Here's the breakdown as I hear it, keeping mind that during low traffic times everything is moved to channel 1. During the next level it is channels 1 & 4. You'll hear them say "6 to 4 and 3 to 1" when moving from 4 channels to 2.
Channel 1: North (TG 3769) is north of the river, except as above.
Channel 3: Central (TG 3773) is central which used to be bordered by the rivers, Broadway & Alhambra, except as above.
Channel 4: South West (TG 3775) is south of Broadway and west of the county portion, except as above.
Channel 6: South East (TG 3779) is south of Broadway and east of the county portion, except as above.
In the definition of the south area being anything south of the American River, channel 3 would cover a portion of that. In the definition that the south area is south of Broadway, it would not. BUT, all it takes is an "incident" and regular traffic gets moved to other available channels.