The Kissimmee River,
located in Central Florida, flows from Lake Kissimmee to Lake Okeechobee.
Historically, the Kissimmee meandered 166 km, exhibiting continuous in-channel flow and
frequent
over-bank flow. The 1.5 to 3 km wide floodplain surrounding the river was typically inundated
for prolonged
periods throughout the year (Toth, 1993).
Between 1962 and 1971, the US Army Corps of
Engineers
constructed a regional flood control project, for which a straight canal (C-38) was dredged
through the river
and floodplain. Dredging of C-38 cut off sections of the old river channel and confined flow
within a 90-km
long, 100-m wide, and 9-m deep waterway. This canal was divided by water control structures
into 5
impoundments with stabilized water levels (Goodrick et al, 1974). Although channelization and
the hydrologic
manipulations provided flood protection for the region, the ecological integrity of the
river/floodplain
ecosystem was degraded, altering vegetative communities and associated fauna.
Prior to channelization, continuous flows confined aquatic vegetation to littoral edges of
the river
channel. Although there is little information available on specific vegetation characteristics of
the river prior
to 1962, some features can be derived from aerial photographs and historical flow regimes. The
morphology
of the river dictated variations in flow along and across the channel. Highest flow velocities
occurred along
outside bends of channel meanders, leading to a deeper channel in these areas. Plant species
growing along the
outer bends were well adapted to deep water with high flows and included Nuphar
lutea
(spatterdock), Sacciolepis striata (American cupscale), and Polygonum
densiflorum
(smartweed). Conversely, inner margins of channel meanders had lower velocities; these
depositional zones
exhibited prominent sand bar formations. Species colonizing inside bends had to tolerate
varying water levels,
and included Sagittaria lancifolia (arrowhead) and Pontederia cordata
(pickerelweed). Other common species distributed throughout the river channel included
Scirpus cubensis (Cuban bulrush), Panicum hemitomon (maidencane),
and
Hydrocotyle umbellata (pennywort). During periods of low flow, floating species,
such
as Pistia stratiotes (water lettuce) and Eichhornia crassipes (water
hyacinth)
likely accumulated along the edge of littoral beds (Toth et al, 1995). Additionally, floating mats
of vegetation
would often accumulate in cut-off oxbows and backwater areas that received little or no flow.
Species common to these mats included Scirpus cubensis, Lemna spp. (duckweed),
Eichhornia crassipes, Pistia stratiotes, and Luziola fluitans (watergrass)
(Pierce et al, 1982).
After channelization, aquatic vegetation communities changed significantly due to stagnant water
conditions
within remnant river channels. One of the most striking effects was increased coverage of the
exotic species
Pistia stratiotes and Eichhornia crassipes. Although these species were
present
prior to channelization, continuous flows likely restricted their growth, particularly during
moderate to high
flow regimes. After flow to remnant channels was eliminated, these species proliferated and now
often cover
the entire width of remnant river channels. Eichhornia crassipes and Pistia
stratiotes
are treated with herbicides several times a year to maintain navigation through these
channels.
Lack of flow to remnant river channels has allowed the distribution and coverage of
Scirpus cubensis to increase, and it is now one of the most abundant species within
remnant
channels. Scirpus cubensis forms mats that may host a variety of other forbs and
small shrubs,
including aquatic species as well as species typical of drier conditions. Secondary colonization
by
Ludwigia spp. (primrose willow), Eupatorium capillifolium
(dog-fennel),
Pontederia cordata, Sagittaria lancifolia, Typha domingensis (cat-tail), and many
other
species is common on the floating mats.
In addition to Scirpus cubensis, other common aquatic species include
Salvinia minima (water spangles), Lemna spp., Hydrocotyle
umbellata,
Polygonum densiflorum, and Nuphar lutea. Of these, the floating and
mat-forming
species are the most abundant within remnant channels. Because there is no flow, species
distribution is
independent of channel morphology, often covering the entire width of the river channel. This is
particularly
common during summer months, at the peak of the growing season. The overabundance of
aquatic vegetation
in remnant river channels, particularly Scirpus cubensis, not only prevents light from
reaching
the water column below, but also contributes large deposits of organic matter to the remnant
channels. In
some areas, the organic layer may reach one meter thick, covering the sandy substrate and
increasing the
biological oxygen demand in the surrounding waters (Koebel, 1995).
Over the past two decades, demonstration projects have been conducted to determine the
feasibility of restoring the Kissimmee River to pre-channelization conditions. These projects
provide some
insight into expected responses of the vegetation community to restoration. During 1984 and
1985, three
weirs were placed across the C-38 canal to force water through adjacent remnant channels. The
reestablishment
of flow in the river channel decreased the width of vegetation mats in mid-channel areas;
however, when flow
subsided Eichhornia crassipes and Pistia stratiotes often rapidly
expanded
(Miller, 1990).
In 1994, a 300-m section of C-38 was backfilled, diverting the majority of flow through
the channel
adjacent to one of the demonstration project weirs. Based on aerial photographs of this channel
six months
after flow was restored, several changes in the macrophyte community were documented. Mats
of
Scirpus cubensis were flushed out by high flows and replaced by narrower littoral
beds
composed primarily of Nuphar lutea and Polygonum densiflorum.
While the
overall width of macrophyte beds decreased, this effect was more prominent on outer bends,
which had
narrower littoral zones compared to the inner bends of channel meanders. Inner bends had a
higher
occurrence of Pontederia cordata and Sagittaria lancifolia, while outer
bends
supported more Panicum hemitomon. Although these projects likely do not show
the full effects
of restoration on the macrophyte community within river channels, they indicate the potential for
restoring
these communities to historic conditions.
Restoration of the Kissimmee River began in June 1999. During the 11-year construction
period
the C-38 canal will be backfilled to reconnect remnant river channels and restore continuous
flow.
Overall, 35 km of C-38 will be backfilled, restoring 70 km of continuous river channel. Two
water control
structures will be removed and water levels will be managed to reestablish historic hydrologic
conditions.
Several expectations for changes due to restoration can be formulated based on the effects
of the weirs
and backfill projects, along with knowledge of the historic Kissimmee River. Once flow is
reestablished,
vegetation coverage is expected to decrease and be limited to the littoral fringes of the river
channel.
Along inside bends of channel meanders, vegetation beds likely will remain within 5 meters of
the bank; along
outside bends of channel meanders, where flows will be higher, vegetation beds will remain
within 3 meters
of the bank.
In addition to changes in macrophyte bed width, dominant plant species also are expected
to change.
Floating and mat-forming species that currently dominate will be replaced by emergent species,
particularly
those that are adapted to flowing conditions and varying water levels. These beds will likely
include
Nuphar lutea, Polygonum densiflorum, Sacciolepis striata, and Panicum
hemitomon.
The distribution of these species will depend on channel morphology. Emergent species
tolerant of varying
hydroperiods will dominate along inner bends, where sand bar formation is likely, while
deep-water emergent
species will be more common along outer margins of channel meanders.
Changes in the macrophyte communities of the Kissimmee River are expected to occur within
one to three
years following backfilling of the C-38 canal (Dahm et al, 1995). Currently, flow has been
reestablished
in two channels that were stagnant for nearly 30 years. As construction continues, the remnant
channels
will be reconnected into a single continuous river channel. Although it is not possible to restore
the entire
length of the Kissimmee River, this restoration effort will reconnect 70 km of river channel and
reestablish
over 14,000 ha of floodplain wetlands. The benefits of this project extend beyond the vegetation
communities
by providing habitat for birds, fish, and other wildlife.
SUMMARY OF MACROPHYTE COMMUNITY CHANGES WITHIN THE
KISSIMMEE RIVER
Historic Conditions
1. Vegetation within river channels was confined to narrow littoral zones.
2. Littoral zones were composed of emersed, submersed, and floating species. Dominant
plants
included Nuphar lutea, Polygonum densiflorum, Sacciolepis striata, and
Panicum hemitomon.
3. The deepest areas occurred along outer bends where flows were highest; these areas
hosted
species better suited to such conditions (e.g. Nuphar lutea and Polygonum
densiflorum).
Inner bends were shallower with characteristic sandbars.
Current Conditions
1. Vegetation extends well into mid-channel areas and often covers the entire channel
width.
There is no variation in vegetation width due to channel morphology.
2. Floating and sprawling species dominate. Although 55 species have been identified in
the
channels, only 7 of these account for over half of the total coverage. The most abundant species
are
Scirpus cubensis, Salvinia minima, Lemna spp., Pistia stratiotes, and
Hydrocotyle umbellata.
3. Distribution of vegetation within the channels is independent of channel
morphology.
After Backfilling & Restoration of Hydrology:
Initial High Flows/Short-term Expectations
1. High flows will flush thick mats of Scirpus cubensis
out of the channel, clearing mid-channel areas.
2. Floating species, such as Pistia stratiotes, also will be removed.
Continuous Flows Over Time/Long-term Expectations
1. Vegetation will be confined to a narrow littoral zone. Along inner bends of channel
meanders,
vegetation will be within 5 meters of the bank. Along outer bends of channel meanders,
vegetation will be
within 3 meters of the bank. Vegetation along straight runs will be within 3 to 5 meters of the
bank.
2. Emersed species, such as Nuphar lutea, Polygonum densiflorum, and
Sacciolepis striata will become dominant. Floating and sprawling species will
remain, but will
diminish and be confined to backwater areas.
3. The composition of macrophyte beds will vary depending on morphology of the
channel.
Outer bends will support deep-water emersed species, such as Nuphar lutea, Polygonum
densiflorum,
and Sacciolepis striata. Inner bends will have marsh vegetation, such as
Pontederia cordata and Sagittaria lancifolia.
References:
Dahm, C.N., K.W. Cummins, H.M. Valett, and R.L. Coleman. 1995. An
ecosystem view of the restoration of the Kissimmee River. Restoration Ecology 3(3): 225 -
238.
Goodrick, R.L, and J.F. Milleson. 1974. Studies of floodplain vegetation and
water level fluctuation in the Kissimmee River Valley. South Florida Water Management
District Technical Publication 74-2.
Koebel, J.W. 1995. An historical perspective on the Kissimmee River
restoration project. Restoration Ecology 3(3): 149 - 159.
Miller, S.J., J. Wood, and L. Perrin. 1990. Vegetation community responses
to restoration. South Florida Water Management District. Kissimmee River
Restoration Symposium. 97 - 110.
Milleson, J.F., R.L. Goodrick, and J.A. VanArman. 1980. Plant communities
of the Kissimmee River Valley. South Florida Water Management District Technical
Publication 80-7.
Pierce, G.J., A.B. Amerson, and L.R. Becker. 1982. Final report: Pre-1960
floodplain vegetation of the Lower Kissimmee River Valley, Florida. United
States Army Corps of Engineers, Biological Services Report 82-3.
Toth, L.A. 1993. The ecological basis of the Kissimmee River restoration
plan. Biological Sciences 56(1): 25 - 51.
Toth, L.A., D.A. Arrington, M.A. Brady, and D.A. Muszick. 1995.
Conceptual evaluation of factors potentially affecting restoration of habitat structure within
the channelized Kissimmee River ecosystem. Restoration Ecology 3(3): 160 - 180.
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