Page:Africa by Élisée Reclus, Volume 3.djvu/527

THE CONGO ESTUAEY. 433 rocky escarpments submerged only to a depth of 600 feet. The Congo Yallev is thus continued for over 300 miles sexwards, enclosed right and left by ridges or embankments, evidently consisting of refuse of all kinds deposited by the fluvial current in its conflict with the surrounding waters. Hence the Congo develops, not a delta as has been stated, but rather a submarine estuary, analogous to the alluvial formations by which the beds of the Rhine and the Rhone influents are continued under the surface waters of Lakes Constance and Geneva. The tidal wave penetrating into the island-studded Congo estuary stems the fluvial current and raises its level, without, however, reversing it. Hence the mangroves, which fringe the banks of most other equatorial estuaries, are almost entirely absent from those of the Congo. The volume of fresh water, which has a fall of over 40 feet between the head of the inlet at Boraa and its mouth on the Atlantic, is far too great and too rapid to be arrested at any point by the marine inflow. The first estimate of the mean discharge, calculated by Tuckey in 1816 at 1,540,000 cubic feet per second, coincides in a remarkable manner with those that have been made in recent times. Stanley found the outflow near Stanley Pool in the month of March, that is, at low water, to represent about 1,310,000 cubic feet, while the high-water marks on the rocks seemed to indicate a discharge of 2,300,000 during the floods. Subsequent more or less trustworthy estimates for the section between Noki and the mouth vary from 1,200,000 to 1,800,000 cubic feet per second, the discrepancy being explained partly by the variations in volume from year to year, partly to the uncertainty attending such experiments, owing to the great breadth of the island-studded estuary, where the fluvial current flows ov^er the heavier tidal wave. The yearly quantity of sedimentary matter brought down by the Congo is estimated by M. Chavanne at 11,250,000,000 cubic feet, sufiicient to build up an island 1,000 feet high and half a mile square at the base. In any case the Congo certainly exceeds in volume all the rivers of the Eastern Hemisphere, and in the New World is surpassed by the Amazons alone, which like it rises in the equatorial zone, and is swollen by innumerable tributaries fed by the tropical rains. Both are characterised by a series of moderate floods and subsidences, corresponding to the oscillations of the chief affluents, which arriving at different periods tend to maintain the main stream at a certain uniform level. This, however, varies in tlie narrows of the regions of the falls as much as 30 feet, and at Yivi, below the last cataract, 14 feet. At its mouth the Congo presents two periods of high water, December and May, the corresponding lowest levels being in March and August. These two floods evidently follow the two rainy seasons of winter and spring, the latter being caused by the rise of the Arawhimi, U-Banghi, Alima, and other affluents on the right bank. The pre- liminary studies that have been made at the mouths of these and the southern tributaries, give a rough idea of their relative importance in this vast hydro- graphic system ; but the exact share of each in the general movement of the Congo waters can be determined only by a long series of patient observations. The navigable highways open to human industry in this basin yield in extent