Stormwater is water from rain and snowmelt. When it doesn’t soak into the ground, it becomes runoff, flowing over hard surfaces like pavement and collecting pollution including fine sediment, fertilizer, paint, motor oil, gasoline, pesticides, trash, leaf litter, grease, soaps, and pet waste. In the Lake Tahoe Basin, stormwater runoff is channeled into storm drains, which often flow directly into the Lake.

Fine sediment generally refers to particles less than 0.063 mm in diameter — smaller than the width of a human hair. The smaller the particle, the more likely it is to stay suspended in the water. Particles 0.020 mm in diameter and smaller are of particular concern for Tahoe because they take a long time to sink to the bottom of the lake, impairing clarity.

Most of the decline in water clarity in Lake Tahoe is from suspended fine sediment particles. Fine sediment is also a source of phosphorus because phosphorus clings to these particles as they travel through the watershed. Phosphorus is food for algae, which also clouds the lake.

Sediment particles in Lake Tahoe eventually sink to the bottom of the Lake. They can be re-suspended, however, by water currents, waves, boats and other activity on the Lake surface. Re-suspension of these particles can also contribute to clarity loss.

A watershed is an area of land where all of the water drains into one place. The Lake Tahoe Basin is an example of a watershed. All of the water that falls within the Lake Tahoe Basin flows into Lake Tahoe.

Turbidity is a measure the of the cloudiness of a sample of water. It is measured by directing a beam of light through the water sample and recording how much light hits a target on the other side. Light passing through the sample is scattered in multiple directions by fine sediment particles in suspension; the smaller the particles and the greater the concentration of those particles, the more light is scattered.