No abstract

This study uses tracer experiments to characterize the fate of a cold plunging river into a Mediterranean reservoir under different stratification conditions. Three tracer experiments in Lake Béznar, Spain, demonstrate that a significant fraction of the inflow may be quickly entrained into the surface mixed layer (SML), rather than flowing to lower depths. We observe that a fraction (and possibly all) of river inflow entrains into the SML when the density current forms intrusions at the top of the metalimnetic layer. Intrusions near the base of the SML form due to lighter interfacial fluid at the top of the density current, with intermediate properties between riverreservior water when there is sufficient dilution of the cold inflow, and abrupt density changes at the base of the SML. Consequently, a layer of fluid splits from the denser part of the current and intrudes just below the SML, while any remaining fluid flows to lower depths. We parameterize this splitting behavior in terms of the transition Richardson number, Ri 12 5 C 3 (r 2 2 r 1 )/(r 0 2 r 1 ), where r 0 , r 1 , and r 2 are densities of the inflow, the SML, and the metalimnetic layer underneath, respectively; and C is the total dilution through the plunge zone and subsequent entrainment. Splitting occurs when Ri 12 $ 1, consistent with previous laboratory experiments. We use this theory to predict the seasonal fate of river water during the stratification of Lake Béznar and identify the conditions that favor the formation of intrusions near the base of the SML.

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In this paper, we discuss observations of temperature variability in the tidal portion of the San Joaquin River in California. The San Joaquin River makes up the southern portion of the Sacramento San Joaquin Delta, the eastern end of San Francisco Bay. Observations made in August 2004 and August 2005 show significant diurnal variations in temperature in response to surface heat exchange. However, to account for observed changes in heat content a sizeable downstream heat flux (approximately 100 W m −2 ) must be added to the surface heat flux. To account for this flux via Fickian dispersion, a flow-dependent dispersion coefficient varying from 500 to 4,000 m 2 s −1 is needed. These values are much larger than would be predicted for a river of this size, suggesting that the complex topology of the Delta greatly enhances longitudinal dispersion. Building on these observations, we present a simple theory that explores how the subtidal temperature field varies in response to changes in flow rate, dispersion, and heat exchange.

The invasive aquatic plant Egeria densa (Brazilian waterweed) is a submersed aquatic plant that has expanded its distribution in both its native and introduced range. Because the plant grows so densely, it can become a problem for management of waterways and habitat restoration projects. It is difficult to remove once established and mechanical and chemical controls have shown limited effectiveness. Here we analyze the distribution of E. densa in the Sacramento-San Joaquin Delta (the Delta) of California, USA, using environmental variables that include mean water velocity, mean water turbidity, and water column depth. We found that increasing water column depth strongly limited E. densa occurrence, especially when depth at mean lower low water (MLLW) exceeds 2 m. The highest probability of occurrence occurred at locations with a water column depth of-1 to 2 m at MLLW. Turbidity had a reliably negative effect on E. densa occurrence; as water clarity has increased in the Delta, it has likely favored the spread of the plant. Neither mean water velocity nor maximum water velocity had a reliable effect on E. densa probability, in spite of scientific and observational evidence that it is sensitive to flows. These results suggest potentially serious problems with restoration projects that emphasize shallow water habitat in the range favored by E. densa. Without some way to manage spread of the plant-through spraying, sediment loading, or gating-channels in such projects are at risk of being taken over by E. densa. However, these results should be interpreted in light of the fact that water outflow in water year 2008 was very low, and that E. densa abundance may be partially controlled by higher water flows than those considered here.