Lakes play a major role symbolically and physically in the Circle series. There has been 3 variants in the book series. These variants are: Green, Black, and Red lakes.
Green was the original version of the lakes in the series. At the start they granted major healing abilities, including enough power to bring Thomas back to life. The green lakes are recognized as Elyon's lakes. Many times going in the green lakes allowed people to hear Elyon. They changed dramatically after Tanis drank the Black water. They changed where in order to remain pure you had to bathe in the lakes regularly. After this change they also lost their major healing powers.
Black pools were actually the first form of lake encountered in the series. At the beginning of Black, Thomas encounters them with Bill. They are the water Teeleh, he uses them to tempt humans into drinking it in which they are then bound to his will. Tanis is the only known one from the villages to ever drink from it.
The Red lakes came into being after the sacrifice of Justin, son of Elyon. After he was killed and his blood spilled into the Green lakes, they become unpure and would not heal the scabbing disease on touch. After this all of the Green Lakes became Red. Then the rules of the lakes changed again, this time being to become saved they would have to dive into lakes and drown. (Metaphorically, they did not actually die, more similar to a baptism) After this they became permanentally cured from the Horde disease.