CHAPTER II.

CHARACTER OF THE WORK.

THE circumstance which gave the most interest to this work, was the fact of its being the pioneer bridge across the Missouri River, and to the distinguishing features of that river the chief difficulties of the undertaking were due.

Of the three great tributaries of the Lower Mississippi, the Missouri is at once the largest, the wildest, and the least known. The Ohio, draining the eastern slope of the Mississippi basin, flowing through a well-settled country, between high banks, over a hard and undisturbed bed, has long proved a most serviceable stream for navigation, and offers no peculiar difficulties to the bridgebuilder. The Upper Mississippi, rising among the plains of the central valley, and flowing for its whole navigable length through a low bottom land between the high bluffs which mark the level of the surrounding country, has in general a sandy and somewhat unstable bed ; but its light fall and easy current render it a good river to navigate, while its regimen is sufficiently fixed to make the task of bridging more properly one of magnitude than of special difficulty. The Missouri, drawing its source from the eastern face of the Rocky Mountains, and flowing with a rapid descent down the westerly slope of the great basin, unites within itself all elements of unstableness and irregularity, combining the impetuosity of a mountain torrent with the volume of a lowland river. The navigable length from Fort Benton to its junction with the Mississippi, is computed by the river pilots at about 3,150 miles, and the area of its drainage is given by Humphreys and Abbot as 518,000 square miles, or more than one-third greater than the united basins of the Ohio and Upper Mississippi. Owing to the lightness of the rain-fall on a large part of this district, the mean annual discharge is far from being proportionate to the extent of the drainage, and the overwhelming floods of exceptional years must be taken as the real examples of the size of

the river; but its greatness is also shown by the character of the water, filled with a light sand brought from the disintegrating rocks among the mountains, by the strange geological mixture found in the gravel and pebbles below its bed, and by the annual summer floods which come in their greatest violence when other rivers are on the decline.

The chief tributaries of the Missouri are the Yellowstone, the Platte, the Kaw or Kansas, and the Osage-the two latter being prairie streams of irregular supply, and the two former, like the upper river, deriving most of their water from the mountains. Each of these rivers has its own characteristics and produces its distinctive freshets. The Yellowstone unites with the upper river to cause the summer flood; the Platte usually pours out its water a little earlier in the spring; while the freshets of the Kaw and the Osage are of less regular occurrence, and dependent largely on local rains. A combination of these freshets, the waters from the melting snows among the mountains being supplemented by heavy rains in the lower countries, has produced the great floods which occur at long but irregular intervals, the last of which took place in 1844. This flood, the only great flood of which we have accurate information, submerged the entire bottom land below the mouth of the Kaw, and has been regarded by the settlers as an event too terrible to occur a second time; but Indian traditions mention other floods of similar character, one of which, occurring towards the end of the eighteenth century, probably in 1785, is said to have considerably exceeded that of 1844.

In many matters of topography the Missouri resembles the Upper Mississippi, while it is substantially identical in these respects with the lower river. Its course lies through a low alluvial deposit of bottom land enclosed on each side by bluffs. The distance between these bluffs varies from a mile and a half to 15 miles or more, the bluffs being generally highest, most rugged, and containing the greatest quantity of rock, when they approach most nearly together. For about 500 miles from the mouth of the river, or nearly to the southern boundary of the State of Nebraska, the bottom land, except where artificially cleared or where its width is very great, is covered with a heavy growth of timber, the cotton-wood being the most common tree, while the sycamore, black walnut, and several varieties of oak and elm also abound; farther north

the timber becomes more scarce, and a large part of the bottom land is open prairie. The average elevation of the bottoms is a few feet above the ordinary high-water level, but below the range of the extreme floods of exceptional years. The river winds to and fro in a circuitous course between the bluffs, with little apparent regularity-the width from bank to bank, measured between the wooded or grass-grown shores, varying from 300 to 1,500 yards, and averaging about half a mile. At low water the channel contracts within much smaller limits, becoming reduced to 600 or 700 feet, and leaving the remaining width a dry and desolate sand bar.

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The usual fall being from 10 inches to a foot in the mile, the current is very rapid, varying with the different stages of water, in an ordinary season, from three miles an hour to eight. The bed of the river, the sand bars and the substratum of the bottom lands are composed partially of sand and partially of a fine silt, having a specific gravity little greater than that of water* siderable quantity of this silt is always held in suspension by the water, and the current, when strong, moves the combined silt and sand with surprising rapidity. The current is most violent during a rise in the river, and the velocity is dependent on the suddenness of the rise, the level of the water being raised from above, and the surface slope thereby temporarily increased. On these occasions the current is often strong enough to deepen the channel 20 feet in a single day, and if impinging on a low bank, to cut several yards into the shore in a single hour. There is a local saying that the Missouri has a standing mortgage on the entire bottom land from bluff to bluff, and the farmer on the Missouri bottom often learns to his sorrow, by the loss of his farm, that real estate is not always immovable property.

The water of the Missouri is found by analysis to contain less solid matter in solution than is found in the water of any other important river of the continent; but it always holds a large amount of silt and fine sand in suspen

follows:

The weight of one cubic foot of different varieties of Missouri River sand and silt was found to be as

sion, which, originally emanating from the Yellowstone and the upper river, is from time to time deposited on the bars and again picked up, till it has finally been carried the whole length of the stream and left to form the bars and delta below New Orleans. Portions of the deposits remain undisturbed for centuries, forming the foundation of timber land and perhaps farms; but many of them are of the most temporary nature, swept away and replaced several times in a season. The character of the deposit also varies materially with different floods; sometimes it is almost entirely a clayey silt, while at others, especially if the flood be a violent one, it is largely composed of heavy sand. Below the silt and sand there is found a layer of coarse gravel and loose stones of varied geological character, and containing occasional relics of animal life, This gravel deposit is a collection of the coarser portions of the annual flood deposits, which, from its greater weight, having been moved but slowly by the current, has in time settled to the lowest limit of scour; it is only found at considerable depths, and is almost entirely wanting in those parts of the stream where the bed rock is frequently swept bare.

The water is most nearly clear during the low water of the winter, and especially when the river is frozen; it is muddiest during the summer flood, when a thickness of half an inch of water becomes a perfectly opaque screen. Such an amount of solid matter can only be kept suspended when in rapid motion, and is at once deposited wherever the current slackens; hence it usually happens that, while the river is cutting away the bottom lands on one side around a bend, a sand bar is forming on the other; and after the flood the channel will be found to have changed its position, while its width remains nearly the same as before. The very violence and power of the river thus confine it between narrow banks, and become the masks of its real size, quickly converting any slack-water into dry land, limiting the width to that actually required for the discharge, and depriving the Missouri of such large areas of calm still water as those which add so much to the beauty and apparent size of the Upper Mississippi. For the same reasons the "travelling sands" usually observable in rivers with sandy bottoms, and which have been described at length by writers on the Mississippi River, play a much less active part in the Missouri, as their existence demands a sufficient width of river to allow the

whole discharge of water to pass in a shallow stream over the crest of the sand bar ; they undoubtedly occur in this river, but are confined to straight reaches of the stream, where the channel is broad and but poorly defined, and to seasons of high water; while their action is slow and unimportant compared with the violent wash and scour on the curves or where the current is rapid. When the river cuts into a timbered part of the bottom land the destruction of the bank lets the trees fall into the water; they usually remain for a short time, seldom as much as a day, at the spot where they fall, forming a temporary protection to the bank and causing local irregularities in the channel; but they soon become free and float down the stream till caught by some obstruction in the bottom; here they soon lose their leaves and smaller branches, and unless set free by the rising water or by the loosening of the obstruction which entangles them, they remain fast and form the snags which so greatly impede navigation. Besides the live trees washed into the river, every flood picks up a large quantity of loose timber and rubbish from the sand bars and low portions of the bottom land, the amount of drift which even a moderate flood brings down being very great.

When the width of the bottom land is not more than two or three miles, the usual course of the river is to follow along the base of one of the bluffs till deflected by some obstacle, then to cross the valley to the other bluff, follow that for a short distance, and then return to the former side, thus pursuing a serpentine course, and alternately inpinging upon each bluff. The meanderings of the river are then more marked and regular than in other streams; the vein of strongest current can generally be distinguished by a casual observer, it crosses the stream diagonally in the straight reach between the curves, and is always strongest on the outside of the curves; the river constantly washes the lower bank as it crosses the bottom land, and thereby impinges on the opposite bluff at a lower point than hitherto, so that unless held by natural projections in the bluffs, or other protection, the pair of reversed curves, resembling a large letter S somewhat flattened, and corresponding to the points where the stream leaves one bluff and where it strikes the other, by cutting away the bottom land and forming fresh sand bars, are continually advancing down the valley. When the bottom land has a greater width than two or three miles, the river is liable to