Why is the lake of chad fresh, and the rivers do not flow out of it?

In the center of Africa there is a lake, which is a reservoir for four states – Chad, Cameroon, Niger and Nigeria. This is Lake Chad. It is considered an endless lake, into which rivers flow, but do not flow out of it. In the area where these rivers flow, water samples show that it is fresh there. However, as you move away from the mouths of the rivers, it will still stop a little brackish, which is typical for closed-drainage lakes, which Chad is considered to be. However, researchers have identified some small underground runoff that prevents more salt from accumulating. In addition, during the rainy season, the lake can overflow its banks, forming a riverbed.