Popular Science Monthly/Volume 25/September 1884/The Upper Missouri River System
|THE UPPER MISSOURI RIVER SYSTEM.|
THE Missouri River, as is well known, is the larger of the two great branches which unite to form the Lower Mississippi, discharging at its mouth 120,000 cubic feet of water per second, while the Upper Mississippi discharges only 105,000 cubic feet per second. It is therefore itself properly the Upper Mississippi. The perpetually turbid character of its waters is a familiar fact to the ordinary reader, even if he has never seen them.
It is proposed to state a few facts, derived from a season's personal observation in the valley of the Upper Missouri and of its nearly equal tributary, the Yellowstone, which may account for this condition, and serve to explain the peculiar form of erosion that characterizes this river system.
The upper portion of these rivers, where they flow through mountain-gorges, form deep cañons, and leap over wild cascades, is, of course, more interesting than their lower portions, where the flow, though rapid, is tolerably uniform through valleys of considerable width and among low sand-bars and islands of their own creation. As a consequence of this, we find that it is this upper portion that has received the chief attention by writers and explorers, who hasten through the duller parts of the country and make only a meager record of them. Another reason for this has been that it is in the region of country about the sources of these rivers that the most profitable mining and agricultural enterprises have been conducted, and large and thriving settlements, even cities, have grown up there, unaided by railroad connections, and communicating with the civilized world by overland routes—not along the river-valley, but across the country from the south, uniting this region with the Salt Lake Basin. It is thus that Helena, Bozeman, Virginia City, and, to a large extent, Fort Benton, now a thriving town, have come into existence, cut off, as it were, on the east, with the great valleys through which the waters of this region are led back to the inhabited parts of the country in a condition akin to unexplored. This was especially the case with the Yellowstone Valley prior to the construction of the Northern Pacific Railroad.
The Yellowstone, from its rapid current of about three miles per hour, its frequent sand-bars, shoals, rapids, and other obstructions, is scarcely navigable at all; while the Upper Missouri, though navigable with great difficulty in high water as far as Fort Benton, or even to its Great Falls, forty miles above that point, possesses a sad history of wrecks, disasters, and failures.
The Yellowstone and Upper Missouri Rivers flow in an easterly direction, nearly parallel to each other and at a distance of about one hundred miles apart, at least for the lower half of their course. Above the Musselshell, which stretches nearly across the intervening space, the country is more or less mountainous, the fall of the water is more rapid, the bottom usually gravelly or rocky, the valleys narrow, and the water clear except in times of flood. Below the Musselshell of the Missouri and the Big Horn of the Yellowstone, nearly opposite, this Mesopotamian region consists of an elevated plain wholly destitude of arborescent vegetation. Its elevation, though not sufficient to be called mountainous, is considerable, and is formed by several distinct rises or terraces. The summit is a level plain, and contains large lakes or marshes in which wild-geese and other water-fowls in immense numbers breed and rear their young. From this plateau long valleys, sometimes of considerable width, descend to the rivers, carrying streams of water which, in some cases, persist throughout the year. The highest part, or divide proper, between the rivers is not central but is nearer the Missouri, which has rugged banks on its south side, with some of the features of the Dakota Bad Lands. Toward the Yellowstone the slope is gradual, and the terraces become lower and lower until the river-valley proper is reached. The right bank of the Yellowstone for most of this distance is similar to the right bank of the Missouri, and toward its mouth the country lying south of the river is not to be distinguished from the true Bad Lands of the LittleMissouri adjacent to it. On the other hand, whatever wide flats or
low country the Missouri here possesses are generally to be found on the north or left bank of that river (see Diagram No. I).
Without attempting a description of those strange and interesting mauvaises terres, which are the favorite theme of popular writers, I shall endeavor to give some idea of the process by which the valleys of these rivers have been formed and of the action of the rivers within their present bed. It is quite evident that the entire configuration of the land-surface of the region has been the result of erosion, and distinct breaks or even low cliffs sometimes occur, showing the edges of the horizontal strata. At intervals of from five to ten miles small streams or creeks fall into the river, often entirely dry in summer, sometimes containing a small quantity of perfectly transparent water, but so charged with alkali as to whiten the pebbles over which it flows, and to render its use by man or beast almost impossible. These creeks, locally denominated coulées—a name given them by the early French explorers—have excavated valleys of different lengths and widths, and between these occur narrow plains, or even mere ridges. Of the immense volume of solid earth and rock that has been brought down by the process of eroding these terraces, creek-valleys, etc., only a minute fraction has been retained, but this has been deposited near the river, forming an alluvial bottom of varying width. This alluvial deposit it is the function of the river perpetually to wear away, while at the same time laying down new matter, with which it is constantly charged, to take its place. The result is, that throughout the lower portions of these rivers, and also in the Missouri Yalley below their junction, the bed of the river is perpetually shifting its position in the general valley. When we contemplate the entire history of the river, the valley must be regarded as due to this process, and its great width relatively to that of the stream itself can only thus be accounted for. But, if we contemplate it only at a given time, as the present, the valley appears to consist of two quite distinct parts, viz., the riverbed and the valley proper, raised above it and gradually sloping back on one or both sides to the foot of the first terrace. If, in time of low water, we compare these two parts, the latter will appear to be stable, while the former will clearly show that it is unstable. There was probably never a time in the river's history when these two distinct features did not exist much as now, though no one can say how many times the river may have worn away the stable portion of its valley on one side while it was forming anew on the other, and afterward receded and carried off the last-formed valley, leaving its previous bed to be again filled up until it has regained all the aspect of permanence which it previously possessed. This crossing and recrossing by the river-bed of the general valley, proceeding simultaneously with the work of lateral erosion, have gradually lowered the valley to its present position and are still lowering it. In a certainsense this applies to all rivers and river-valleys, but nowhere perhaps
on the globe does there exist a better example from which to study these principles of surface erosion than in the Upper Missouri River system. This will be best seen when we consider a little more closely the proper bed of the river.
For two or three months of each year, between March and June, the river is high, and this state of high water is tolerably uniform from year to year, so as to be in a manner normal. Supplied chiefly from melting snows at greater and greater altitudes as the season advances, it persists with only slight fluctuations until the supply is exhausted, when the water slowly falls to its low-water mark, where it remains the rest of the year with only a small amount of variation, because the rainfall is so light. There thus exist two distinct and somewhat uniform conditions of the water, each occupying its regular part of the year. Owing to this regularity of high water, the maximum bed of the river produced by it is somewhat uniform and clearly marked, while it also bears a tolerably uniform relation to the deeper channel represented by the low-water state. Examined in time of low water, this river-bed seems to be three or four times as wide as the river itself. The stream, then, usually flows in serpentine curves which cross and recross the bed. The bed itself is also crooked much as is the channel, only its curves are as much longer as it is wider. The whole valley is usually also winding with much more ample curves, and the river-bed crosses and recrosses it in a manner similar to that in which the channel crosses and recrosses the bed. The river itself generally hugs one of the banks of the bed, but it is always at a curve, or bend, such as will tend to wear the bed on the convex side and thus render it more crooked. The distance traversed by the channel in crossing from one side of the bed to the other is small, compared with the distance traversed while in close contact with the bank of the river-bed, which it is perpetually extending into the general valley. The reason why it does not constantly grow wider is, that on the abandoned side the surface is being constantly raised by deposits of material which the water, more sluggish on this side, can no longer hold. As the river shifts its position in the valley, a strip of land of varying width is formed each year to be gradually assimilated to the permanent valley (see Diagram No. II).
If, now, we take the more general view and regard the entire valley as one homogeneous product, we can better study the process by which it has been formed. Beginning with the channel of the river we shall find that, except where crossing the bed, its cross-section presents a figure approaching more or less closely to a right-angled triangle with the right angle at the bottom, or deepest place. One side will then be formed by a steep wall or bank, which may become perpendicular above the surface of the water, but is not usually so below. The other side of the triangle represents the general bottom of the river, which gradually grows more shallow toward the remote side of the river-bed. At the deepest point, fresh erosion or corrosion is taking place, while the steep bank adjacent is being rapidly worn away (see Diagram No. III).
The features to be described can only be satisfactorily observed in time of low water. The bank above the river on the deep side is then generally very high, often rising perpendicularly twenty feet or more above the surface of the water. This high bank, thus exposed to the view of the navigator in the river, affords a most excellent opportunity of studying the manner in which the material composing the general valley has been deposited, the various agencies that combined to form the deposit, and the approximate time required for the accumulation of a given thickness of this alluvium.
These walls of loose earth are always very conspicuously stratified, the layers having various thicknesses and different colors. As many as a dozen distinct strata can usually be seen, often very definitely marked off from one another. The color of these layers enables the observer to determine, with considerable certainty in any case, whether it was due to a wash from the neighboring hills, whose color can be directly compared, or to a deposit from the river itself, brought in time of flood from points higher up, or, as is often the case, from vegetable, mold which long immunity from disturbance has allowed to accumulate. Some idea of the time occupied in the total deposit may be formed from the presence of forests of cottonwood (Populus monilifera, Ait.) which line the river. These trees are sometimes of great size, measuring three or four feet in diameter, and, although the cottonwood is a rapidly growing tree, there can be no doubt that many of the trees are two or three hundred years old. But the mere presence of these forests standing upon the surface of the latest stratum of the general valley is by no means the only time-measure we have. A careful observer, though merely walking among them, might perceive that some of them have their bases buried to some little depth with alluvial earth or vegetable mold. This fact, which would escape any one who was not specially looking for evidences of it, becomes striking when the edges of the strata are viewed from the river.
As the river wears away the previously formed deposits of its valley, it at length approaches the portion that has bad time to become covered with these forests. Undaunted, it attacks this portion also, and begins the work of felling the trees. Their roots are laid bare, the solid earth on which they have stood for ages is swept away, and one after another these ancient giants succumb to the rapacity of the waters, and fall powerless into the raging current. Every step in the process by which this result is accomplished may be seen by watching these eroded banks while floating down the stream. The river, as it passes one of these doomed forests, is choked with snags, throughwhich the surging waters roar, and among which it is extremely difficult and often dangerous to guide a boat. These snags are of all
ages, from the old "sawyers" that have bowed before the current with rhythmic regularity perhaps for centuries, to the freshly-felled monarchs still bearing their green leaves of the season.
But the fact of chief interest is the presence of trees on the brink of these eroded walls, whose still living and healthy trunks are laid bare to a depth of several feet below the present surface of the ground. In some cases the subterranean portion occupies as many as four or five feet of the base of the trunk, descending through a number of distinct strata. But even at much greater depth there are frequent and unmistakable relics of ancient forests long since destroyed, or, as it were, buried alive. At depths of ten or twelve feet below the present surface, old stumps, with roots and remains of trunks, are brought to light by the inroads of the river. The trees which these represented must have been buried deeper and deeper, in the same manner as existing ones are proved to be undergoing burial, until, unable longer to perform the functions of circulation, they died, and all decayed except these deeply buried parts. Sometimes even these are gone, and naught remains beyond a reddish stain against the vertical wall to mark the spot where once there flourished upon the then surface of the valley a large and healthy tree (see Diagram No. IV).
The method thus far described of studying the mode of formation of the river-valley is that of analysis—the observation of the action of the water in disintegrating it. But we may also employ the method of synthesis, and study the manifest process of valley-building which takes place simultaneously. The river is always loading up on one side, and unloading on the other. The deepest part of the river near the high banks, as it sweeps round the great bends, is also the swiftest. The current grows slower and slower in the direction of the opposite shore, and at the same time the water grows more and more shallow, until at last a sand-bar is reached gradually rising out of it. If this proves to be the mainland, the case is simple, and we will first consider this simple case. This sand-bar was formed at the last period of high water in the spring and early summer. It therefore consists of sand only, without vegetation. It may have a width of fifty or a hundred feet when it ceases, and a distinct rise occurs, with a little terrace of sand, thickly covered with seedling willows, all belonging to one species (Salix longifolia, Muhl.), and bearing no other vegetation. The sand is still damp, being saturated with water from the river. This land is two years old. A short distance farther back another similar terrace is reached, bearing a thicket of this same willow, but it is now two to four feet high, and fruit-bearing. The land is here three years old. Another remove brings us to a third terrace, having larger willows and some other vegetation, such as is not injured by periodical floods flowing over it. This four-year-old soil is darker in color and firmer. Itmay complete the river-bed proper, or there may be still another terrace. As we recede from the river, these old river-bed marks become
gradually obliterated, and the valley seems to slope away with a gentle upward curve to the foot of the lowest hills. As soon as we are fairly out of the present river-bed the little willow gives way entirely to a large one (Salix cordata, Marshall), popularly known as the diamond willow. This species often grows very dense and in large clumps, forming an almost impenetrable thicket. It monopolizes the soil, and renders approach to the river difficult. It is at a point still more remote that the growth of cottonwoods begins, and these may form a belt half a mile to a mile in width. From the outer edge of these cotton- wood-forests the plain commences, and stretches back, not only across the remainder of the valley, but far away in an uninterrupted sea of grass, until another river system is reached (see Diagram No. III).
Such, in its general outline, may be conceived to be the normal character of the Missouri and Yellowstone Rivers after they pass the mountainous part of their course and enter the portion where wide valleys prevail. But there are, of course, many deviations from this normal type. The fires may have destroyed the cottonwoods and willows that line the river and occupy most of its bed, and an unbroken plain may extend down to the sand-bars upon its banks. These sandbars may form islands around which quite brisk currents flow even in the dry season. Sometimes, as at Spread-Eagle Bar, on the Missouri, a number of such bars occur, with shallow currents between them, wearing them away along clean-cut faces, and shifting their position from place to place, giving great width to the river. Large islands are often formed, which have accidentally escaped the denuding process, and, being beyond the reach of fires, become covered by a heavy growth of timber. Sometimes the bed of the river lies between two similar high banks, more or less central in the valley, showing that, instead of continuing to approach the bluffs on one side, its erosive action has from some cause been arrested or reversed. In such cases there is occasionally found a nearly equal current against each bank, but usually, even here, the main channel is snug against one of the walls, which it is rapidly carrying away, while the opposite wall has an ancient or obsolete appearance, with shoals or bars at its base. Of course, the entire configuration of the country is modified by the occurrence at short intervals of tributary streams with their valleys. These streams, in spring, contain considerable water; but, throughout the summer and autumn, most of them are perfectly dry, at least at their point of junction with the river, whatever water they receive from rains or springs being evaporated in their passage across the arid plains. One is greatly astonished to find no water, or only a rivulet, at the mouths of what are called rivers, and which drain hundreds of square miles of country.
But the Missouri and Yellowstone themselves never go dry. They are large and rapid streams at the dryest seasons of the year, and their turbid waters surge past like a resistless tide. They wear down their valleys by slowly crossing and recrossing them, like a turner's chisel. Once at their limit on a given side, they may be imagined to halt and turn back. The form of the bottom is changed and the point of greatest activity transferred from one side to the other; the sand-bars are first removed, and then the willow-belt is carried away; next they attack the forest of cottonwoods, and mercilessly sacrifice these; still undaunted, they invade the higher parts of the valley, wear away wide stretches of plain, and slowly march up to the foot of the adjacent hills and mountains, which they also attack and undermine, until, checked by the increasing quantity of débris, and driven back by the very magnitude of their own trophies, they beat a retreat, only to repeat for the thousandth time the process which we have thus hastily sketched.