Save The Lake!
Red line is Carrizal River, Green Area is the lake and Blue line is the Grijalva River. (Sorry for the low quality picture :P )
So, build a canal to feed water on the lake from the red stream and build a canal to release water to the blue stream, you don't need pumps for the city(which is inside those 2 rivers and around the lake), making some calculations we can now what amount of water we have to let flow in the 2 streams so we can clean the lake and keep it beautiful for the sake of us all.
The following picture is from a part of the lake which constantly gets polluted, last time it got some sewage discharge and killed almost all fish in it, even maggots infested the lagoon, really disgusting.