Delta Smelt Concerns Result in Changes in SWP and CVP Operations

State and federal export facility operations were modified in May and June in response to concerns over the distribution and high salvage of delta smelt at the SWP and CVP Delta pumping facilities. Since we have no direct measure of delta smelt losses at these facilities we use salvage of delta smelt as a surrogate for "take. " 1999 was an above-normal (San Joaquin Basin) to wet (Sacramento Basin) water year (DWR 1999), but the distribution of young-of-year (YOY) delta smelt was more typical of a dry year hydrology with a greater proportion of the population remaining in the Delta through spring and early summer. It is uncertain why delta smelt remained in the Delta for so long this year, but water temperature may have been an important factor (Dale Sweetnam, personal communication). Delta smelt spawn in areas of fresh water under tidal influence. In dryer years, spawning is often concentrated on the Sacramento River side of the Delta, especially in the Cache Slough area. In wetter years, spawning is widespread and can occur as far west as the Napa River, as it did this year. Similar to 1997, a large YOY delta smelt population in the central Delta resulted in higher take at the SWP and CVP facilities. The elevated take levels were surprising since this year's Delta hydrograph showed a similar pattern to 1996 (Figure 1). Delta exports were considerably higher in late May and June 1996 than they have been this year, yet delta smelt salvage in 1996 was less than half of the 1999 levels (Figure 2)