A watershed is the area of the land where all of the water flows to one place, a common waterway. All of the precipitation and streams in the Upper Truckee River Watershed flow into the Upper Truckee River and eventually into Lake Tahoe.
There are 63 watersheds that empty into Lake Tahoe. The Upper Truckee River Watershed is the largest watershed in the Lake Tahoe Basin, at 142 square kilometers.
The boundaries of a watershed are defined by topography (ridges, mountains, gullies, etc). The southern boundary is the top of Carson Pass. To the north, the watershed flows into Lake Tahoe at the Upper Truckee Marsh, near Tahoe Keys. The western boundary is defined by Angora Ridge.
The Upper Truckee River delivers over half of the fine sediment that enters Lake Tahoe. Fine sediment (less than 20 micrometers) remains suspended in the water column rather than settling out and refracts light. Scientists estimate that fine sediment is responsible for 2/3 of the loss of Lake Tahoe’s famed clarity.
The watershed has undergone significant modification and degradation. Wetland areas, crucial for water purification, flood control and wildlife habitat, have been filled in with sediment for development. California alone has lost 90% of its historic wetlands. Urbanization and an increase in impervious surface coverage slows the infiltration of storm water into groundwater. Urban runoff is the leading source of fine sediment and excess phosphorus, both of which cause a decrease in lake clarity. The Upper Truckee River has also been modified. The river has been realigned and channelized from its original, more sinuous state. Channelization increases the velocity of water in the river, resulting in increased erosion. Over a 53 year period, some parts of the Upper Truckee River have decreased in length by 26% due to stream channelization.
The Upper Truckee River is classified as an impaired waterway by the Clean Water Act due to excessive levels of iron, phosphorus, and pathogens. Neighboring Trout Creek Watershed is classified as an impaired waterway due to iron, phosphorus and nitrogen levels. Fertilizers from residential areas and golf courses, rich in nitrogen and phosphorus, enter the lake and lead to increased algal populations in Lake Tahoe and negatively impact lake clarity.
