Home Archived April 24, 2019

  California Water Science Center

Assessment of the Role of the Invasion of Egeria Densa in the Clearing of Sacramento-San Joaquin Delta Waters

In the last several years (2002-2004), abundance indices calculated by the Interagency Ecological Program (IEP) have been at record lows for delta smelt Hypomesus transpacificus and age-0 striped bass Morone saxatilis and near-record lows for longfin smelt Spirinchus thaleichthys and threadfin shad Dorosoma petenense. While several of these declining species have shown evidence of long-term declines, there appears to have been a precipitous “step-change” to very low abundance by at least 2002-2004 even though flow conditions were moderate. Similar flow conditions have supported modest production of these species in the past. These changes are particularly important to managers because the delta smelt is a federal and state endangered species.

Studies by interagency teams suggest that changes in specific conductance and turbidity (as measured by secchi depth) are associated with declines in pelagic habitat for delta smelt, striped bass, and threadfin shad. Concern is perhaps greatest for delta smelt, a rare and delicate endemic species listed as threatened under both the California and U.S. Endangered Species Acts. Laboratory studies have shown that delta smelt require turbidity for successful feeding, so the observed decreases in turbidity may decrease feeding success. Also, clearer water may increase vulnerability of the larvae and other life stages (e.g., juvenile, adult) of all species to predation by other fishes. The abundant and widespread inland silverside may be particularly important as a predator on delta smelt eggs and larvae. There is no information on the effect of turbidity with respect to feeding success of threadfin shad or striped bass but increased vulnerability to predators likely applies to all species, including non-POD species. The decline in turbidity coincides with the spread of Egeria densa, an introduced aquatic macrophyte, throughout much of the Delta. It is well known that aquatic vegetation serves to attenuate waves and currents and that this results in increased sedimentation. The particular rate of sedimentation is strongly related to plant architecture, canopy height, and plant density. Thus, the relationship between turbidity and the spread of E. densa needs to be evaluated.

Hypothesis which will be tested include:

  • Egeria colonization at a specific location causes a step decrease in turbidity with time.
  • Turbidity decrease at sites with Egeria is greater than at sites without Egeria.
  • Sites with or close to Egeria have less turbidity than sites without Egeria.
  • Turbidity decreases as distance from an Egeria bed decreases.
  • Turbidity in the Delta has decreased as Egeria has spread.

Sites without Egeria or far from Egeria and sediment load to the Delta measured at Freeport and Vernalis will be used to identify trends in turbidity and SSC that are independent of Egeria so that turbidity trends are not falsely attributed to Egeria. For example, we know that sediment supply to the Delta has decreased by about one-half in the later half of the 20th century (Wright and Schoellhamer 2004). We will try to identify control sites not likely to be affected by Egeria and use them to normalize data from sites affected by Egeria. In this way, we may search out turbidity anomalies that are associated with Egeria.


Wright, S.A., and D.H. Schoellhamer. 2004. Trends in the Sediment Yield of the Sacramento River, California, 1957 – 2001: San Francisco Estuary and Watershed Science. v. 2, no. 2, article 2. http://repositories.cdlib.org/jmie/sfews/vol2/iss2/art2

Project Chief: David Schoellhamer
Phone: 916-278-3000
Email: dschoell@usgs.gov

Accessibility FOIA Privacy Policies and Notices

USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://ca.water.usgs.gov/projects/CA07C_1.html
For Page Information: Send Us a Message
Page Last Modified: Thursday, 29-Dec-2016 23:06:18 EST