The Napa River and its estuary are an important spawning and rearing area for longfin and Delta smelt, especially in wet years. Wet years, with their high Delta outflows (Figure 1) and modest Napa River flows (Figure 2) provide spawning habitat for the smelt in the Napa River and its estuary (Figures 3-6).
Wet year 2019 shows use by longfin (Figure 3), but little use by Delta smelt (Figure 7), which likely reflects their low population abundance.
Because the smelt populations have strongest recruitment in wet years,1 the Napa River estuary likely is an important contributor to their overall population health and abundance. The Napa River estuary deserves more attention in smelt recovery strategies. However, that should not take away from improving upper Bay-Delta estuary habitat conditions in all water year types.
Figure 1. Delta outflow in recent wet years 2011, 2017, and 2019.
Figure 2. Napa River flows 2009-2019.
Figure 3. 20-mm Survey results for Longfin smelt March 2019. Source
Figure 4. 20-mm Survey results for Longfin smelt March 2017.
Figure 5. 20-mm Survey results for Delta smelt April 2011.
Figure 6. 20-mm Survey results for Delta smelt April 2017.
Figure 7. 20-mm Survey results for Delta smelt April 2019.