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

3.3.2 Wetted Perimeter Inflection Point
A useful technique for evaluating the relation between the quantity of stream
habitat and the rate of streamflow involves an evaluation of the "wetted
perimeter" of the stream bottom. Wetted perimeter is defined as the distance
along the stream bed and banks at a cross section where there is contact with
water. According to Annear and Conder (1984), wetted perimeter methods for
evaluating streamflow requirements assume that a direct relationship between
wetted perimeter and fish habitat exists in streams. By plotting the response of
wetted perimeter to incremental changes in discharge, an inflection can be
identified in the resulting curve where small decreases in flow result in
increasingly greater decreases in wetted perimeter. This point on the curve
represents a flow at which the water surface recedes from stream banks and fish
habitat is lost at an accelerated rate. Stalnaker et al. (1995) describe the wetted
perimeter approach as a technique for using "the break" or inflection point in the
stream's wetted perimeter versus discharge relation as a surrogate for minimally
acceptable habitat. They note that when this approach is applied to riffle (shoal,
Figure 3-1) areas, "the assumption is that minimum flow satisfies the needs for
food production, fish passage and spawning."
We view the wetted perimeter approach as an important technique for evaluating
minimum flows and levels near the low end of the flow regime. Studies on
streams in the southeast have demonstrated that the greatest amount of
macroinvertebrate biomass per unit reach of stream occurs on the stream bottom
(e.g., Benke et al. 1985). Although production on a unit area basis may be
greater on snag and root habitat, the greater area of stream bottom along a
reach makes it the most productive habitat under low flow conditions. The
wetted perimeter inflection point in the channel provides for large increases in
bottom habitat for relatively small increases of flow. This point is defined as the
"lowest wetted perimeter inflection point" or LWPIP. It is not assumed that flows
associated with the LWPIP meet fish passage needs or address other wetted
perimeter inflection points outside the river channel. However, identification of
the LWPIP permits evaluation of flows that provide the greatest amount of
inundated bottom habitat in the river channel on a per-unit flow basis.
3.3.3 In-Channel Habitats for Fish and Macroinvertebrates
Maintenance of flows greater than those allowing for fish passage and
maximization of wetted perimeter are needed to provide aquatic biota with
sufficient resources for persistence within a river segment. Feeding, reproductive
and cover requirements of riverine species have evolved in response to natural
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