nothing wrong with the water flowing into the small tarn/lake. Just don't take it out of the lake.
At the risk of putting too fine a point on it - getting to the inflow point of this little tarn is difficult and time-consuming, so for all practical purposes you have to use an area where a hundred other people may use it on any given day - hence the need to filter.
As for the water flowing across a couple of switchbacks about 1/3 of the way up - even when it is flowing in general, it's often frozen in the early hours as you begin the climb. Even when it's thawed, you have to block the trail in order to get water. So ... because of the uncertainities/hassles of this location, I regard it as an emergency water source, and filter 3 liters at the little tarn.
Of course, YMMV. The above is the basis for my decisions. Everyone is free to make their own choices.
Whether to filter/not to filter is often a prickly issue for some, but not me. Just like I don't play in traffic, jump off 3 story buildings, try to outrace a cop, snack on roadkill - I don't drink from questionable water sources.
