Physical Characteristics of Rivers

PHYSICAL CHARACTERISTICS OF RIVERS River general the size of rivers above any tidal limit and their average fresh-water discharge are proportionate to the extent of their basins, and the amount of rain over those basins. They vary in extent according to the configuration of the country, ranging from the insignificant drainage-areas of streams rising on high ground near the coast and flowing straight down to the sea, to immense parts of continents, where rivers rising on the slopes of mountain ranges far inland have to traverse vast stretches of valleys and plains before reaching the ocean.

The comparative size of the principal river systems of the world is shown in the following table : Available Rainfall.—The rainfall varies considerably in dif ferent localities, both in its total yearly amount and in its dis tribution throughout the year; also its volume fluctuates from year to year. Even in small river basins the variations may be considerable according to differences in elevation or distance from the sea, ranging, for instance, in the Severn basin, with an area of only 4,350 sq.m., from an average of under 3o in. in the year to over 8o in. The proportion, moreover, of the rain falling on a river basin which actually reaches the river, or the available rainfall in respect to its flow, depends very largely on the nature of the surface strata, the slope of the ground and the extent to which it is covered with vegetation, and varies greatly with the season of the year. The available rainfall has, indeed, been found to vary from 75% of the actual rainfall on impermeable, bare, sloping, rocky strata, down to about 15% on flat, permeable soils.

Fall of Rivers.

The rate of flow of rivers depends mainly upon their fall, though where two rivers of different sizes have the same fall, the larger river has the quicker flow, as its retarda tion by friction against its bed and banks is less in proportion to its volume than that of the smaller river. The fall of a river corresponds approximately to the slope of the country it traverses.

As rivers rise close to the highest part of their basins, generally in hilly regions, their fall is rapid near their source and gradually diminishes, with occasional irregularities, till, in traversing plains along the latter part of their course, their fall usually becomes quite gentle. Accordingly, in large basins, rivers in most cases begin as torrents with a very variable flow, and end as gently flow ing rivers with a comparatively regular discharge.

Variations in the Discharge of Rivers.

In tropical countries, subject to periodical rains, the rivers are in flood during the rainy season and have hardly any flow during the rest of the year ; whilst in temperate regions, where the rainfall is more evenly distributed throughout the year, evaporation causes the avail able rainfall to be much less in hot summer weather than in the winter months, so that the rivers fall to their low stage in the summer and are liable to be in flood in the winter. In fact. with a temperate climate, the year may be divided into a warm and a cold season, extending, in the northern hemisphere, from May to October and from November to April respectively. The rivers are low and moderate floods are of rare occurrence during the first period, and they are high and subject to occasional heavy floods after a considerable rainfall during the second period. The only exceptions are rivers which have their sources amongst mountains clad with snow, and are fed by glaciers; their floods occur in the spring and summer from the melting of the snows and ice, as exemplified by the Rhone above the Lake of Geneva, and the Arve which joins it below. But even these rivers are liable to have their flow modified by the influx of tributaries subject to different conditions, so that the Rhone below Lyons has a more uniform, discharge than most rivers, as the summer floods of the Arve are counteracted to a great extent by the low stage of the Saone flowing into the Rhone at Lyons, which has its floods in the winter when the Arve on the contrary is low.