In any event, you're on the right track: Find the clearest-looking water and start with that. My preference is to find a little pool in a moving stream. That way the tube float isn't bobbing around too much, but the water still comes from a moving source. Keep in mind that dirt and grit can cause real problems with a filter, so you want to avoid filtering water that looks dirty if at all possible. The PUR filter folks suggest a few good tricks to help with this (www.purwaterfilter.com). One is to draw some water into a bucket or water bottle and let it settle for several minutes. Another, strain it first through a paper coffee filter (which can be re-used if you're careful). Either of those steps will go a long way toward prolonging the life of the filter.