You will be hard pressed to find something that meets all those requirements and also stays the correct size. You might look at Carpinus betulus

Are you looking for year-round screening? A fence might be a better option. I garden in an area just a bit beyond the range of Taxodium, but I sure admire it when I run across one. I've seen some very nice specimens in Iowa...yes, Iowa. There's a supposedly small one doing business as Shawnee Brave. I don't know if it grows knees? Mmm, I wonder if anyone has attempted selection for prolific knees?

Musclewood (Carpinus caroliniana)? Dura Heat River Birch?

I'd second the Carpinus, also known as Blue Beech.

There are many varieities of Japanese maple (Acer palmatum) that will stay within those size requirements and nearly any will produce great fall color.

z6 is about half the US ... do we need anymore specificity on this one???

ken

Dogwoods! Cornus kousa, cornus mas, cornus officinalis. They do produce fruit, though I wouldn't call them messy. Count me in as a proponent of carpinis caroliniana also. Shawnee Brave bald cypress was selected for its narrow, dense, columnar habit. It is not a small tree. All the ones I've seen around here are street or parking lot trees, and some of those have reached 30 feet tall. Grown in a wider, wetter area, they'll get huge. I have a young one myself and it appears to be grafted, so this has caused me to wonder if Shawnee Brave's supposed lack of knees is due to genetics, or the fact that it's almost never grown in conditions that favor knee growth.

In years my Cornus kousa fruits, it is messy. However, mine only has a heavy fruit year about one out of 5. I don't know if that is typical for C. kousa. I do like dogwoods in general. The Carpinus suggestion is a good one, and you can find a Japanese maple to suit your size needs. Just be sure to research varieties using a reliable source rather than just relying on the tag since there are many choices and tags can be unreliable.

Thanks all. I think we're going to go with the Japanese maple.

Good luck with your new tree. If you are planting it in an area with poor drainage, it would be wise to build up a wide mound of soil to plant the tree in. At the very least, make sure you plant it with the root flare a few inches above ground level. I've noticed that my trees always sink after planting as the planting soil compacts over time.

How long does it flood for? File Japanese Maple as one that will fail if its feet are wet. Personally I'd look for something else if you have occasional flooding that doesn't drain away fairly quickly.