Page:South African Geology - Schwarz - 1912.djvu/42

 the tide flows up to 12 ml. from the mouth, and similar conditions exist in most of the rivers flowing into the Indian Ocean. Very few rivers flow into the Atlantic Ocean in South Africa. Where the river is perennial the fresh water meets the salt water and mingles with it, producing brackish water, which is characteristic of most of the waters of estuaries in well-watered lands; but the estuaries or tidal portions of the South African rivers are filled with almost pure salt water for the greater portion of the year, as the rivers flow intermittently, and are frequently dry for months or even years.

As sand is being carried continually up and down the shore, by coastwise currents, it tends to accumulate in the openings formed by the rivers, and builds up bars. These may be simply tracts of shallow water at the mouth of the river, on which the waves break, and which may be swept away by a flood coming down the river, or they may accumulate so much that they entirely cut off the river from the sea, in which case lagoons are formed. Such is the nature of the famous Gxaxa lagoon. The Durban and St. Lucia Bay lagoons are similar, but the channels between the lagoons and the sea are still open to a certain extent, and in the case of the former the channel has been widened artificially by dredging.

Boulders and pebbles rarely reach the mouths of the rivers, as the grinding along the course of the streams is sufficient to reduce them to sand, and what is eventually discharged is fine sand.

The river system of South Africa is a very simple one. On the south, where alone it has been properly studied, there is a central ridge, elevated 6000 ft. above sea level, which runs from Cape Town to Delagoa Bay.