Introduction 
Adaptation of estuarine organisms to a wide range of annual 
and seasonal fluctuations in biochemical and biological charac¬ 
teristics is the result of centuries of evolution, in response 
to the probabilistic nature of runoff variation. This process 
has resulted in the ability to populations of estuarine orga¬ 
nisms to recover from extreme hydrological conditions, e.g., 
drought-produced, catastrophic declines in runoff leading to 
salt intrusion, sporadic algal blooms, anoxia, etc. (Hedgpeth, 
1970; L'vovich, 1974; Bronfman, 1977; Mann, 1982; and Rozengurt, 
1974, 1983b). It is evident that the maintenance of estuarine 
characteristics such as biological productivity and flushing 
capacity are determined by the natural cycles of fluctuations of 
freshwater supply to the system (Baydin, 1980). This inflow is 
a renewable but limited resource. 
Geophysical and climatological properties of the watershed 
determine its volume and are important physical limitations that 
should be considered an essential component of overall estuarine 
resource management. Natural flow is a most essential factor to 
be considered in analyzing any system (Champ et al. 1981; 
Cronin, 1967; Lauff, 1967; Officer, 1976; and Vorovich et al.) 
to determine what guantity of water can be diverted without 
seriously damaging the estuary. However, the definition of 
"natural flow" has been confused with a more limited concept of 
"historic flow" based on the residual regulated flow, i.e., what 
is left after upstream and within Delta diversions. From this 
perspective "historic flow" is the unregulated runoff that 
occurred during some past period, according to hydrological 
definitions established by UNESCO (1974), and Sokolov and 
Chapman (1974). Both recommend performing basin analyses on 
unimpaired flow fluctuations over periods of at least 50-60 
years. 
To avoid confusion, it would be best to use the term 
"historic" or "natural" when the figure concerned is the 
unimpaired flow for all recorded years, and state the period 
during which these baseline observations were made. Residual 
flow should be considered as the net "regulated" rather than 
"historic" flow. 
Background 
In estuaries which have a mean inflow significantly higher 
than their total volume, the prevailing fluctuations of mean 
freshwater supply (5 year running means of natural annual or 
spring runoff under natural conditions) vary within 25 percent 
of normal 50-60 year averages. Hence, if diversion within a 
cycle, especially during periods of less than average flow, does 
not exceed the natural deviations from the average flow, the 
cumulative supply of the watershed may compensate for these 
36 
