In flood-time rivers bring down a large quantity of detritus, derived mainly from the disintegration of the surface-layers of the hills and slopes in the upper parts of the valleys by glaciers, frost and rain. The power of a current to transport materials varies approximately as the square of its velocity, so that torrents with a rapid fall near the sources of rivers can carry down rocks, boulders and large stones. These are by degrees ground by at trition in their onward course into shingle, gravel, sand and silt which are carried forward by the main river towards the sea, or partially strewn over flat alluvial plains during floods. The size of the materials forming the bed of the river or borne along by the stream becomes less as the reduction of velocity diminishes the transporting power of the current. In the Po, for instance, pebbles and gravel are found for about 140 m. below Turin, sand along the next ioo m., and silt and mud in the last II() m. When, however, the fall is largely and abruptly reduced, as in the case of rivers emerging straight from mountainous slopes upon flat plains, deposit necessarily occurs owing to the sudden reduction of velocity. If the impeded river is unable to spread its detritus over the plains, its bed becomes raised by deposit, causing the river in flood-time to rise to a higher level. The materials, moreover, which are carried in suspension or rolled along the bed of the river tend to deposit when the flow of the river slackens and is finally brought to rest on encountering the great inert mass of the sea. This is the cause of the formation of the bars and, especially in the absence of a tide and any littoral current, of the deltas with their shallow outlets which occur at the mouths of many rivers.
A depression along part of a valley, with a rocky barrier at its lower end, causes the formation of a lake in the course of the river flowing down the valley. The intervention of a lake makes the river, on entering at the upper end, deposit all the materials with which it is charged in the still waters of the lake. The river issues at the lower end as a per fectly clear stream with a regular discharge, because the floods, in flowing into the lake, are spread over a large surface, and so produce only a very slight raising of the level. This effect is illustrated by the river Rhone, which enters the lake of Geneva as a turbid, torrential, glacier stream, and emerges at Geneva as a sparkling, limpid river with uniform flow, though in this par ticular case the improvement is not long maintained, owing to the confluence a short distance below Geneva of the large, rapid, glacial river, the Arve.
The influence of lakes on rivers is, indeed, wholly beneficial, in consequence of the removal of their burden of detritus and the regulation of their flow. Thus the Swine outlet channel of the Oder into the Baltic is freed from sediment by the river having to pass through the Stettiner Haff before reaching its mouth. The St. Lawrence, again, deriving most of its supply from the chain of the Great Lakes possesses a very uniform flow.
The discharge of the rainfall erodes the beds of rivers along the lowest parts of the valleys ; but floods occur too intermittently to form and maintain a channel large enough to contain the augmented flow. A river channel, indeed,
generally suffices to carry off the average flow of the river, which, whilst comprising considerable fluctuations in volume, furnishes a sufficiently constant erosive action to maintain a fairly regular channel. But rivers having soft beds and carrying down sedi ment erode their beds during floods and deposit alluvium in dry weather. As the velocity of a stream increases with its fall, the size of a channel conveying a definite average flow varies inversely with the fall, and the depth inversely with the width. A river chan nel, accordingly, often presents considerable irregularities in sec tion, forming shallow rapids when the river flows over a rocky barrier with a considerable fall, and a succession of pools and shoals when the bed varies in compactness and there are dif ferences in width, or when the river flows round a succession of bends along opposite banks alternately.
A river flowing through a flat alluvial plain has its current readily deflected by any chance obstruction or by any difference in hardness of the banks, and generally follows a winding course, which tends to be intensified by the erosion of the concave banks in the bends from the current impinging against them in altering its direction round the curves. (For Prof. James Thomson's theory of the concave banks of rivers see his Collected Papers [1912], also the article HYDRAULICS and Hunter's Rivers and Estuaries.) Large rivers bringing down a considerable amount of detritus shift their courses from time to time, owing to the obstructions produced by banks of deposit.
The rise of rivers in flood-time depends not merely on the amount of the rainfall, but also on its distribu tion and the nature of the strata on which it falls. The upper hilly part of a river basin often consists of impermeable strata, some times almost bare of vegetation. Consequently the rain flowing quickly down the impervious, sloping ground into the water courses and tributaries feeding the main river produces rapidly rising and high floods in these streams, which soon pass down on the cessation of the rain. The river Marne, draining an imper meable part of the upper Seine basin, is subject to these sudden torrential floods in the cold season. On the contrary, rain falling on permeable strata takes longer in reaching the rivers, and the floods rise more gradually, are less high, continue longer and sub side more slowly. A river fed by several tributaries, some from impermeable and others from permeable strata, experiences floods of a mixed character. An example is the Seine at Paris, below the confluence of the torrential Marne and Yonne, where the floods of the gently flowing Upper Seine and other tributaries with permeable basins also contribute to the rise of the river.
High floods are caused by a heavy rainfall on land already sodden by recent rains at a period of the year when evaporation is inactive, and especially by rain falling on melting snow. A fairly simultaneous rainfall over the greater part of a moderate-sized river basin is a tolerably common occurrence ; and under such conditions, the floods coming from the torrential tributaries reach their maximum height and begin to subside before the floods from the gently flowing tributaries attain their greatest rise.