Fine‐Sediment Loadings to Lake Tahoe¹

Abstract

Over the past 35 years, a trend of decreasing water clarity has been documented in Lake Tahoe, attributable in part to the delivery of fine‐grained sediments emanating from upland and channel sources. The overall objective of the research reported here was to determine the amount of fine sediment delivered to Lake Tahoe from each of the 63 contributing watersheds. The research described in this report used combinations of field‐based observations of channel and bank stability with measured and simulated data on fine‐sediment loadings to estimate fine‐sediment loadings from unmonitored basins throughout the Lake Tahoe Basin. Loadings were expressed in the conventional format of mass per unit time but also in the number of particles finer than 20 μm, the latter for future use in a lake‐clarity model. The greatest contributors of fine sediment happened to be those with measured data, not requiring extrapolation. In descending order, they are as follows: Upper Truckee River [1,010 tonnes per year (T/year)], Blackwood Creek (846 T/year), Trout Creek (462 T/year), and Ward Creek (412 T/year). Summing estimated values from the contributing watersheds provided an average, annual estimate of fine‐sediment (<0.063 mm) loadings to the lake of 5,206 T/year. A total of 7.79E + 19 particles in the 5‐20 μm fraction were calculated to enter Lake Tahoe in an average year with the Upper Truckee River accounting for almost 25% of the total. Contributions from Blackwood, Ward, Trout, and Third creeks account for another 23% of these very fine particles. Thus, these five streams making up about 40% of the basin area, account for almost 50% of all fine‐sediment loadings to the lake. Contribution of fine sediment from streambank erosion were estimated by developing empirical relations between measured or simulated bank‐erosion rates with a field‐based measure of the extent of bank instability along given streams. An average, annual fine‐sediment loading from streambank erosion of 1,305 T/year was calculated. This represents about 25% of the average, annual fine‐sediment load delivered to the lake from all sources. The two largest contributors, the Upper Truckee River (639 T/year) and Blackwood Creek (431 T/year), account for slightly more than 80% of all fines emanating from streambanks, representing about 20% of the fine sediment delivered to Lake Tahoe from all sources. Extrapolations of fine‐sediment loadings to the unmonitored watersheds are based on documented empirical relations, yet contain a significant amount of uncertainty. Except for those values derived directly from measured data, reported results should be considered as estimates.

Hydrologic budget and dynamics of a large oligotrophic lake related to hydro-meteorological inputs

“…5). The monthly average precipitation contours for these wet months shows that precipitation is highest on the west side of the lake and lowest in the east side mid-lake area; agreeing with the available isohyetal maps for annual precipitation (Simon, 2008). Although precipitation distribution on the lake may vary from event to event, hourly gridded precipitation data are not available.…”

“…The one parameter that is known to display significant spatial variability is precipitation, and hence we have adjusted precipitation distribution over the lake on the basis of the isohyetal lines to provide a more realistic input. The isohyetal map of Lake Tahoe used for the TMDL (Lahontan and NDEP, 2010; Simon, 2008) shows that precipitation over the lake varies nearly 50% from the shore to the mid-lake. Note that weather data was used from nine SNOTEL meteorological stations around the lake for the watershed model, LSPC.…”

“…Shown are the midlake and index in-lake measuring stations and the meteorological station at Tahoe City. Isohyetal map for the Lake Tahoe basin showing contours of equal annual precipitation (redrawn from Simon, 2008). station (Fig.…”

Use of fish functional traits to associate in-stream suspended sediment transport metrics with biological impairment

“…Watersheds undergoing channel adjustment generate greater annual yields of suspended sediment than geomorphologically stable systems, where in the unstable systems 60% to 90% of load sources can be from channel incision and bank retreat (Simon and Darby 1999;Simon and Rinaldi 2000;Simon 2008). Simon et al (2004) developed ecoregion-level suspended sediment TMDLs by quantifying SSC and SSY exceedance frequencies for RGA-classified stable and unstable channels.…”

The role of the surface fine-grained laminae in low-gradient streams: A model approach

“…Past modeling of sediment transport reported in the literature has included many studies that address streambed and streambank erosion (Mosselman, 1998;Viney and Sivapalan, 1999;Simon et al, 2000;Langendoen et al, 2001;Liu et al, 2002;Kinsey-Henderson et al, 2005;Duan and Nanda, 2006;Jarritt and Lawrence, 2007;Simon, 2008;Smith and Dragovich, 2008;Wilkinson et al, 2009;). However, we found no previous studies that included the erodible semi-buoyant SFGL storage as a sediment source.…”