Good morning! My property has a very gentle slope - so gentle you can only see it when you stand back and look hard - but water finds that slope every time it rains hard, and rushes down it to make a big pool in a flatter spot. Huge. Like a lake about an inch deep. There's an obvious rim right under the trees where the water comes through what almost looks like natural swales but then finds just open space and pours out. I'd like to extend it further to the right (north) and make a long swale out of it to stop the flooding every rain - but not sure what to do with it once the digging is done. It's still really shady in most parts, so can I get any benefit out of a swale right on the edge of woods? (oak, hawthorne, wild plum, etc)
I would say you can get a lot of benefit right at the edge of the woods in our climate. Plant on the berm of such a swale and the trees would provide some shelter from the worst of the summer heat. Texas has such intense summer sun that most plants require some shelter. (I've seen sunburn on tomatoes and peppers.) If the shade is dense enough to need more light for fruiting, it should be possible to prune the existing trees to allow more light, without having to remove the whole tree. This is for the warm season gardening.
So long as those aren't live oaks, you will have a perfect location for winter gardening with on site free mulch. Most greens (spinach, cole crops, swiss chard) grow well throughout the winter. Onions and garlic also. I haven't tried yet, but I've heard that many root crops (beets, turnips) grow well.
I'm pretty sure there is no area of our state that doesn't benefit from any strategy we can employ to increase water retention.