I panicked a little (I hadn't brought my cell phone and was miles away from home), but I unclipped her reins and attached them to her halter. Again, she didn't act freaked out at all. She moved around a little but knew she couldn't get out by herself.
I pulled at hard as I could and told her to walk on. She braced against the lead and managed to hook a foot on the bank. With a little more pulling, she crawled out. She lay on the dry ground and panted for a minute while I rubbed her shoulder and told her she was good. "Yeah... I totally meant for that to happen. You passed the challenge." She gave me the "Yay, master is pleased!" look and got back on her feet.
I checked her legs. Nothing wrong. I re-mounted and rode her back up to the creek. She wasn't scared. She probably would have gone in again if I asked. We rode for the rest of the day without a problem.
It's amazing that she still trusts me. She will still do whatever I say. We crossed other unknown creeks --creeks I had ridden through, but never on her-- later that day. She didn't even question my orders.