What used to characterise Lake Poopó in Bolivia, effectively dry since some time before the end of 2015?

Answer 1

It was highly saline
The lake was fed by a trickle from the Rio Desaguadero which is the outlet for Lake Titicaca. When the water level of Lake Titicaca is below 3,810 m (12,500 ft), the flow of Desaguadero River is so low it could no longer compensate for water losses due to evaporation from the surface of Lake Poopó.