Enormous Underground River Discovered in the Amazon

It is possible to have underground river, but the the size and depth described raises the questions. What is the hight of the land surface from mean sea level? It should be much more than 2 miles otherwise the flow to sea coast may be defying gravity.

In India, river Ganges and Yamuna meet at Allahabad. The meeting point is called "Triveni Sangam" means "three river meeting point". It is believed that the third river (known as Saraswati as per ancient literature) is extincted. But some of the geo-scientists came forward with an idea (after some research) that in fact the third river Saraswati is an invisible underground river which meats to other two rivers at Allahabad. Some evidences were also presented by them for its rout, but I do not have enough information about it.