Under siege from sediment and algae, a new draft federal study of watersheds and streams in the Lake Tahoe Basin is offering a glimmer of hope for the lake’s clarity.
Computer models project a decrease over the next 50 years in the level of sediment delivered to the lake from the Upper Truckee River on the South Shore, and from streams along the lake’s Western Shore. The sediment comes from mountainsides and other land areas where runoff contribute water to the streams, as well as from erosion along the stream channels.
“This is particularly significant for the western streams because they currently produce some of the highest loadings to the lake,” the authors wrote.
The projections contradict claims by other researchers who say future loads in these streams are likely to increase in the future.
The draft report says falling sediment loads from the Upper Truckee River appear to be a continuation of trends during the past 24 years.
Streams on the North and East Shores produce the lowest sediment loads in the basin, and those that were studied were actually net collection points for sediment over time.
That’s despite recent measurements that rank Second and Third creeks in Incline Village among the top five highest sediment contributors, which also include the Upper Truckee River and Blackwood and Trout creeks.
The authors say this reflects Incline Village’s high development levels since the 1960s, which has drastically fallen in recent years under Tahoe Regional Planning Agency regulations. The long-term trends for these show dropping levels.
Other North and East Shore streams, such as Logan House, Dollar, Glenbrook, and Edgewood creeks, have some of the lowest loads in the basin.
Fine-scale suspended sediments, together with algae nourished by water- and airborne compounds, are the two biggest threats to Lake Tahoe’s diminishing clarity.
Both scatter light and cause the lake to appear murky below 78 feet for the past two years.
The study, titled “Sediment Loadings and Channel Erosion: Lake Tahoe Basin,” was performed by the U.S. Department of Agriculture’s National Sedimentation Laboratory, and commissioned by the U.S. Army Corps of Engineers.
It will be used by the Total Maximum Daily Load program, which in 2007 will release plans for reversing environmental harm done to Lake Tahoe under a mandate from the 1972 federal Clean Water Act.
The TMDL goal is to return the clarity depth to the 1967-1971 average annual of 97 feet. Clarity was measured this year at 78 feet.
The sediment loading report is the last of three Corps studies TMDL needs to assess the amount of nutrients and sediments flowing into the lake.
The assessment will be used by Pathway 2007, a multiagency effort that, in addition to helping TMDL, also will aid the regional plan updates of the Tahoe Regional Planning Agency and U.S. Forest Service’s Lake Tahoe Basin Management Unit.
Some researchers point to the arrested clarity loss as evidence that recent measures to address the problem are working, and that the Pathway 2007 efforts are worth the effort.
John Cobourne, a researcher with the University of Nevada Cooperative Extension program, finds these results encouraging, but says keeping these gains in place and building on them could require a shift in behavior at the lake.
“If we all (think of) the protection of Lake Tahoe as our personal business, instead of thinking that somebody else will do it,” said the longtime Tahoe researcher, “then it could happen.”
Other major findings of the study include:
o Streambank erosion is an important contributor of suspended-sediment from streams disturbed by human action.
o Disturbed watersheds contribute considerably more suspended sediment than nearby undisturbed counterparts.
o The Upper Truckee River is the greatest contributor of suspended-sediment and fine-grained sediment in the Lake Tahoe Basin.
o Upper Truckee River sediment delivery could be significantly reduced by controlling streambank erosion adjacent to the Lake Tahoe Golf Course and downstream from the Lake Tahoe Airport.
o The January 1997 major runoff event engorged western streams and the Upper Truckee River most severely, but had minor effects on northern streams.
o The most significant effect of the January 1997 runoff event was to flush stored sediment from valleys, which lowered sediment loads in following years.
For more information contact Phil Brozek at (916) 557-7630 or Meegan Nagy at (916) 557-7257 from the US Army Corps of Engineers. The study may be found at www.spk.usace.army.mil/civ/tahoe.