If you just fish in Texas and not in other Navionics regions, the least expensive card is the Navionics Hotmaps Premium South region.
You can view the current maps on the Navionics website's Web App.
"good detailed creek and river channels" ain't going to happen since the map software will not stack contour lines on each other to indicate ledge drops.
At best you will get the channels highlighted if a location is a somewhat recent survey. You turn that feature on in the Chart menu, Chart Detail>Custom, and turn on Nature of Seabed
It would look like this sample at Kentucky Lake: