We did a hike in southern Utah called Grand Gulch a few years ago. Water in that canyon was always a concern. After a rain there would be pools of water held in solid rock basins, from 1 liter to swimming pool size. Where the stream bed was sandy instead of solid rock, the water might be in nasty pools as big as a bath tub or smaller, covered with an oily film, and stinky. If that was the only water available, you filter it as best you can, and drink it. The water in the picture below would be considered very clean water for Grand Gulch.
On that trip, we had Katadyn filters, a Sawyer filter with a syringe backflush, an MSR Hyperflow, a Steripen, cloth bandana as a prefilter, and Aqua Mira drops. The Katadyn and Hyperflow quickly clogged. The Sawyer worked best in clear water, but took a lot of backflushing. The Steripen was very hard to use in bright daylight. The Aqua Mira always worked, but took 20 minutes to get drinkable water. On that trip I thought a prefilter, like an automotive in-line gas filter, would be perfect. I never used one because I also thought it might be made with some nasty chemicals.
MSR has added a nice tool that helps filter silty water that is often found in the desert: the MSR SiltStopper Prefilter.
The picture above shows a SiltStopper after 5 days use in Dark Canyon. Most of that red silt was sucked up the first time I used the filter. The intake tube got too close to the mud on the bottom of the creek, and sucked up a lot of red dirt. The white tube to the left of the dirty SiltStopper carries 3 spare prefilters. I used the prefilter with the Katadyn water filter shown.
The picture above shows the cover removed from the SiltStopper, which shows the red dirt it caught. Also shown is the Katadyn filter cartridge, which is in pretty good shape considering the amount of dirt in the prefilter. I like this little prefilter. Its a must for use when the water has silt or particulates.