THE RIVER PLAINS. 25 down the delta, they become more and more sluggish, and raise their beds still higher above the adjacent plains. Each set of channels has a depressed tract or swamp on both sides, so that the lowest levels in a delta lie often about half-way between the rivers. The stream overflows into these depressed tracts, and gradually fills them up with its silt. The water which rushes from the rivers into the swamps is sometimes yellow from the quantity of silt or sand which it carries. When it has stood a few days in the swamps, and the river-flood subsides, the water flows back from the swamps into the river- channels ; but it has dropped all its silt, and is of a clear dark- brown hue. The silt remains in the swamp, and by degrees fills it up, thus slowly creating new land. River Estuaries. — The last scene in the life of an Indian river is a wilderness of forest and swamp at the end of its delta, amid whose malarious solitudes the network of channels merges into the sea. Here all the secrets of land-making stand dis- closed. The streams, finally checked by the dead weight of the sea, deposit their remaining silt, which rises above the surface of the water in the shape of banks or curved headlands. The ocean-currents also find themselves impeded by the down-flow from the rivers, and drop the burden of sand which the tides sweep along the coast. In this way, while the shore gradually grows out into the sea, owing to the deposit of river silt, islands or bars are formed around the river mouths from the sand dropped by the ocean-currents, and a double process of land- making goes on. The Bivers as Irrigators and Highways. — The great Indian rivers, therefore, not only supply new ground by depositing islands in their beds, and by filling up the low-lying tracts or swamps beyond their margins, but also by forming banks and capes and masses of land at their mouths. They slowly construct their deltas by driving back the sea. The land which they thus create, they also fertilize. In the lower parts of their course, their overflow affords a natural system of irrigation and manuring ; in the higher parts, man has to step in, and to bring their water by canals to the fields. They form, moreover, cheap highways for carrying the produce of the country to the
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