I wouldn't recommend using chemicals to get rid of snails. You have to know that anything that kills snails can't be good for any of the other living organisms in your tank. You've received lots of good suggestions for getting rid of them, including tempting them onto food that can be removed and snail eating loaches. However, I wanted to point out that it sounds like you have Malaysian trumpet snails in your tank and if you're planning to switch to a sand-substrate planted tank, you may not want to get rid of them after all. Sand compacts down and, unless agitated with some regularity, you run the risk of developing pockets of anaerobic bacteria which, when disturbed at a later date, will release toxic gases into your water. Malaysian trumpet snails will burrow down into the sand and tunnel around under it, keeping your substrate well aerated and greatly reducing the need to be stirring around in it when you do water changes (very preferable if you're trying not to uproot plants).
As for the wood from the river, I would actually recommend baking it first to kill any living organisms on it. If you get wood that was fully submerged in the creek, it won't need to be water logged. However, you will need to soak it, changing the water a couple of times per day, until the water is pretty clear. Wood releases tannic acid into your water and will drop your pH; you want to soak most of it out before adding it to your tank to avoid dramatic water chemistry changes.
