Timelapse video has captured just how quickly floodwaters rose along the Llano River in Kingsland, Texas.

The video, which was sped up and taken on Friday, shows the water rush in and rise within 30 minutes.

Authorities overseeing the search for flood victims said they will wait to address questions about weather warnings and why some summer camps did not evacuate ahead of the flooding that killed at least 104.

The devastation along the Guadalupe River, outside of San Antonio, has drawn a massive search effort as officials face questions over their preparedness and the speed of their initial actions.

The floods grew to their worst at the midpoint of a long holiday weekend when many people were asleep.

The Texas Hill Country in the central part of the state is naturally prone to flash flooding due to the dry dirt-packed areas where the soil lets rain skid along the surface of the landscape instead of soaking it up.

Friday's flash floods started with a particularly bad storm that dropped most of its 30 centimetres of rain in the dark early morning hours.

After a flood watch notice midday Thursday, the National Weather Service office issued an urgent warning around 4 am that raised the potential of catastrophic damage and a severe threat to human life.

By at least 5.20am., some in the Kerrville City area say water levels were getting alarmingly high.

The massive rain flowing down hills sent rushing water into the Guadalupe River, causing it to rise eight metres in just 45 minutes.

The flash floods have erased campgrounds and torn homes from their foundations.