In the UseNet Backcountry group, I just came across a Los Angeles Times article supplied by happywhenhiking in spokane.
The idea that going deeper in lake water will yield less bacteria is called into question. There is an on-going debate on many outdoor forums about whether you need to filter water in many places. And the debate is usually tied into how to get as lightweight as possible on the trail.
After being hammered the past decade with wilderness water safety, I'm a little gun-shy of not filtering. However, to help you make your own choice, here is part of the article on treating or not treating water:
Bob Derlet drinks his water straight — without fancy filters or chemical treatments. He leans face down into Delaney Creek, which flows directly down into Tuolumne Meadows from the Sierra Crest, taking healthy gulps from the rushing stream, and then fills his water bottle. It's nearly noon on an early summer day, and temperatures are hovering in the mid-80s. After a rigorous two-mile ascent in altitudes around 9,500 feet, the pristine mountain water is indescribably refreshing: no chemical aftertaste of tap water and chilled to perfection by the Sierra's melting snowpack.
"No one camps above here. There's no livestock or park animals so there's little chance of contamination," says Derlet, gesturing toward Mt. Dana in the distance and the lush, grassy alpine meadow surrounding the creek.
Be light. Be safe. Be one with the pack.