Alafia River Minimum Flows and Levels - Southwest Florida Water ...

subsidy of water and nutrients that supports high rates of primary production in
river floodplains (Conner and Day 1976, Brinson et al. 1981). This primary
production produces large amounts of organic detritus, which is critical to food
webs on the floodplain and within the river channel (Vannote et al. 1980, Gregory
et al. 1991). Floodplain inundation also contributes to other physical-chemical
processes that can affect biological production, uptake and transformation of
macro-nutrients (Kuensler 1989, Walbridge and Lockaby 1994).
Soils in river floodplains exhibit physical and chemical properties that are
important to the overall function of the river ecosystem (Wharton et al. 1982,
Stanturf and Schenholtz 1998). Anaerobic soil conditions can persist in areas
where river flooding or soil saturation is of sufficient depth and duration. The
decomposition of organic matter is much slower in anaerobic environments, and
mucky or peaty organic soils can develop in saturated or inundated floodplain
zones (Tate 1980, Brown et al. 1990). Although these soils may dry out on a
seasonal basis, typically long hydroperiods contribute to their high organic
content. Plant species that grow on flooded, organic soils are tolerant of anoxic
conditions and the physical structure of these soils (Hook and Brown 1973,
McKevlin et al. 1998). Such adaptations can be an important selective
mechanism that determines plant community composition. Because changes in
river hydrology can potentially effect the distribution and characteristics of
floodplain soils, soil distributions and their relationship to river hydrology are
routinely investigated as part of MFL determinations for District rivers.
Compared to instream evaluations of MFL requirements, there has been
relatively little work done on river flows necessary for meeting the requirements
of floodplain species, communities or functions. Our work on the upper Peace
and Alafia Rivers suggests that direct and continuous inundation of floodplain
wetlands by river flows is in many cases not sufficient to meet the published
inundation needs of the dominant species found in the wetlands. There are
probably several reasons for this apparent inconsistency. Some floodplain
systems are likely to include seepage wetlands, dependent on hydrologic
processes other than direct inundation from the river. Other wetlands may occur
in depressional areas where water is retained after subsidence of river flows.
The District's approach to protection of flows associated with floodplain habitats,
communities and functions involves consideration of the frequency and duration
of direct connection between the river channel and the floodplain. As part of this
process, plant communities and soils are identified across the river floodplain at a
number of sites, and periods of inundation/connection with the river are
reconstructed on an annual or seasonal basis. These data are used to
characterize the frequency and duration of direct connection/ inundation of these
communities to or by the river and to develop criteria for minimum flow
development.
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