If, however, we make allowances for roughness of bottom, bendings of channel, and sudden descents at cataracts, and say the declivity is even seven inches per mile, those 800 miles between the east coast and the great falls would require less than 500 feet to give the observed velocity, and the additional distance to this point would require but 150 feet of altitude more.

If my observation of this altitude may be depended on, we have a steeper declivity for the Zambesi than for some other great rivers.

The Ganges, for instance, is said to be at 1800 miles from its mouth only 800 feet above the level of the sea, and water requires a month to come that distance.

But there are so many modifying circumstances, it is difficult to draw any reliable conclusion from the currents.
The Chobe is sometimes heard of as flooded, about 40 miles above Linyanti, a fortnight before the inundation reaches that point, but it is very tortuous.

The great river Magdalena falls only 500 feet in a thousand miles; other rivers much more.
The forests became more dense as we went north.