In a March 2020 post, I described where the remnants of the endangered Delta smelt population spawn and rear in the Sacramento Deepwater Ship Channel (Ship Channel) in the north Delta (Figure 1). In this post, I describe how the rearing conditions in the Ship Channel are poor. This can be seen by comparing habitat conditions in the Ship Channel with those in the lower Sacramento River channel at Freeport several miles to the east in late spring 2020.
Net flow (cfs) in the Ship Channel remains near zero, since the gate at the north end of the channel near Sacramento remains stuck in the closed position as it has been for several decades (Figure 2).
Water temperature (oC) is significantly higher in the stagnant flows of the Ship Channel than at Freeport, often reaching into the lethal range 23-25°C for Delta smelt (Figure 3).
Salinity (conductivity) is much higher in the Ship Channel because of discharges from urban sewage treatment plants and agricultural operations (Figure 4).
Turbidity is much higher in the Ship Channel due to higher plankton production and port-bound ship traffic in the relatively shallow and narrow Ship Channel (Figure 5).
Dissolved oxygen levels are much lower in the Ship Channel because of warmer water, high concentrations of suspended organic sediments, and higher plankton production (Figure 6).
Interpretations and Conclusions
Delta smelt are attracted to the Sacramento Deepwater Ship Channel in winter and early spring to spawn in the relatively warm, low salinity, turbid, and more productive water. The adult smelt can also easily tidal-surf up the ship channel without having to content with strong downstream currents of the Sacramento River channel. Their eggs hatch early to an awaiting abundant plankton food supply. However, in spring the Ship Channel lacks net downstream flows to carry the young smelt to the Bay. By late spring, water temperatures in the Ship Channel reach lethal levels for the young smelt.
Opening the gate at the north end of the Ship Channel would help to alleviate the problems by providing net flow with cooler water temperatures, and by flushing and diluting the stagnant waste waters in the Ship Channel. An operable gate at the head of the Ship Channel would allow adaptive management of the habitat conditions for smelt.