Another 'answer' used by several folks around here (and the orig. drift boat anchors) is a section of inner tube. Cut a section about 2.5 feet long, lace the bottom shut with nylon (or sum-such) cord. Make an anchor harnes (similar to a "sea anchor" and fill it with 'beach rocks' when you get to the stream.

Off the river ... dump out the rocks. Sounds too simple, but for pontoon boats actually works quite well. Remember, 95% of your 'anchoring' will be in very slow/no moving current.