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 the fall is but slight, they tend, on the contrary, to raise their beds by depositing sand and mud brought down from higher levels. Hence, in the lowest part of their course, many of the most celebrated rivers, the Nile, the Po, the Mississippi, the Thames, &c., run upon embankments, partly of their own creation.

When not interfered with by man, rivers under such conditions sooner or later break through their banks, and, leaving their former bed, take a new course along the lowest part of their valley, which again they gradually raise above the rest. Hence, unless they are kept in their own channels by human agency, such rivers are continually changing their course.

Finally, when the river at length approaches the sea, it in many cases spreads out in the form of a fan, forming a very flat cone or "delta," as it is called from the Greek capital Δ, a name first applied to that of the Nile, and afterwards extended to other rivers. This is due to the same cause, and resembles, except in size, the comparatively minute cones of mountain streams.

The estuary of the Thames is swept by the tides, and the deposits of the river carried away to sea as fast as they are brought down. At the mouths of the Po, on the contrary, the tide is very small; at those of the Mississippi it never surpasses a yard, and even at the mouth of the Ganges it does not generally rise more than ten feet.

In flat countries the habits of rivers are very different. For instance, in parts of Norfolk there are many small lakes or "broads" in a network of rivers—the Bure, the Yare, the Ant, the Waveney, &c., which do not rush on with the haste of many rivers or the stately flow of others, which steadily set themselves to reach the sea, but rather seem like rivers wandering in the meadows on a holiday. They have often no natural banks, but are bounded by dense growths of tall grasses, Bulrushes, Reeds, and Sedges, interspersed with the spires of the purple Loosestrife, Willow Herb, Hemp Agrimony, and other flowers, while the fields are very low and protected by artificial dykes, so that the red cattle seem to be browsing below the level of the water; and, as the rivers take most unexpected turns,