I believe there is no solution to the loss of life as has played out. A certain percentage of the population will believe it will never happen to them. They will ignore warnings and say it's not going to happen or won't be so bad no matter how the National Weather Service and local officials warn. You see it in hurricanes, floods, tornadoes and winter storms. Some people will just not heed the warnings. No matter what the government does, some will ignore it and some that do will pay the price with their life.
The thing rarely mentioned is the camps that dismissed early because of the predicted bad weather nor the camps that moved kids to higher ground before the warning was issued thanks to the weather service harping on this for a couple of days prior. What happened at Camp Mystic is a mystery we may never solve. My point is not every camp lost campers.
Earlier in a post someone indicated the police and/or fire department should have driven the roads with loudspeaker on to warn people. First, this is a large county. The rural areas had few patrol cars and fire engines.
The volunteer Fire Department did that in one of the areas (I forget the name) and the campers were saved. I get the low water crossings. There are a lot in West Texas too.
The advantage of being "local" is they should be aware of where the most likely spots to flood are and how to get around them safely.
The flooded area is generally within 2 or 3 miles of the river. You don't need to warm the whole county. The local authorities should know who needs warned.
Before COVID in Pickens County Georgia when the conditions were right for tornados volunteer storm spotters were "activated". I don't know if this was a NWS, state or county function but these brave souls would give accurate information for tornados in the west end of the county.
I would suggest if this area has volunteer "storm spotters" that they be asked to "watch the river" as far upstream nessasary to give at least an half hour warning when a flood watch has been issued too.
Another possibility would be for the flood gauges upstream to automatically "call" the local 911 centers if the water has risen to a critical level. I am not sure of the connectivity of these gauges but there should be real time monitoring by the (Army Corp of Engineers, NOAA or TVA: who ever owns the river level gauges) whenever a "flood watch" has been issued not just here but nationwide. This area might need extra flood and water gauges on the tributaries too. It might be necessary for the State of Texas to pay for these if the the Texas Congress and Senators can't get the US Government to do this. IMHO this would be good pork.
With some folks exercising their property rights to an extreme and the lack of building codes in rural areas, there should be a law that if your property has sleeping accommodations and is in the flood plain, you have to have a working alarm for high water* if you want electric power. The same way Georgia requires proper septic systems if you want electric power to your new home or trailer. Your insurance should give you a discount for flood / high water alarms (like smoke detectors) for property in the flood zones.
The volunteer flood watchers and high water /flood detector for buildings and camp sites with sleeping accommodations could be done quickly and cheaply.
*Amazon has several model under $100. My favorite is the Topvico with the 120 db alarm for under $25. Nine volt battery not included.