The Sacramento-San Joaquin Delta in California is one of the most modified estuaries in the world. Its hydrology has been extensively altered by the network of levees, dams, and canals which have made it vulnerable to invasion by eliminating the competitive advantage of low-nutrient brackish-adapted native species. We examined if the extreme California drought has favored the spread of non-native species. We acquired AVIRIS-ng data at 2.5m pixels for the entire delta (~2500 km2) in November 2014 and classified this data using Random Forest Learning Algorithm for invasive species and communities. Previous HyMap data collections in summer 2004-2008 showed that two aggressive freshwater invasive species adapted to high nutrient and low salinity conditions flourished in the Delta: the floating species, Eichhornia crassipes (water hyacinth) and the submerged species, Egeria densa (Brazilian waterweed). A third invasive floating genus Ludwigia spp. (water primrose) was just beginning to establish itself in the northwest portion of the Delta.
We collected 1036 field data points from water channels and tidal marshes across the delta to use in training and validation of the classifier. We used previous field data and HyMap (an imaging spectrometer) imagery to classify data from 2008 to 2014. We confirmed that the total area invaded by floating macrophytes in the Delta has increased many-fold over this period, from 320 to 2610 hectares. The composition of this functional group has also changed. Native Hydrocotyle cover has been significantly reduced and Ludwigia has replaced it as the co-dominant with Eichhornia crassipes. Submerged macrophyte cover has stayed relatively constant and is still dominated by Egeria densa. This study provides insight and understanding of the drought’s impact on invasive potential of an ecosystem.