Seasonal nearshore sediment resuspension and water clarity at Lake Tahoe

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Finding Details

In summer, we observed that the time series is characterized by a strong diurnal pattern with wind intensity peaking each day at mid-afternoon. By contrast, in winter we observed low-wind periods punctuated by high-energy storm events. Both low-wind and high-wind periods were sustained for days at a time with no regular pattern. Generally, the onset of  wind events ended the thermal stratification of the water column within several hours. Based on these results, the thermal structure did not greatly impact the transfer of momentum in the water column.

We considered wind conditions that may derive the maximum bottom shear stress and found that wind driven surface waves resulted in potential sediment resuspension up to a water depth of 9 m. This influence extended to a lake area of 8.3 km2 for a lake water level equal to the natural rim and decreased both as the lake water level increased to the maximum legal limit (7.2 km2) and decreased to 2 m below the natural rim (4.7 km2).

We determined that wind-waves do not greatly disturb the sediment–water interface at the study site (i.e., the south end of Lake Tahoe 1000moffshore at a water depth of ∼5 m). The processes we observed were subtle and nuanced compared to similar phenomena at shallow, wind-exposed lakes like the Salton Sea (Chung et al. 2009a, 2009b).  With increasing bottom shear stress attributed to wind-waves, the suspended sediment concentration of particles of median diameter 1.25–16 μm did not change. For particles of median diameter 100–250 μm, we saw an increase (same order of magnitude) in SSC for those instances when the bottom shear stress exceeded the critical shear stress. In general, we attribute the relative lack of fine material available for resuspension to the coarse and granular lakebed sediment at the nearshore of Lake Tahoe.

An important consideration is how these research findings can be translated to management strategies. The results suggested that wind-driven nearshore sediment resuspension does not produce an increase in particle loading of the size class identified to most negatively impact water clarity; however, the resuspension of coarser material has the potential to increase the internal loading of nutrients and contaminants from the lakebed. Alexander and Wigart (2013a, 2013b) demonstrated an association between the average daily increase in turbidity and high-intensity boating in the southern nearshore of Lake Tahoe. The authors attributed the decline in apparent water quality to resuspended sediment and released nutrients from boating-induced wave action and turbulence (Alexander andWigart 2013a, 2013b). We recommend assessing the potential for internal loading of nutrients by both resuspension and hypolimnetic anoxia, which has not yet been investigated. In addition, it would be worthwhile to extend the geographic scope to include field measurements of wind-driven sediment resuspension from other nearshore areas of the lake, particularly to assess the availability of fine sediment.