QA-CAS-1: My disc landed in a creek that has been declared casual. May I place a rock or a broken limb behind my mark, to stand on in order to keep my feet dry?

No. If you choose not to take casual relief back along the line of play, then you must take your stance as you would anywhere else on the course. The only time you are allowed to move obstacles is to move casual obstacles out of your lie. If you do not want to play the lie as is, or take casual relief, you can take optional relief, or abandon the throw, at the cost of a penalty throw.