For travel in 2019, you will only require your Drivers License, regardless of how you plan to travel. There is absolutely no need for a US citizen to carry their passport for this trip.
Beginning October 1, 2020 there MAY be a need to carry your passport if you are flying. This is the current start date for the requirement to have a REAL ID complaint ID in order to fly. If your Drivers License is REAL ID Compliant (most California drivers licences are NOT - other states will vary) then even from this date you will be fine with just a drivers license.
However if your drivers license is NOT REAL ID Compliant, then from this date you will require a passport or other suitable ID - at least in theory. (Exactly how this ends up being implemented, and what will happen for people without such ID, is at this stage unknown)