They slow the water, so sediment falls out. They have well-established vegetation, soils and nitrogen chomping microbes. Still, their usefulness in the Tahoe Basin is also in question, according to Hanes. Runoff must be stored for a while for all the natural processes to work. In some meadows, water is channeled, flows rapidly through, and isn t there long enough for fine soils to settle out or for vegetation to suck up nutrients.
Smaller marshes suffer similar limitations.
But, at high altitudes with cold winter conditions, another problem occurs. The largest flows of runoff occur in fall, winter and early spring before plants start growing. If runoff water caught by wetlands isn t retained until the warmer growing season, not much nutrient is likely to be removed from it. And during the cold season, microbes that transform nitrates into harmless, gaseous nitrogen and nitrous oxide aren t very active. Incidentally, nitrous oxide is harmless for the lake, but it s a greenhouse gas.