Professional Paper 1656A
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Light, Helen M., Darst, Melanie R., Lewis, Lori J., 2002, Hydrology,
Vegetation, and Soils of Riverine and Tidal Floodplain Forests of the Lower
Suwannee River, Florida, and Potential Impacts of Flow Reductions: Professional
Paper 1656A, 124 p.
ABSTRACT:
A study relating hydrologic conditions, soils, and vegetation of floodplain
forests to river flow was conducted in the lower Suwannee River, Florida, from
1996 to 2000. The study was done by the U.S. Geological Survey in cooperation
with the Suwannee River Water Management District to help determine the minimum
flows and levels required for wetlands protection. The study area included
forests within the 10-year floodplain of the Suwannee River from its confluence
with the Santa Fe River to the tree line (lower limit of forests) near the Gulf
of Mexico, and covered 18,600 hectares (ha) of forests, 75 percent of which
were wetlands and 25 percent uplands. The floodplain was divided into three
reaches, riverine, upper tidal, and lower tidal, based on changes in hydrology,
vegetation, and soils with proximity to the coast.
The Suwannee River is the second largest river in Florida in terms of average
discharge. Median flow at the confluence of the Suwannee and Santa Fe Rivers is
approximately 181 cubic meters per second (m3/s) or 6,480 cubic feet per second
(ft3/s) (1933-99). At the upper end of the riverine reach, river stages are
unaffected by tides and have a typical annual range of 4.1 meters (m). Tides
affect river stages at low and medium flows in the upper tidal reach, and at
all flows in the lower tidal reach. Median tidal range at the mouth of the
Suwannee River is about 1 m. Salinity of river water in the lower tidal reach
increases with decreasing flow and proximity to the Gulf of Mexico. Vertically
averaged salinity in the river near the tree line is typically about 5 parts
per thousand at medium flow.
Land-surface elevation and topographic relief in the floodplain decrease with
proximity to the coast. Elevations range from 4.1 to 7.3 m above sea level at
the most upstream riverine transect and from 0.3 to 1.3 m above sea level on
lower tidal transects. Surface soils in the riverine reach are predominantly
mineral and dry soon after floods recede except in swamps. Surface soils in
upper and lower tidal reaches are predominantly organic, saturated mucks. In
the downstream part of the lower tidal reach, conductivities of surface soils
are high enough (greater than 4 milli-mhos per centimeter) to exclude many tree
species that are intolerant of salinity.
Species richness of canopy and subcanopy plants in wetland forests in the lower
Suwannee River is high compared to other river floodplains in North America. A
total of 77 tree, shrub, and woody vine species were identified in the canopy
and subcanopy of floodplain wetland forests (n = 8,376). Fourteen specific
forest types were mapped using digitized aerial photographs, defined from
vegetative sampling, and described in terms of plant species composition. For
discussion purposes, some specific wetland types were combined, resulting in
three general wetland forest types for each reach.
Riverine high bottomland hardwoods have higher canopy species richness than all
other forest types (40-42 species), with Quercus virginiana the most important
canopy tree by basal area. The canopy composition of riverine low bottomland
hardwoods is dominated by five species with Quercus laurifolia the most
important by basal area. Riverine swamps occur in the lowest and wettest
areas with Taxodium distichum the most important canopy species by basal area.
Upper tidal bottomland hardwoods are differentiated from riverine forests by
the presence of Sabal palmetto in the canopy. Upper tidal mixed forests and
swamps are differentiated from riverine forests, in part, by the presence of
Fraxinus profunda in the canopy. Nyssa aquatica, the most important canopy
species by basal area in upper tidal swamps, is absent from most forests in the
lower tidal reach where its distribution is probably restricted by salinity.
Hydric hammocks, a wetland type that is rare outside of Florida, are found in
the lower tidal reach and are flooded every 1-2 years by either storm surge or
river floods. Lower tidal mixed forests and swamps have continuously saturated
muck soils and are differentiated from upper tidal forests, in part, by the
presence of Magnolia virginiana in the canopy. Lower tidal swamps have the
highest density of canopy trees (about 1,200 trees per hectare) of all
floodplain forest types, with Nyssa biflora the most important canopy species
by basal area.
Water use in the Suwannee River basin in Florida and Georgia is expected to
increase over time because of anticipated growth and development in the region
and adjacent areas. If increased water consumption reduced river flow, river
stage would decrease and salinity would increase, resulting in a variety of
impacts on forest composition, wetland biogeochemical processes, and fish and
wildlife habitat.
Forest composition in the floodplain is primarily determined by duration of
inundation and saturation, depth and frequency of floods, and salinity.
Long-term flow reductions would result in shallower flood depths, allowing
drier and more tidal species to invade wetland forests of the riverine and
upper tidal reaches. If flows were reduced 2.8-56 m3/s (100-2,000 ft3/s), an
estimated 52-1,140 ha, respectively, would change to a drier forest type, and
36-788 ha, respectively, would change to a more tidal forest type. The greatest
impacts would occur in swamps, where important swamp species such as Taxodium
distichum and Nyssa aquatica could have increased competition not only from
drier or more tidal species, but also from opportunistic bottomland hardwoods
or invasive exotic species. Reduced flows could also result in a conversion of
some wetland forests to uplands, increasing vulnerability to human disturbance,
and decreasing tree basal area, species richness, and diversity of wildlife
habitat.
Salt-intolerant species would move upstream if flow reductions increased
salinity in the lower tidal reach. If flows were reduced 2.8-56 m3/s (100-2,000
ft3/s), the area of forests along the tree line that would convert to marshes
is estimated to be 72-618 ha, respectively. Loss of forests at the tree line
would result in a loss of complex vertical structural diversity and woody
micro-habitats that are used by many animals. These changes are already
occurring due to sea level rise, but changes would occur more quickly if
salinities increased as a result of flow reductions.
The amount of inundated and saturated area in the floodplain forest of the
riverine reach would decrease if flows were reduced. The greatest impacts
would result from flow reductions that occurred at low flows, when inundated
and saturated areas in the floodplain are limited. Drier conditions would
result in oxidation of organic matter in swamp soils, which would reduce the
soil's water-holding capacity and ability to retain water during droughts.
Drier soils would increase vulnerability of the floodplain to fire and could
also reduce the ability of riverine forests to remove nitrates and other
pollutants from river water. Loss of inundated areas resulting from flow
reductions at low flow would eliminate aquatic habitats that are critical to
the survival of floodplain fishes and aquatic invertebrates, and are important
to many other animals that use the floodplain. If flow reductions occurred
during high flows, main channel fishes could decrease in diversity and
abundance because they are seasonally dependent on flooded forests for food,
shelter, and reproduction. In addition, aquatic organisms in the river and
estuary could be adversely affected because they depend on particulate organic
detritus and other floodplain exports as food sources.
TABLE OF CONTENTS:
Preface
Glossary
List of Scientific Names Used and Common Name Equivalents
Abstract
Introduction
Purpose and Scope
Acknowledgments
Setting
Methods of Study
Data Collection
Study Sites
Hydrologic Measurements
Soil Sampling
Vegetation Sampling
Data Analysis
Long-Term River Flow and Stage
Defining and Mapping Forest Types
Flow-Dependent Characteristics of Floodplain
Impacts of Flow Reductions
Hydrologic Characteristics of the River
Flow
Stage and Tidal Range
Storm Surge
Salinity
Topography and Hydrology of Forested Floodplain
Land-Surface Elevations, Hydrologic Conditions, and Forest Types at Transects
Salinity in Floodplain Water Bodies of Lower Tidal Reach
Soil Characteristics
Taxonomic Classification
Texture and Saturation
Conductivity in Lower Tidal Reach
Forest Composition and Distribution
Important Tree Species
Forest Type Composition
Oak/Pine Uplands
Riverine Wetlands
Upper Tidal Wetlands
Lower Tidal Wetlands
Characteristics of Forest Type Composition
Flow-Dependent characteristics Of Floodplain
Inundation and Saturation
Flood Depths
Salinity
Potential Impacts of Flow Reductions
Changes in Floodplain Forest Composition
Change to Drier Forest Types
Upstream Movement of Tidal Forests
Upstream Movement of Marshes and Salt-Tolerant Forests
Summary of Forest Composition Changes
Loss of Inundated and Saturated Area in Floodplain Forests
Ecological Consequences of Flow Reductions
Summary
References
APPENDIXES
I. Median monthly high, median daily high, and median daily low stages at transects and gages
in tidal reaches of the lower Suwannee River, Florida
II. Median land-surface elevations of forest types at transects in the floodplain of the
lower Suwannee River, Florida
III. Floodplain hydrology observations and measurements made at study sites in floodplain
forests of the lower Suwannee River, Florida
IV. Salinity of ponds and tidal creeks in the floodplain of the lower Suwannee River, Florida
V. Soil profile descriptions for floodplain forest types of the lower Suwannee River, Florida
VI. Percentage of inundated and saturated area for each forest type and transect in the
riverine reach in relation to flow in the lower Suwannee River, Florida
VII. Flow duration curves for five hypothetical flow reductions compared to the existing flow
in the lower Suwannee River, Florida
FIGURES:
1-2. Maps showing:
1. Drainage basin of the Suwannee River in Florida and Georgia
2. Study area with location of reaches, gaging stations, and study sites in the floodplain
of the lower Suwannee River, Florida
3. Flow chart showing basic study components and analytical approach for describing hydrology,
vegetation, and soils of the floodplain and estimating impacts of flow reductions in the
lower Suwannee River, Florida
4-7. Photographs showing:
4. Measurement of horizontal distances with meter tape extended between numbered wooden
stakes in UTswl forest on MS transect in the lower Suwannee River floodplain, Florida
5. Surveying with a tripod-mounted level in LTsw2 forest on TK transect in the lower
Suwannee River floodplain, Florida
6. Examination of soil auger contents in UTsw2 forest at KN transect in the lower Suwannee
River floodplain, Florida
7. Measuring and identifying canopy trees in UTsw1 forest on MS transect in the lower
Suwannee River floodplain, Florida
8-9. Graphs showing:
8. Daily mean stage at gages and riverine transects in relation to flow in the lower
Suwannee River, Florida
9. Daily high and low stage at gages and tidal transects in relation to flow in the lower
Suwannee River, Florida
10-12. Flow charts showing:
10. Methods for defining and mapping forest types in the lower Suwannee River floodplain, Florida
11. Methods for calculating area of each riverine forest type that is inundated or saturated
in relation to flow in the lower Suwannee River, Florida
12. Methods for calculating flood depths for each forest type at each transect in the lower
Suwannee River floodplain, Florida
13-16. Graphs showing:
13. Mean and median monthly flows of the lower Suwannee River, Florida, 1933-99
14. River stage during the 1997, 1998, and 1999 water years at four gaging stations on the
lower Suwannee River, Florida
15. Land-surface elevations and forest types at floodplain transects in relation to
long-term hydrologic conditions in the lower Suwannee River, Florida
16. Forest type elevations in relation to daily high river stage from 1985 to 1999 at
transect locations in the upper and lower tidal reaches of the Suwannee River, Florida
17-19. Photographs showing:
17. Tidal creek at low tide in LTsw2 forest on TK transect in the lower Suwannee River
floodplain, Florida
18. Shore of the Suwannee River, Florida, near DM transect at very low tide
19. A well-defined hummock that is large enough to support several trees on BC transect in
the lower Suwannee River floodplain, Florida
20. Graph showing salinity of surface-water samples collected at selected sites in lower tidal
forests in the floodplain of the lower Suwannee River, Florida
21. Photograph showing isolated pond at BC transect in the winter in the lower Suwannee River
floodplain, Florida
22-26. Graphs showing:
22. Texture of soils in floodplain forests of the lower Suwannee River, Florida
23. Soil conductivity ranges in lower tidal forests of the Suwannee River floodplain, Florida
24. Area of uplands and wetlands that are presently or were historically forested in the
10-year floodplain of the lower Suwannee River, Florida
25. Distribution of selected canopy tree species in relation to distance from the mouth of
the Suwannee River, Florida
26. Area of wetland forest types in the floodplain of the lower Suwannee River, Florida
27-38. Photographs showing:
27. Buttressed trunk of a Taxodium distichum tree growing on the banks of Rock Bluff Spring
Run in the riverine reach of the lower Suwannee River, Florida
28. Clumps of Forestiera acuminata trees in the riverine reach of the lower Suwannee River
floodplain, Florida
29. Carya aquatica with a flared base growing in Rblh1 forest at the LL transect in the
lower Suwannee River floodplain, Florida
30. Rblh2 forest in winter on the MS transect in the lower Suwannee River floodplain, Florida
31. Large Quercus virginiana in high bottomland hardwood forest in the riverine reach of
the lower Suwannee River floodplain, Florida
32. Swollen bases of Nyssa aquatica and Taxodium distichum in UTsw1 forest at the MS transect
in the lower Suwannee River floodplain, Florida
33. UTmix forest on the KI transect in the lower Suwannee River floodplain, Florida
34. A stunted stand of Fraxinus profunda trees growing along East Pass near the tree line
in the lower Suwannee River floodplain, Florida
35. LTmix forest at SN transect, which receives regular tidal inundation from a small
tributary of Sandfly Creek in the lower Suwannee River floodplain, Florida
36. Root mat on the bank of East Pass in the lower Suwannee River, Florida
37. Sabal palmetto and Pinus taeda dominate the canopy of hammocks in the lower tidal reach
of the Suwannee River floodplain, Florida
38. Sabal palmetto trees growing on slightly higher ground in a marsh on East Pass in the
lower Suwannee River, Florida
39-42. Graphs showing:
39. Species richness of canopy and subcanopy trees in floodplain forests of the lower
Suwannee River, Florida
40. Basal area of canopy trees in floodplain forests of the lower Suwannee River, Florida
41. Average size of canopy trees in floodplain forests of the lower Suwannee River, Florida
42. Density of canopy trees in floodplain forests of the lower Suwannee River, Florida
43. Photograph showing dense stand of trees in LTsw1 forest on DM transect in the lower Suwannee
River floodplain, Florida
44-49. Graphs showing:
44. Density of subcanopy trees in floodplain forests of the lower Suwannee River, Florida