QX tried various routings involving ACV, starting with the DH8-100/200 on the PDX run. (I saw it landing once while at Patrick's Point State Park, but I don't know if it was a round robin then with another city.)
Later they ran the Q400 on round-robins with RDD: PDX-RDD-ACV-PDX and PDX-ACV-RDD-PDX. At some point I think SEA may have replaced PDX. At the end, they did LAX-ACV-RDD-LAX, LAX-ACV-RDD-PDX/SEA, LAX-RDD-ACV-north, etc.. On the LAX-ACV/RDD-north trips, they had overlaying flight numbers. eg. LAX-ACV-RDD as flt 123 and ACV-RDD-north as flt 456 so they didn't sell the LAX-north as a through flight. A fair amount of bussing from RDD to ACV when ACV fogged in.
Feel free to correct me as memory fades, and some of this started before I moved to the coast (CEC). [Way back in 1979-81, I worked pre-UAx WestAir at SMF with the C402 flying SFO-SMF-ACV-CEC and reverse, along with SFO-SMF-CIC-RDD. Between Valley tule fog and coastal fog, we ended up not knowing where the planes were going to end up, come back or cancel. Ugh....]
In the 1990s, Horizon also used CEC and OTH as en route stops from PDX as well.
When I did UAX customer service in the late 2000s, the Horizon flights were usually the only possible way to get passengers who misconnected or had a cancellation to RDD/ACV home (CEC sometimes, too). During peak seasons, the ACV/RDD/CEC flights were all very full and also usually weight restricted to 26-28 pax (sometimes more or less) out of 30 seats on the EMB-120. No way to fit extra pax/bags, so we'd usually send them up to PDX to then take a Q400 on Horizon down to ACV (also useful for CEC pax as only UAX flew there at the time) or RDD. As UAX flew SMF-ACV/CEC at that time, there were plenty of pax being flown around creatively during IROPS. Stuff like SFO-SMF-ACV on an EMB-120 then ACV-RDD on a Q400 or SFO-CEC-SMF to get to SMF after SMF canceled (non-American pax traveling in from long haul flights rarely wanted to deal with renting a car and driving 2 hours if we could fly them there, domestic travelers would usually rather drive). Too bad those flights aren't around anymore, they were very handy. Some of the multi-stop routes actually even connected SFO to SEA via different flight numbers but the same aircraft (and usually the same crew). I used to sneak commuting employees on those flights when SFO-SEA/PDX was oversold. Too bad since then these routes have more or less gone away with AS/UA.