[Carry_Okie]

Which of course assumes that a "clean" lake is "natural."

Nutrient levels in the lake probably go in cycles with forest maturity. During the period a few decades after the Comstock (once the miners had left and the mud and ashes had settled), the average vegetation was probably relatively young and the lake unusually clear. Now we see a nitrate release because the forest can't consume it as fast by growth as the fungi produce it feeding on dead vegetation. In the Tahoe basin I am more concerned about the condition of the forest than whether or not tourists want a lake that is clear.

[alloysteel]

The cleanest and purest of natural waters is that which has flowed through a marsh or fen, which absorbs most of the nutrients, sunken through deep sand, and has been in underground aquifers for varying periods of time, perhaps centuries, essentially "fossil" water. Flowing streams only clense to the degree excess nutrients are absorbed by the vegetation in the stream bed and on the stream banks.