They have all dried up by a change of climate, possibly similar and cotemporaneous with that which has dried up the Dead Sea.
The journey of Ezra was undertaken after a fast at the River Ahava.
With nearly 50,000 people he had only about 8000 beasts of burden.

He was ashamed to ask a band of soldiers and horsemen for protection in the way.

It took about four months to reach Jerusalem; this would give five and a half or six miles a day, as the crow flies, which is equal to twelve or fifteen miles of surface travelled over; this bespeaks a country capable of yielding both provisions and water, such as cannot now be found.

Ezra would not have been ashamed to ask for camels to carry provisions and water had the country been as dry as it is now.
The prophets, in telling all the woes and miseries of the captivities, never allude to suffering or perishing by thirst in the way, or being left to rot in the route as African slaves are now in a well-watered country.

Had the route to Assyria been then as it is now, they could scarcely have avoided referring to the thirst of the way; but everything else is mentioned except that.
Respecting this system of Lakes in the centre of Africa, it will possibly occur to some that Lake Nyassa may give a portion of its water off from its northern end to the Nile, but this would imply a Lake giving off a river at both ends; the country, too, on the north-north-west and north-east rises to from 4000 to 6000 feet above the sea, and there is not the smallest indication that Nyassa and Tanganyika were ever connected.