For development of MFLs, it is necessary to identify a benchmark period from which flow

deviations may be evaluated. Often changing flow trends are presumed to be the result of

anthropogenic stresses on a system. This is possibly because variation in flow is frequently

assumed to be the result of random independently and identically distributed random variables

such as rainfall in flood risk analysis (Olsen et al. 1999). However, the effect of multidecadal

oscillation on river flow patterns must also be recognized as natural climatic variations. As

Enfield et al. (2001) observe, the Atlantic Multi-decadal Oscillation has a pronounced effect on

rainfall in the continental United States and wet-season rainfall in peninsular Florida is

negatively correlated with the Atlantic Multi-decadal Oscillation (McCabe and Wolock 2002).

Kelly (2004) examined stream flows in Florida and identified a step-trend similar in timing to the

step-trend describing the AMO (Enfield et al. 2001). This resulted in the identification of two

benchmark periods, one from 1940-1969 and one from 1970-1999, which are characterized by

relatively higher and lower wet season river flows in peninsular Florida, respectively. For

determination of minimum flows, records from both benchmark periods are analyzed, and used

to develop percent-of-flow reduction criteria. Minimum flows are determined by calculating the

percent flow reduction for each benchmark period that would result in no more than a 15%

reduction in habitat. For the Block 3 period, this means identifying the flow reduction that

would result in no more than a 15% loss in the number of days of days that floodplain features

are inundated by the river, during each of the two benchmark periods and utilizing the more

limiting of the two.

 In establishing minimum flows, the District defines a 15% change in habitat availability as

a criterion for identifying significant harm, or unacceptable change. For the high-flow period or

Block 3, this change is expressed as a temporal difference in the number of days specific water