I watered mine a few times right after I planted them and never again since. That was at least 5 or 6 years ago.
The Hertiage birch never gets watered and it is in full sun. I don't the name of the other river birch but it only gets what may spray from watering the hostas (in the shade). Not very much. But it is in the path that gets lots of rain water run off from the roof.

cactusfreak -- So, that does sound pretty drought-tolerant. Also, are you saying one of your River Birch trees is in part shade/shade? And, it still grows well?

I can understand your confusion. While River Birch (Betula nigra) is certainly tolerant of moisture as evidenced by it's ability (and apparent preference) for growing near rivers, it does not require those growing conditions. While I think that planting it in concrete islands is taking it a bit too far, it is obviously happy to grow in suburban gardens as well.

I think it's a nice choice.

Well, both of you seem pretty confident and...again...hundreds of concrete islands can't be wrong! Hahaha! :) It just seemed so counterintuitive to me, but I do understand what esh is saying about prefer-vs-require on the growing conditions. I planned to put the trees in beds within my St. Augustine (don't shoot me...I have had good success with SA and once it's established, honestly is more drought-tolerant than people think...especially when there is a little shade to help out...hence the intended River Birches), so it would get more water than, say, my rock garden out by the street.

Yes, one of them is in almost total shade.
You can see just the trunk in this picture.
There is a huge sweet gum tree over shadowing it and a giant red tip, along with jasmine vine.
Note, just some of my many rain barrels.
They are all connected together and the drain pipe runs under the ground below the LP gas tank behind the trellis. It has a spigot on the end.

cactusfreak -- I'm convinced. River Birch, it is. Related question: did you do the rainbarrels yourself? I have been looking into that a little (wouldn't be able to do it for the front yard, but could do it on one side of our house and considering it also for a potential vegetable garden down the road).

My DH and I did them. We have about 17 that are connected with downspouts and spigots and 7 more that just sit under the spouts. I have a water transfer pump. It sits outside the barrels and a hose goes into the opening to pump water.
The ones in the picture are high enough that I can just attach a hose or the soakers to them. It takes a while but the water will come out slowly.
If you look around you can get the barrels free. I only paid $10 each for mine but the fittings added up to a lot.
Brass spigots are at least $5.00 each.
The ones on the other side of the house are connected to a soaker hose or I can use the spigot.
I have had these 5 years or more.
We are looking into getting a well. In case anyone is interested the price quoted is $11,000. if they hit water within 500ft. This includes house and irrigation pumps.
Since I have 2 acres, over 3000 hostas and many ferns and a large greenhouse and tropicals, a pond and pool. I will need all the water I can get.
25 mins a day every other day would do just my containers.
I had all these plants way before the drought.
Don't you think I should be 'Grandfathered' in before the new rules.?

What new rules?

I have not seen any free barrels around, but my husband thinks we can put a set-up together with materials from the local home improvement center, for less than purchasing "real" rain barrels online.

OK, I went and looked it up. I know what you are talking about now with the 25 mins of hand-watering every other day. I'm in Central GA and we're still just doing odd/even. Of course, I haven't watered in months now anyway and this could all change, based on how much rain we get...or don't get...this spring.

I only have front yard grass in the center, with beds all around (and probably going to add two more beds with the aforementioned River Birches anyway). I am slowly turning the outermost bed (near the street) into a rock/xeriscape garden. I have to be careful, due to the utilities out there, of course; but I started filling it last year with juniper, crape myrtle and lantana. This year, I've added a couple of mugo pines and some sedum; and will also be adding some ornamental grasss and etc. I had several large rocks delivered last week, in fact. My neighbors are looking at me a little quizzically, but I think they are starting to see that it really will look good and give them something nicer to look at than the ugly culvert of days past. :)

They're beautiful fast-growing trees. However, they are a favorite site of bagworms and have large surface roots. As for drough tolerance, true that they didn't die last year, but they got brown early and stayed that way for awhile before dropping leaves.

I have 3 that I am cutting down this year.