flood on the Brazos was over 100 years ago, in 1913. It occurred at a similar time of year, albeit November instead of October. That was the benchmark event against which most of the Harvey Brazos River floods were measured.
Texas rivers are notoriously drought and flood prone. That 1913 event was also the last time that the Brazos and Colorado rivers merged south of Columbus.
I remember all the news reports around 2011 and 2012 wondering if Lake Travis would ever fill back up.
The locals have seen this ebb and flow before, but this one is pretty historic.
The attached link has a clip of a bridge being washed away, and a photo showing the ridiculousness of the Llano River flood.
Travis goes up and down, but down stream they keep them constant level. If they have to start letting water out of Travis it's a problem downstream.
I don't see that other than maybe exceeding the banks a bit, Lake Austin and Town Lake should be fine, as 4 flood gates are open at Mansfield Dam from Lake Travis to Lake Austin, and an equal amount at Tom Miller Dam from Lake Austin to Town Lake (cannot adopt the "Lady Bird Lake" newcomer moniker), with flood gates also open at Longhorn Dam to let the water flow from Town Lake down the Colorado River.
While we're getting a respite from heavy rains today, it is expected to start again late tonight.
Lake Travis as of 9 AM was 126% full. It is now at 697'. Full is 681' and the top of the dam is 714'. The forecast is to reach at least 700'. This is a flood of historic proportions.
My house has a direct view of Mansfield Dam, but fortunately we are up on a bluff. As you correctly point out, it is Lake Austin and Town Lake that run the risk of continuing necessary discharge from Lake Travis.
John Fogerty had a rain obsession.