Places to find & collect rocks.

My dad loves collecting rocks and uses them for personal projects. He loves all kinds but specifically he’s looking for river rocks at the moment. He would like to go to a stream or river in the area or even somewhere outside the Houston area that could be part of a day trip and collect them. Does anyone have any suggestions of where he could go to find them? Picture is just to show the various kinds he enjoys, but not specific to what he’s looking for at the moment.