It was bound to happen while hiking the PCT in a severe drought year; after 1500 miles I was finally forced to filter my drinking water out of a swampy, stagnant, pool. For the first 750 miles a water report helped hikers navigate through the arid sections. Hikers update the water report via email and text message, and anytime the adjective "stagnant" appeared on the water report I planned accordingly and carried extra water.
In Northern California no such water report exists, so I rely on the information found on my trail maps, which is often quite accurate, but my luck finally ran out. Two noted creeks had no water which created an unknown 18 mile section with no flowing water. Adjacent to the second dry creek I found a low boggy area with standing water. The water was clear enough that I could see the muddy bottom, but I could also see all the tadpoles and a variety of water bugs and insects that thrived in the over-sized puddle. I was careful to scoop up water without disturbing the muddy deposits, and I avoided capturing any bugs.
After filtering the water I could see the silty sediments that collected in my dirty water bag and on the filter o-ring. I was reluctant to take the first sip of my freshly filtered puddle water which turned out O.K. I remember on one backpacking trip in Colorado, my friend Dustin and I were forced to filter stagnant water which had a strong sulfur smell even after the water was filtered. My puddle water, however, tasted fine; but even after filtering the water, a faint yellow tint filled my hydration bag. Regardless, I needed the water to cook dinner and eight to ten miles stood between me and the next spot where I hoped flowing water could be found.