past ages this filled up with sediment which formed banks as it extended the land into the ocean. Excavations in every portion of this vast region afford evidence of this. We have the same formation and soil in layers, with logs and remains of trees, as deep as it has been penetratedsay 400 feet. If we can reclaim the territory alluded to, its equal is not to be found in the State, as it is subdivided by numerous natural canals which traverse every portion, and you can scarcely find an acre in the vast area that is not within two miles of a navigable stream. This question involves over a hundred million dollars' worth of land in the best sugar region of Louisiana. E. W. FULLER. St. Martinsville, La., Nov. 28, 1860.