THE IMPACT OF WATER DIVERSIONS 
ON THE RIVER-DELTA-ESTUARY-SEA ECOSYSTEMS 
OF SAN FRANCISCO BAY AND THE SEA OF AZOV 
Michael A. Rozengurt, Michael J. Herz, 
and Michael Josselyn 
Paul F. Romberg Tiburon Center for Environmental Studies, 
San Francisco State University 
Abstract 
A review of the long-term impact of river diversions on the 
hydrological and biological features of the estuarine ecosystems 
of San Francisco Bay and the Sea of Azov (once Russia's richest 
fishing ground) indicates that despite differences in scale and 
in climatic, hydrographic, and physiographic regimes, the eco¬ 
logical status of these systems, involving the River-Delta- 
Estuary and adjacent coastal zone, depends on cumulative river 
runoff fluctuations. In the past, before human intervention, 
these systems were naturally maintained by stochastic processes, 
but as a result of regulation of river flow during the last 
30-40 years, these conditions have become primarily determinis¬ 
tic, artificially manipulated by man. Analysis of the relation¬ 
ships between water supply variables and parameters such as 
salinity and catch of anadromous fish in both San Francisco Bay 
and the Sea of Azov indicates that steady reduction of annual 
and spring freshwater supply by diversions exceeding 30 percent 
of the natural limits of the dominant fluctuations of these 
estuarine ecosystems has resulted in drastic declines in the 
fisheries. 
Unprecedented changes in ecological conditions have 
appeared 5-7 years after a period of 10-15 years of relatively 
stabilized seasonal stream flows. These flows were at 30-65 
percent of the mean historical water supply. The residual 
inflow onto the estuary cannot entrain enough water to flush 
wastes and excess salt into the sea and cannot provide the 
optimal ranges of nutrients, salinity, and other dissolved 
constituents necessary for the survival of estuarine species. 
In the Sea of Azov, flow reductions have resulted in 
increased salt intrusion from the Black Sea and have led to a 
massive invasion of scvphozoan medusae , resulting in radical 
declines in the economic and recreational significance of the 
Sea since the late 1970s. 
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