You should let the duck go. Too many people rescue baby animals, especially birds, thinking the parents are nowhere to be found...but by taking the animals in, you're taking away any hope of the parents finding and caring for the animal.
TPWD recommends not rescuing wildlife in this manner, or if you ever do, taking it immediately to a licensed, certified wildlife rehabilitation specialist.
Please read the following:
http://www.tpwd.state.tx.us/expltx/jrnat/rehab.htm