Wetlands Research Program Technical Report WRP-DE-4

A Hydrogeomorphic Classification for Wetlands

by Mark M. Brinson

August 1993 – Final Report

Approved For Public Release; Distribution Is Unlimited

US Army Corps 

of Engineers

Waterways Experiment 

Station
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The following two letters used as part of the number designating technical reports of research published

under the Wetlands Research Program identify the area under which the report was prepared:

Task Task

CP Critical Processes Restoration & Establishment

DE Delineation & Evaluation !; Stewardship & Management

The contents of this report are not to be used for advertising,

publication, or promotional purposes. Citation of trade names

does not constitute an official endorsement or approval of the use

of such commercial products.

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%# PRINTED ON RECYCLED PAPER
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Wetlands Research Program Technical Report WRP-DE-4

August 1993

A Hydrogeomorphic Classification

for Wetlands

by Mark M. Brinson

Biology Department

East Carolina University

Greenville, NC 27858

Final report

Approved for public release; distribution is unlimited

Prepared for U.S. Army Corps of Engineers

Washington, DC 20314-1000
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Watemvays Experiment Station Cataloging-in-Publication Data

Brinson, Mark M.

A hydrogeomorphic classification for wetland / by Mark M. Brinson ;

prepared for U.S. Army Corps of Engineers.

101 p. : ill. ; 28 cm. — (Technical report ; WRP-DE-4)

Includes bibliographical references.

1. Wetlands — Classification. 2. Geomorphology. 3. Hydrology.

4. Wetland ecology. 1.United States. Army. Corps of Engineers. Il. U.S.

Army Engineer Waterways Experiment Station. Ill. Wetlands Research

Program (U.S.) IV. Title. V. Series: Wetlands Research Program technical report ; WRP-DE-4. VI. Series: Technical report (U.S, Army Engineer Waterways Experiment Station) ; WRP-DE-4.

TA7 W34 no.WRP-DE-4
Corps of Engineers Research Report Summary, August 1993

Wetland Functions

A Hydrogeomorphic Classification for Wetlands (TR WRP-DE-4)

ISSUE:

Under the Corps 404 Regulatory Program, the

permit review process requires assessing the

effect of a project on wetland functions. Many

of the currently available methods fail to address

critical technical and programmatic requirements.

RESEARCH:

The hydrogeomorphic classification of wetlands is intended to lay a foundation for and

support ongoing efforts to develop methods for

assessing the physical, chemical, and biological

functions of wetlands. Strengths of the classification include its ability to clarify the relationship between hydrology and geomorphology

and wetland function, and its open structure,

which allows adaptation in various types of

wetlands and geographic regions of the country.

SUMMARY:

This report outlines a classification of wetlands

based on the wetland hydrogeomorphic properties of geomorphic setting, water source, and

hydrodynamics. Indicators of function are discussed as derivatives of the three basic properties, along with the ecological significance of

each of the properties. Development of “profiles” that reveal the functions that wetlands are

likely to perform is discussed.

AVAILABILITY OF REPORT:

The report is available on Interlibrary Loan Service from the U.S. Army Engineer Waterways

Experiment Station (WES) Library, telephone

(601) 634-2355.

To purchase a copy call the National Technical

Information Service (NTIS) at (703) 487-4650.

For help in identifying a title for sale call (703)

487-4780.

NTIS report numbers may also be requested

from the WES librarians.

Please reproduce this page locally, as needed.
—

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . .

l—Introduction . . . . . . . . . . . . . . . . . . . . . . .

Purpose . . . . . . . . . . . . . . . . . . . . . . . . .

Meaning of Ecosystem Function . . . . . . . . . . .

Classifications with Hydrogeomorphic Approaches

Ontogeny of the Present Classification . . . .

2—Description . . . . . . . . . . . . . . . . . . . .

Geomorphic Setting . . . . . . . . . . . . . . .

Water Sources . . . . . . . . . . . . . . . . . .

Hydrodynamics . . . . . . . . . . . . . . . . . .

3—Indicators of Functioning . . . . . . . . . . . .

Water Characteristics as Indicators . . . . . .

Other Indicators of Functioning . . . . . . . .

4—Profile Development and Reference Wetlands

Profile Development . . . . . . . . . . . . . . .

Reference Wetlands . . . . . . . . . . . . . . .

5—Toward Functional Assessments . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . .

Appendix A: Glossary . . . . . . . . . . . . . . . .

SF 298

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1

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68

Al
List of Figures

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13.

Figure 14.

Diagram that conceptualizes the relationship

between wetland functions and wetland values . . . . . .

Four major hydrologic types of wetland types

in Wisconsin . . . . . . . . . . . . . . . . . . . . . . . . . .

Hydrogeologic classification for East Anglian fens . . . .

Factors controlling expression of plant community

physiognomy and species composition of sea-level

controlled wetlands in North Carolina . . . . . . . . . . .

Classifications of wetlands based on their

functional properties . . . . . . . . . . . . . . . . . . . . .

Interaction of breaks in slope with groundwater

flow as a source of water for wetlands . . . . . . . . . . .

Major types of wetlands found in Europe . . . . . . . . .

Six major types of fringe wetlands and their

position relative to changing sea level and vertical

movement of coastal marshes . . . . . . . . . . . . . . . .

Principal sources of water . . . . . . . . . . . . . . . . . .

Hydrography of wetlands and streams

associated with wetlands . . . . . . . . . . . . . . . . . . .

Several approaches for quantifying hydroperiod

in wetlands that reveal the dynamics of flooding . . . . .

Diagram that allows expression of the

relative contribution of three water sources—

precipitation, groundwater discharge, and lateral

surface flow-to a wetland . . . . . . . . . . . . . . . . . .

Categories of hydrodynamics based on dominant

flow pattern . . . . . . . . . . . . . . . . . . . . . . . . . .

The relationship between seepage conditions (groundwater flow) and water salinity in prairie potholes . . . . .

4

8

10

11

14

26

28

31

33

34

36

37

44

52

iv
Preface

The work described in this report was authorized by Headquarters, U.S.

Army Corps of Engineers (HQUSACE), as part of the Delineation and

Evaluation Task Area of the Wetlands Research Program (WRP). The work

was performed under Work Unit 32756, for which Mr. R. Daniel Smith

was Principal Investigator. Mr. John Bellinger (CECW-PO) was the WRP

Technical Monitor for this work.

Mr. Jesse A. Pfeiffer, Jr. (CERD-C), was the WRP Coordinator at the

Directorate of Research and Development, HQUSACE; Mr. William L.

Klesch (CECW-PO) served as the WRP Technical Monitor’s Representative; Dr. Russell F. Theriot was the Wetlands Program Manager. Mr. Ellis

J. Clairain, Jr., was the Task Area Manager.

This work was performed by Dr. Mark Brinson at East Carolina University, North Carolina. Mr. R. Daniel Smith, Wetlands Branch, Environmental

Laboratory (EL), was the Project Manager under the general supervision of

Mr. E. Carl Brown, Chief, Wetlands Branch, EL; Dr. Conrad Kirby, Chief,

Ecological Research Division, EL; Dr. John Keeley, Assistant Director,

EL; and Dr. John Harrison, Director, EL.

The hydrogeomorphic classification of wetlands described in this report was intended to lay a foundation for ongoing efforts to develop methods for assessing the physical, chemical, and biological functions of

wetlands. Strengths of the classification include clarification of the relationship between hydrology, geomorphology and wetland function, as well

as the open structure, which allows adaptation in various types of wetlands

and geographic regions of the country. The classification is not intended to

replace or displace other wetland classifications such as the U.S. Fish and

Wildlife Service’s Classification of Wetlands and Deepwater Habitats,

which are well suited for the purposes for that they were designed.

Much of the credit for the hydrogeomorphic classification can be attributed to the many pioneers who have demonstrated the relationship between

ecosystem structure and function. Dr. J. Henry Sather saw the critical need

to further develop functional assessments, and was instrumental in initiating

the development of a hydrogeomorphic procedure. The manuscript benefited

from the detailed written comments of Mr. Garrett Hollands and Drs. Frank

Golet, Katherine Ewel, Daniel Hubbard, and more recently, Dr. Robert
Beschta and Ms. Sydney Bacchas. Earlier work with Drs. Ariel E. Lugo

and Sandra Brown helped to solidify the author’s perspective on

hydrogeomorphic classification. Credit goes to Dr. Robert Christian for

providing alternative viewpoints and for suggesting the paper by Slobodkin et al. (1980) for consideration.

Participants of the Stone Mountain Workshop, who discussed the classification and the emerging assessment procedure, were as follows: Dr. Alan

Amman, Dr. Candy Bartoldus, Dr. Virginia Carter, Mr. Ellis J.Clairain, Jr.,

Dr. David Cooper, Dr. Lewis Cowardin, Mr. Charles DesJardins,

Dr. Katherine C. Ewel, Mr. Lloyd Fanter, Dr. Frank Golet, Dr. Jan Hoover,

Dr. Courtney Hackney, Mr. Garrett Hollands, Dr. Dan Hubbard, Dr. Roy

Johnson, Mr. Jack Killgore, Ms. Barbara Kleiss, Ms. Kathy Kunz, Dr. Joseph

Larson, Dr. Lyndon Lee, Dr. Edward Maltby, Mr. Thomas A. Muir,

Mr, Richard Novitzki, Dr. Jean O’Neil, Mr. Bruce Pruitt, Dr. J. Henry Sather,

Mr. Rick Schroeder, Dr. Paul Shuldiner, Mr. Bill Sipple, Mr. R. Daniel

Smith, Dr. Arnold van der Valk, and Dr. William Wilen. Norman Van

Home prepared some of the figures, and Eileen Nordlie edited several

drafts of this report at East Carolina University.

At the time of publication of this report, the Director of WES was

Dr. Robert W. Whalin. The Commander was COL Bruce K. Howard, EN.

This report should be cited as follows:

Brinson, M. M. (1993). “A hydrogeomorphic classification

for wetlands,” Technical Report WRP-DE-4, U.S. Army

Engineer Waterways Experiment Station, Vicksburg, MS.

vi
1 Introduction

Purpose

This manuscript introduces a classification approach for wetlands 1 that

places emphasis on the hydrologic and geomorphic controls. These controls

are apparently responsible for maintaining many of the functional aspects

of wetland ecosystems. The approach places emphasis on the importance

of abiotic features of wetlands for such functions as the chemical characteristics of water, habitat maintenance, and water storage and transport.

An attempt has been made to keep it robust enough to accommodate all

wetland types. It is hoped that it also is flexible enough to accommodate

the continua that exist among wetland types, between wetlands and uplands, and between wetlands and deepwater ecosystems. An effort has

been made to keep the classification simple enough so that the user can

learn it quickly, and, in the process, progressively can gain insight into

the functioning of wetland ecosystems through practice. The approach is

completely open to revision and correction as additional information becomes available.

The focus on abiotic features of wetlands is not meant to ignore or

trivialize the importance that organisms play in the structure and function

of wetland ecosystems. In contrast, it is hoped that by using the approach,

it will lead to a better understanding of the relationship between organisms

and the environment. Other classifications, for very good reason, have

placed great emphasis on the structure and species composition of the

plant community. For example, the Fish and Wildlife Service’s Wetland

Classification system (Cowardin et al. 1979) relies largely on vegetative

cover because the type of plant cover (or the lack of it) is the kind of information that can be reliably interpreted from aerial photographs. This

allowed the classification to meet one of its major goals of providing the

basis for tracking changes in the surface area of wetlands over time through

the National Wetland Inventory. However, the present goal is to place emphasis on features of wetlands that are relatively independent of the biogeographic distribution of species. Species composition of plant communities,

1 A glossary of terms is presentedin Appendix A.

Chapter 1 Introduction

‘1
in theory, should be irrelevant to this classification because it relies almost

exclusively on geomorphic, physical, and chemical descriptors. In practice,

however, vegetation often provides important clues of hydrogeomorphic

forces at work in an ecosystem. Also, vegetation structure, especially the

distinction between forested wetlands and marshes, may play a fundamental

role in the capacity of the wetland to serve as habitat for birds, mammals,

and other groups. Hence, familiarity with the adaptations and tolerance

limits of plant and animal species is a necessary skill for successful classification within a given biogeographic region.

Ideally, this classification should interface logically with existing regional

classifications that place emphasis on hydrogeomorphic descriptors. If

such regional classifications do not exist, it is hoped that this approach

can provide a convenient template upon which to build a locally or regionally useful system. Once those tools are developed, the biotic components

should be drawn into the classification process.

The classification is limited to aggregating wetlands with similar functions. It is not intended to be a “valuation” procedure that ranks one wetland relative to another for specific functions. While there may be some

merit to using this classification as a starting point for ranking functions,

assessment procedures are beyond the scope of this report. Chapter 5 discusses how the transition can be made from classification to assessment

procedures. Neither does the classification rank wetlands according to

their capacity to provide a service of value to society. This does not mean

that functions of wetlands are “value free,” but that ecosystem function is

based only on factors essential to the maintenance of the wetland itself

and its associated ecosystems. Factors that contribute to the well-being of

society are omitted in part because such values are prone to vary over

time and geographic region (Lugo and Brinson 1979).

As presented here, the classification lacks the resolution to distinguish

among the many types of wetlands that commonly are recognized within a

geographic region (i.e., bald cypress versus water tupelo swamps; red maple

versus buttonbush; willows versus balsam poplar). This is intentional for

two reasons: (a) hydrogeomorphic classifications by their very nature are

not designed to be sensitive to species composition of vegetation, and

(b) this report describes a generic approach to classification and not a

specific one to be used in practice. Rather, the approach is described so

that an array of existing wetlands in a geographic region can be assigned

hydrogeomorphic classes that will reveal better their ecosystem functions.

Unlike classifications that depend solely on information that can be collected within the wetland, this classification requires that factors external

to the wetland be recognized. One cannot classify, for example, a hectare

of seasonally flooded bottomland without at least implicitly recognizing

that it is part of a larger floodplain and watershed complex. The attributes

of a particular hectare of wetland, such as the source of the water for a

site, are intrinsic properties of that hectare even though they are derived

from a much larger geographic area.

Chapter 1 Introduction
—

—

One of the distinguishing properties of the classification is the emphasis

on what might be considered “first principles” of wetland function. While it

has become trite to say that hydrology is the most important variable that

distinguishes wetlands from other ecosystems and wetlands from each

other, there has been insufficient quantitative work to reveal why and how

hydrology influences wetland type. One of the tools that can be developed during the process of classification is a “functional profile” that is

derived from the ecological significance of the functions determined during the classification process. This concept will be described more fully

in the section “Profile Development.”

Meaning of Ecosystem Function

In the literature on the assessment of wetlands, it is common to refer to

“functions and values.” The implication is that wetlands are functioning

in a way that society perceives as valuable, so there is no effort to make a

distinction between the two terms. In addition, “values” has been used to

indicate that certain functions are “valuable” to wildlife, whereas the terms

“essential” or “beneficial” are more neutral and appropriate terms. Taylor,

Cardamore, and Mitsch (1990) draw a distinction between functions and

values by pointing out that values are the goods and services that emanate

from functions. The present classification stops short of discussing values

because the intent is to classify wetlands according to their hydrogeomorphic properties, not their potential value to society. By limiting the

analysis to science, issues can be avoided that deal with which value is

more important than others. As it turns out, however, most of the functions

that are attributable to wetlands also have a corresponding societal value.

It is possible that the bias invoked in placing emphasis on the better understood functions may cloud the capacity to see and understand more fundamental functions that have yet to be articulated.

An example of the difference between functions and values is the removal of nitrate from surface and groundwater by wetlands. It is common

that nonpoint sources of nitrate are intercepted from agricultural and urban

landscapes by wetlands (Kuenzler 1989). The societal service is improved

water quality because of lower nitrate concentrations. Clean water is perceived to have societal value as recognized in legislation such as the

Clean Water Act. The ecosystem function is the removal of nitrogen by

denitrification. Denitrification (as well as other attendant microbial and

nonmicrobial processes) is the critical mechanism that allows this to

occur.

Another way to distinguish among mechanisms, functions, values, and

related properties is to recognize that functions exist in the absence of

society and are normally part of the self-sustaining properties of an ecosystem. The relationship among these properties is illustrated in Figure 1.

Chapter 1 Introduction

3
—

—

~ : ~

GEOMORPHIC SEITING

&

f?’ 5 & & “\e@+= &

*o *& CRITICAL PROCESSES

5~c

& MECHANISMS >

4J~*

v

STRESSORS

EXTRACTABLE

RESOURCES

CONSTRUCTIVE

HUMAN LIFE DESTRUCTIVE

Figure 1. Diagram that conceptualizes the relationship between wetland functions and

wetland values. The dashed line se~arates the geomorphic settin9, which contains wetland functions, from societal interaction-with wetlands. Items above the

dashed line can continue in the absence of society; those below show the “uses”

of wetlands by society. Critical processes and mechanisms (e.g., photosynthesis,

microbial activity, and decomposition) and ecosystems functions (e.g., primary

productivity, biomass accumulation, and nutrient cycling and retention) may become resources for human life support. The term extractable resources is meant

to include intangibles, commodities, and all other goods and services that contribute to the human life support system. Note that human life support relies both on

wetlands in their geomorphic settings and fossil fuels. Feedbacks initiated by societal values can be either constructive or destructive. While values are merely

perceptions, they establish how the life supporl system interacts with the wetland

resource. Adapted and modified from Twilley (personal communication, 1990,

University of Southwestern Louisiana, Lafayette, LA), Taylor, Cardamore, and

Mitsch (1990), E. Maltby (personal communication, 1990, University of Exeter,

Exeter, U.K.), and Whigham and Brinson (1990)

4

Chapter1 Introduction
—

—

However, others argue that human uses should be incorporated into classifications. 1 While this has much merit given the dependence of some

societies on wetland functions, especially those in the tropics (Welcomme

1979), the approach would be better reserved for classifications designed

to distinguish different types of human uses rather than hydrogeomorphic

classes, the goal sought by the present document. Regardless of the functions and whether they are perceived as having utility during a given decade or by a particular culture, an anthropocentric goal of management for

ecosystems worldwide should be to maintain ecological processes, preserve the genetic diversity, and utilize species, populations, and ecosystems in a sustained way (Lubchenco et al. 1991).

Classifications with Hydrogeomorphic

Approaches

A number of wetland classification systems use hydrology and geomorphology as the basis for differentiating types of wetlands. Some of these

will be reviewed briefly below. One classification of lakes and one of rivers are included because they possess many of the features that are useful

to functional classifications of wetlands. The examples given below are

not exhaustive, but they provide an overview of past efforts to deal with a

large variety of wetland types. Mader (1991) has reviewed the literature

on classifications for forested wetlands.

Wetlands

Most classifications of wetlands are designed for use with a restricted

range of types or restricted geographic coverage. The approach of Gosselink

and Turner (1978), however, can be characterized as geographically neutral. They argue that the hydrologic characteristics of wetlands influence

four ecosystem attributes: species composition of the plant community,

primary productivity, organic deposition and flux, and nutrient cycling.

The major “hydrodynamic characteristics” that they propose are water inputs, water outputs, type of water flow, and hydropulses (i.e., seasonality)

(Table 1).

The “hydrogeologic” evaluation of O’Brien and Motts (1980) was designed for wetlands of New England and the glaciated northeastern United

States. The geologic factors are the composition and thickness of surficial

material and the composition of bedrock. Hydrologic factors are hydrologic position (perched, water table, or artesian), permeability of organic

layer, depth of surface water, transmissivity of underlying aquifers, groundwater outflow, and water quality. Topographic factors are position within

1 Personal Communication, 1990, E. Maltby, University of Exeter, U.K.

Chapter 1 Introduction
.

Table 1

Major Hydrodynamic Characteristics of Freshwater Marshes’

Raiaed- SurkanConvex Maedow Convex Lotlc Tidal Lentlc

Water Inputs

Capillary + +

Precipitation + + + + + +

Upstream Little + + + +

Downstream

I

+

Type of Water Flow

Capillary +

Subsurface + + + + + +

Surface I Islow + + I+

Overbank I I I 1+ 1+ 1+

Water Outputs

Percolation + +

Evepotranspiration + + + + + +

Downstream Little + + +

Hydropulees Seasonal Seasonal Seasonal Seasonal Tidal Variable

‘ From Gosselink and Turner (1978).

a drainage, absolute size, and size relative to the drainage basin. Various

combinations of these factors can be synthesized into major geologic

types common to the New England area. The authors present two such

geologic types that combine hydrologic and topographic positions to yield

a relatively small number of classes. They suggest that further efforts

may ultimately allow mapping of hydrologic types.

Another classification, applicable also to glaciated regions, is outlined

by Hollands (1987) who stated that “In reality, the wetland [vegetation]

is only a green fuzz that grows on top of and as a result of this hydrogeologic setting.” He identifies (a) six dominant or combined hydrologic

types: open water, vegetated without cranberries, active cranberry bog, inactive cranberry bog, perennial stream, ephemeral stream; (b) four surface

inflow-outflow situations: inflowing stream only, outflowing stream only,

inflowing and outflowing streams, no streams; and (c) three groundwater

characteristics: discharge dominated, recharge dominated, and both recharge

and discharge. Combinations of these types and situations are described

as located within a surflcial geologic setting. Hollands also provides a

6

Chapter 1 Introduction
—

—

methodology that can be used to acquire the information needed to apply

the classification to other geographic regions.

Novitzki (1979) described the hydrologic characteristics of Wisconsin’s

wetlands with regard to water source and landform. He recognized four:

surface water depression, groundwater depression, surface water slope,

and groundwater slope (Figure 2). Surface water depressions receive precipitation and overland flow. Losses are through evapotranspiration (ET)

and downward seepage into a surficial aquifer. Groundwater depression

wetlands, in contrast, intercept the water table, so they receive groundwater in addition to direct precipitation and overland flow. Groundwater

slope wetlands differ from the groundwater depressions by having an outlet and also tending to occur on slopes where groundwater has stronger

flow than would normally be encountered in depressions. The size of

these wetlands corresponds to the quantity of groundwater discharge. Surface water slope wetlands receive water from lake or river flooding, and

the water can readily drain back into lake or river as the stages fall. They

may be flooded infrequently, as in the cases of floodplains, or permanently,

as in the case of lakeside wetlands.

A system has been developed for the hydrologic characteristics of the

East Anglian fens (Gilvear et al. 1989). Seven major classes are distinguished

based on the relative contribution of water source (Figure 3): (a) surface

water runoff and riverine flooding (two subclasses), (b) leaky aquifer with

some surface water inputs, (c) surficial aquifer sequences with some surface inflow, (d) both surficial and aquifer sources, (e) leaky main aquifer

although some surface water input, (f) groundwater inputs from an unconfined main aquifer, and (g) sources totally from the surficial aquifer. In

addition to identifying the water sources, characteristics are given for the

surficial stratigraphy, water chemistry, catchment size, and vegetation.

The wetlands of Amazon inundation forests have been classified by

Prance (1979) based on water quality and flooding regime (Table 2).

River types are divided into white water, a term that encompasses turbid

waters that receive their high-suspended sediment load from eroding lands

in the steep headwaters of the basin, and either clear water or black water

which originate in the lower Amazon basin. The black waters differ from

clear waters by having high humic content and low-ion concentrations.

Within each of two categories, the flooding regime can be annual (the

higher order streams), daily (tidally influenced reaches near the mouth),

and irregular (smaller catchment areas that respond to more localized rainfall than the annual regime).

The Canadian system (National Wetlands Working Group 1987) presents

five commonly recognized wetland classes (bog, fen, marsh, swamp, and

shallow water) and further divides these on the basis of form, of which

70 are identified using surface morphology, surface pattern, water type,

and morphology of underlying mineral soil. The lowest level uses the

physiognomy of the vegetation.

Chapter 1 Introduction
—

I

————.. ___ ___ __ I

Water table usually

below wetland level

a. Surface water depression

OVERIAND

FLow

----

-

GROUND–WATER

INFLOW

b. Groundwater depression

Figure 2. Four major hydrologic types of wetland types in Wisconsin

(Continued)

Chapter 1 introduction
—

—

0“

Water table

---

--

---

GROUND-WATER >

INFLOW

c. Groundwater slope

OVERlAND

FLOW

LAKE OR RIVER

FLOODWATER

—.- --

——— ——— -

.

W/ater table usually

below wetland level

d. Surface water slope

Figure 2. (Concluded)

Chapter1 Introduction 9
—

——

Surface Wafer

WATER SOURCE

Ground Water

I / \

Y’Y7-’Y%’!P

Figure 3. Hydrogeologic classification of Gilvear et al. (1 989) for East

Anglian fens. Description of wetland classes are as follows:

(a) those fed by surface water runoff and wetlands that receive

river flooding, (b) those receiving aquifer discharge in addition

to some surface water, (c) those fed by surficial groundwater in

addition to some surface water, (d) those receiving both surficial

groundwater and aquifer discharge, (e) those fed predominately

by aquifer discharge with minor surface water input, (f) those

fed by unconfined main aquifer, and (g) those receiving total

surficial groundwater. Precipitation inputs are assumed similar

in all examples

Table 2

Classification of Amazon Inundation Forests’

River and Foreat Type

White Weter Cleer or Black Water

Flooding Regime V6rzea Foreet lgap6 Forest

Annual Seasonal v~rzea Seasonal igap6

Daily Tidal v~rzea Tidal igap6

Irregular Floodplain v~rzea Floodplain igap6

‘ Adaptad from Prance (1979).

10

Tidal wetlands can be divided into those that receive mostly fresh water

and those that are exposed to brackish water. In a typical salt marsh, zonation of vegetation appears to correspond to differences in flooding frequency

Chapter 1 Introduction
—

—

(i.e., tall-form Spartina alterniflora in the regularly flooded brackish zone;

short-form S. alterniflora in the irregularly flooded zone; and other halophytes listed below in the higher elevations that are infrequently flooded

by tides). The short-form S. alterniflor-a sites often develop hypersaline

pore waters in summer. This additional stressor partially explains the

growth form. Where flooding from tides becomes extremely rare, lowsalinity marshes develop that are composed of a mixture of species depending in part on geographic location (Juncus gerardi, J. roemerianus,

S. patens, Distichlis spicata, and S. cynosuroides). Tidal freshwater wetlands can range from marshes (Odum et al. 1984) to forested wetlands.

Because of a somewhat unique environmental setting in the sounds of

North Carolina, Brinson (1989) developed a classification for sea-level

controlled wetlands that first differentiated tidal from nontidal, and then

freshwater from saltwater (Figure 4). This allowed recognition of hydrologically distinct marshes—ones in tidal regimes (described above) and

others in nontidal regimes—both of which were colonized by similar

assemblages of halophytes.

SEA-LEVEL CONTROLLED

COASTAL WETLANDS

*

NONTIDAL REGION TIDAL REGION

FRINGE

A

NONilDM

SWAMP BRACKISH

FOREST MARSH

(9urn, :ypress) (black needle-

,Z rush)

&k

NONTIDAL

FRESHWATER

MARSH

+

+ Freshwater

TIDAL SALTMARSH TIDAL MARSH

ASSOCIATIONS

J \<;::Frp

Irrecjulariy

Flooded 3 pper Intertidal

Zone zone (low march

(high mareh\black with saltrna~h

needlerush) cordgrass)

Figure 4. Factors controlling expression of plant community physiognomy

and species composition of sea-level controlled wetlands in

North Carolina. From Brinson (1989)

Lakes and Streams

As indicated above, classification of lakes from a hydrologic perspective might provide some insight to the differentiation of wetlands. The

classical temperature-mixing classes (Hutchinson 1957) are of little use

because they rely on thermal stratification of the water column, a property

of little significance in the shallow waters typical of wetlands. However,

Chapter 1 Introduction

11!
—

Winter’s (1977) classification of hydrologic settings in the north-central

United States uses parameters that would apply also to wetlands. These

include regional slope, regional position, local relief, ratio of drainage

basin to lake area, presence of inlets and outlets, substrate composition,

water quality, and the precipitation-evaporation balance. His analysis by

principal components methods showed that many of the variables mentioned above had an important influence on classification.

Stream classifications based on morphological characteristics are particularly relevant when they give insight into wetlands in floodplains and riparian zones. Leopold, Wolman, and Miller (1964) still stands as a useful

reference for floodplain and stream channel processes. One of the more recent syntheses of stream classification is that of Rosgen (1985). He identifies 25 stream types based on continua within six factors: gradient, sinuosity,

width/depth, dominant particle size in channel, channel entrenchment and

valley confinement, and landform that includes soil characteristics and descriptors of stability. Additional types are identified for estuarine streams

(i.e., deltas—four of them) and glacial streams (two). Stream subtypes require further description of organic debris (10 descriptors), channel width

(13 sizes), depositional features (8 bar types), riparian vegetation (10 or

more if mixtures occur), flow regime (4 general and 5 specific categories),

and meander patterns (8 types). Naiman et al. (1992) provides a historical

synthesis on principles of stream classification.

Ontogeny of the Present Classification

Early explorers such as Humboldt and Darwin were very much aware,

as illustrated in their writings, of the strong effect of climate on vegetation form and pattern. Merriam (1894) was among the first in the United

States to illustrate the parallel properties that gradients in altitude and latitude had on the distribution of plants and animals. Continental maps produced for soils and natural vegetation illustrate at a glance patterns of

temperature and moisture (Kuchler 1964). Both Holdridge et al. (1971)

and Walter (1973) developed elegantly simple approaches to the classification of terrestrial ecosystems based on temperature, precipitation, and derivatives of the two. All of these efforts had one thing in common: they

virtually ignored wetlands that are subject to edaphic control rather than

climatic control.

The need for functionally based classifications of wetlands is twofold.

The first is to simplify our concept of wetlands, recognizing that while

each one may be unique, each can be placed into categories in which similar wetlands share functional properties. The exercise of reducing the apparent complexity of an array of ecosystems that change rapidly over time

and space should not be taken lightly. The result of this simplification

should be improved communication among researchers and managers, and

perhaps even with the public, by focusing on processes that are fundamental to the sustained existence of these ecosystems. The other need for

12 Chapter1 Introduction
—

—

functionally based classifications is to foster the development and the

redevelopment of paradigms that clarify the relationship between ecosystem structure and function. For example, the Environmental Monitoring

and Assessment Program (EMAP)-Wetlands program, which is to follow

the “health” of the nation’s wetland resources over time, needs to have indicators that are sensitive to the condition of specific wetlands. To what

extent do qualitative or quantitative changes in hydroperiod cause wetlands to change and in what way? Are the tools and procedures available

to predict the long-term effects of changing the nutrient regime of a wetland? What are the thresholds of the factors that are likely to create a

change in state from one wetland type to another? These are all legitimate

questions that could be answered from recognizing a set of common denominators to which wetlands respond.

The progenitor of the current classification is the one developed by

Odum, Copeland, and McMahan (1974) for coastal ecosystems. They proposed a classification “according to the most prominent processes dominating the functional activity of the system.” It was based on a theory of

classification “that includes biological, geological, chemical, and physical

classification factors, energy being a common denominator.” Within each

of the six major categories, up to 18 types were identified along with their

characteristic energy or source of stress. The major categories (with some

examples of representative types) are as follows: (a) naturally stressed

systems of wide latitudinal range (high energy beaches and sedimentary

deltas), (b) natural tropical ecosystems of high diversity (coral reefs and

tropical seagrass beds), (c) natural temperate ecosystems with seasonal

programming (marshes and bird and mammal islands), (d) natural Arctic

ecosystems with ice stress (sea ice and ice-stressed coasts and glacial

fjords), (e) emerging new systems associated with man (sewage waste and

pulp mill wastes), and (f) migrating subsystems that organize areas. The

monumental task of organizing this document has not been updated for

coastal systems in over 20 years. (The work was actually conducted in

1968 and 1969. The description and literature synthesis of each type of

ecosystem was written by one or more authors.)

While “Coastal Ecological Systems of the United States” covered much

more than just wetlands, a mangrove-specific classification emerged from

studies on the hydrologic, structural, and other properties of mangrove

swamps in Florida (Lugo and Snedaker 1974). They proposed five groupings of mangroves based on source and quality of water, the mechanism of

flow through the system, and the zonation of vegetation (Figure 5a).

Somewhat later, Brown, Brinson, and Lugo (1979) illustrated that freshwater forested wetlands appeared to differ according to the amount of water

flow. Structural indices, primary production rates, and sediment properties

were different between flowing-water wetlands and still-water wetlands

(Figure 5b). These efforts led to the possibility that three core factors—

hydroperiod, hydrologic energy, and nutrient level—were responsible for

much of the variation occurring in wetlands (Figure 5d), at least for forested

ones (Lugo, Brinson, and Brown 1990b). Three hydrogeomorphic types

resulted by subsuming two of the five types for mangroves within other

Chapter 1 Introduction 13
—

—

Basin forest

Fringeforest

{

Riverine forest

1

Overwoshforest

Dwarf forest

a. Five types proposed for mangroves in Florida by Lugo and Snedaker (1974) as

illustrated in Wharton et al. (1977)

Figure 5. Classifications of wetlands based on their functional properties (Sheet 1 of 4)

14 Chapter 1 Introduction
—

‘III HD%%

I

/

>

Fluvial Deposition

b. Flowing versus still-water proposed by Brown, Brinson,

and Lugo (1 979) for forested wetlands

CORE FACTORS

,. . . . . .,,, ..., . . . . .............. .. ..... ... ,.,.

l'::;:.HYdro!oglc5 ::Energy!:::::\::;\ ::::::::::\:;::- .. ..... . ]

...............:.:.:..,.., .;...,.. ..............,’, .,,...., ,, ..,..,, ,, ...

.,...,:,.,. .:.:.:.:.:.:.:.: .;.:.;.:.,...:.: .:.;:.:Hydro~erio~;:\:::; . !::::;::;::l .............. . .......

:::Nitrierit:;Availabi !i?y:::-,.,.,.:.:.,.......... ..,,.,,.,,.,,.,.,,

~ I SALINITY I LOW LOW ] MOD MOD i LOW LOW

.f FIRE FtECWWY LOW HIGH Low MOD LOW HIGH

3

HERBIVORY LOW HIGH LOW LOW LOW Low

is BOTTOMLAND NORTHERN MANGROVE TEMFW?ATE BOGI HARDWIOD RIVERINE I FOREST SALWH I MUSKEG FOcoslh

FOREST MARSH

c. The use of modulating factors to characterize specific types of wetlands

beyond the core factors (Brinson and Lee 1989)

Figure 5. (Sheet 2 of 4)

Chapter1 Introduction 15
x

d.

,4 I

Hydrologic Energy Hydrologic Energy

*

Riverine, basin, and fringe arranged around three core factors

proposed by Lugo, Brinson, and Brown (1990b)

Zonatlon

Strings

Islands

e. Six generic Iandform patterns that have equivalents

for wetlands (Kangas 1990)

Figure 5. (Sheet 3 of 4)

16 Chapter 1 Introduction

Strip

Background

Centers
—

—

NUTRIENT ACCESS

RAIN ONLY

AERIAL VIEW SIDE VIEW

c1.$ . . .

.. . .. . . . . . . .

“.:::.:....;: .

... . .. .

.. :..:.. .

. . ..

SLIGHT DRAINAGE

DRY SEASON

L

,;~.: .;,:$, > .“ /,

., . .. . .

.,..... ..* ”;. .“$.,.. .

,: . . .

● **, ,..’~ ● ,”

,..,,”‘.*O ..”” ,.oe .O,. . .

;’2;0’,.. ”;”.,“.:;”’-.<,

LARGER RUNOFF \

AREA

n

.. /

#o 0:

00>0 0

~:o>”.

/ \

STRAND FLOW

RIVER a

FLOODPLAIN

II

. 1. .. . .

“.”:: o :O.,.., . .

. . .

.:..:.1. O 1 ~..”.

OO1..’. .. .

‘o“;. O -.C.

,,. ,

4’

● w.

oo~::.. :’J$J: o,-

0 I“x’

,.~

● Oo ,.. .”. :;:::L‘d:+::..:.

(iimrir

EVERGREEN

Q+!!!54

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BOG .. . . .

... .-.

---...

I 1

DECIDUOUS

t?,

HIGH WATER

I 1

FIJ3W

PEAT 6

SEDIMENT

f. Cypress wetlands of Florida arranged along increasing water and nutrient flows

Figure 5. (Sheet 4 of 4)

Chapter 1 Introduction 17
—

—

categories. The concept of only three primary controls did not, of course,

explain more than the most gross aspects of the variation among all wetlands. Modulating factors like fire, herb ivory, frost, and other variables

were necessary to further characterize specific wetlands and their vegetation (Figure 5c).

During this same period, other classifications emerged that have been

discussed in the previous section. However, one developed by Kangas

(1990) stands out as embodying landscape properties that could be extended beyond wetlands. This system recognizes four spatial distributions

of energy (sheet, point, front, and line) and combines them with six basic

ecosystem forms. The resulting patterns are represented by the following

ecosystem types (Figure 5e): zones (tidal salt marshes), string (string

bogs), islands (perpendicular to strings), strip (streams and floodplains),

background (large bogs, which may have other types embedded within

them), and center (cypress domes in Florida). These are similar to the pattern recognized for cypress wetlands of Florida (Odum 1984), which are

arranged in order of increasing water and nutrient flows (Figure 5f). Fire

could be added as a factor that controls ‘vegetation structure and resets

succession.

18 Chapter1 Introduction
2 Description

The core of the classification has three components: (a) geomorphic

setting, (b) water source and its transport, and (c) hydrodynamics. Geomorphic setting is the topographic location of the wetland within the surrounding landscape. The types of water sources can be simplified to

three—precipitation, surface or near-surface flow, and groundwater discharge. Hydrodynamics refers to the direction of flow and strength of

water movement within the wetland. While the three components are treated

separately, it is apparent that there is considerable interdependency. Such

redundancy may be useful if it serves to reduce errors of interpretation

and to reinforce the underlying principles that explain wetland functions.

Geomorphic Setting

Implicit in the hydrology of a particular wetland is its landscape position, or “geomorphic setting,” which will accommodate the flows and storages of water. From a broad and long-term geomorphic perspective, water

flows and wetland position are inextricably linked. Consequently, it is difficult to describe geomorphic setting without also discussing hydrology.

However, water source and hydrodynamics will be discussed in detail in

separate sections.

Background

The geomorphic settings listed in Table 3 (column 1) are elaborations

of the depressional, riverine, and fringe categories developed originally

for mangroves by Lugo and Snedaker (1974). They have been simplified

from the original report by Lugo and Snedaker in descriptions by Brinson

(1988) and Lugo, Brinson, and Brown (1990a). Each category tends to

have a distinctive combination of hydroperiod, dominant direction of water

flow, and zonation of vegetation. Extensive peatlands have been added

here as a separate category because of the strong feedback between biogenic accretion and hydrology.

Chspter 2 Description

19
Table 3

Examples of Geomorphic Setting as a Property of Hydrogeomorphic Classification

Examples of

Geomorphic Setting’ Qualitative Evidence* Quantitative Evidence3 Functions4 Ecological Significance

Depressional Wetlands6

No apparent inlet or outlet. Topographically isolated from Drydowns frequent; water table Retains inflow; loss primarily by Inaccessible to aquatic organisms

other surface water bodies. significantly below wetland much evapotranspiration (ET) or dependent on streams.

of the time. infiltration. Endemism likely.’

Positioned on local topographic Outlet may be defined by Drydowns frequent and water

high. Surface outlet only.

Temporary flood storage; outlet System open to upstream

contours or intermittent stream table significantly below wetland

symbol.

may overllow during high water immigration and downstream

much of the time. (surface-water dominated) or flow emigration of aquatic organisms.

continuously (groundwater Potential for recolonization by

supported). Outlet controls aquatic organisms if drydowns

maximum depth. cause local extinctions.

Located in marginally dry climate Inlets and outlets may be defined If water has low conductivity, Retains inflow; loss primarily by Geographic location critical to

(e.g., prairie pothole region). by contours or intermittent stream wetland is recharging underlying ET or infiltration. Maybe subject

Variable inlets and outlets.

migrating waterfowl as flyway

symbol. aquifer. If high conductivity, to wide fluctuation in water depth. position indicates. Changes in

groundwater is discharging to vegetation create varied waterfowl

wetland (Sloan 1972). habitat. May be vulnerable to

eutrophication and toxin

accumulation because of long

residence time of water. Probable

import and export of detritus.

(Sheet 1 of 4)

‘ Array of geomophic settings is derived from the fringe-riverine-basin/depressional categories of Lugo, Brinson, and Brown (1990b). Extensive peatlands were originally not a

separate category.

2 Normally assessed by direct observation.

~ Normally requires records of discharge and stage height to illustrate seasonal and interannual variation. Reliable indicators maybe used also.

Mechanisms for maintaining ecological significance.

5 Other ecologically significant functions maybe present; only examples are given.

6 If wetland contains open water, waterfowl habitat may be inferred, For prairie pothole region, soil properties provide excellent indicators of hydrology (Hubbard 1988). Playa lakes

~f the southern High Plains reviewed by Bolen, Smith, and Schramm (1989).

Zedler (1987).

.

‘II
Table 3 (Continued)

Examples of

Geomorphic Setting Qualitative Evidence Quantitative Evidence Functions Ecological Significance

Depressional Wetlands (Continued)

Both surface inlet and outlet; large Inlets and outlets may be defined Water budget dominated by Temporary flood storage; Potential for fish population

catchment sustains marginal by contours or intermittent stream lateral surface flows or strong drainage back to stream shortly

riverine features. symbol.

recruitment through migration.

groundwater discharge. after flooding (surface-water Probable import and export of

supported) or continuous detritus.

saturation (groundwater

supported).

Located on break in slope. Soil saturated most of time. Chemistry indicative of Inflow steady and continuous; Provides surface moisture during

groundwater; discharge from loss by ET seasonal. Renewal of dry periods; contributes to beta

slope base or face. Piezometric pore waters maintains higher diversity of landscape.

confirmation. redox than typical for constant

saturation. Low-surface storage

capacity.

Extensive Peatland

Ombrotrophic bog. Peat substrate; saturated most of Peat confirmed by organic content Surface storage may facilitate Wetland-upland interactions minor

time. Plant species indicate and thickness. Ombrotrophy storm runoff; groundwater relative to wetland-atmospheric

ombrotrophic bog; surface flows evident from low pH and low-ion conservation occurs when water exchanges.s Upland habitats

negligible. content. table is below surface. Peat scarce. Species composition

deposits control topography and unique to bog conditions.

geomorphic surface.

Rich fen. Peat substrate; saturated most of Peat confirmed by organic content Subsurface water supply Represents conduit for lateral

time. Graminoid species indicative and thickness. Minerotrophy maintains saturation to surface water movement without

of groundwater supply. evident from circumneutral pH and hydraulic gradient to maintain channelized flow. Moderate levels

and high-ion content. flow. of primary production and organic

matter export.

(Sheet 2 of 4)

B Brinson (1991).

11’11
Table 3 (Continued)

Examples of

Geomorphic setting Qualitative Evidence Quantitative Evidence Functions Ecological Significance

Riverine Wetlands (floodplain, not channel)

Streamside zones of intermittent Headwater position; first order Flows not continuous; flow lacks Interface of landscape where Riparian zone critical to

streams. stream. headwater flooding and overbank groundwater and surface water maintaining buffer between

flow properties. sources change phases to fluvial upland and stream flow.

environment.

High-gradient:

downcutting Portionsg Bedrock-controlled channel. Substrate lacks alluvium (soil Scour precludes extensive wetland May impede wildlife movement

maps). Flow may be continuous development. Unvegetated

but likely flashy.

and cover if corridor too narrow.

reaches allow light penetration to Maintains important in-stream

support aquatic production. riffle habitat.

aggrading portions Substrate controlled by fluvial Stratigraphy shows interbedding Wetland on coarse substrate Unstable substrate in high-enargy

processes. and coarse particle size (gravel maintained by upslope environment colonized by pioneer

and larger). groundwater source.’” species. Streamside vegetation

contributes to allochthonous

organic supply.

Middle-gradient Iandform. Channelized flow, evidence of Flow likely continuous with Channel processes establish Alluvium is renewed by surface

oxbows, meander scrolls, etc., moderate- to high-base flows. variation in topography, accretion and point bar

consistent with fluvial processes. hydroperiod, and habitat deposition; interspersion of plant

interspersion on a floodplain. communities contributes to beta

diversity.”

Low-gradient alluvial. Floodplain As above, but in low-gradient Flow continuous with cool season Flood storage; conserves Major habitat for wildlife and

of bottomland hardwood. Iandform. flooding. High-suspended groundwater discharge. biodiversity; strong

sediments in stream. biogeochemical activity and

nutrient retention.

(Sheet 3 of 4)

9 Geomorphic processes and their ecological significance in riparian ecosystems are reviewed by Gregory et al. (1991).

‘0 Major water supply to streamside wetlands is provided from upslope by groundwater discharge (Ruddy and WNiams 1991 ).

11Me&ler and Damman (1g85) describes dependence of understory herbaceous vegetation on annual flood re9ime.

‘!”1 1
Table 3 (Concluded)

Examplee of

Geomorphic Setting Qualitative Evidence Quantitative Evidence Functions Ecological Significance

Riverine Wetlanda (floodplain, not channel) (Continued)

Low-gradient nonalluvial (i.e., low flows not channelized or channels Manning coefficient normally Conduit for drainage in otherwise Wetland possesses both depresin suspended sediments): Florida shallow; if peatland, flow limited to high.’2 Vegetation and sediment preapitation-dominated wetland. sional and riverine attributes

cypress strands and sloughs, peat acrotelm (upper 20 to 30 cm) redox differ from surroundings if Flow facilitates nutrient availability. because of weak lateral flows.

water tracks). (Clymo 1983, 1984). peatland.

Fringe Wetlanda

Shoreline of large lake (i.e., Subjected to seiches. Lake level Amplitude and frequency of wind- Lake serves as water supply for Provides shoreline stabilization

Iacustrine). controls position. generated fluctuations. Year-to- wetland and establishes under moderate wave action;

year trends in zonation follow hydroperiod gradient for wetland transition habitat utilized by both

climatic cycles. zonation. aquatic and terrestrial organisms.

Coastal sea-level location (i.e., Subjected to astronomic tides; Elevation relative to tides and Wetland responsive to tides and Barrier to saltwater encroachment;

estuarine). sea-level controlled. changing sea level. changing sea level. accommodates sediment

deposition; open to estuarine

organisms for feeding and

recruitment.

(Sheet 4 of 4)

‘2 Arcement and Schneider (1989).

-’II
—

—

Depressional wetlands include such landforms as kettles, potholes, vernal pools, and Carolina bays. Because they frequently occur high in drainages, they are typically more dependent on atmospheric exchanges than other

wetland types. In dry climates, depressions are either dry much of the time,

as in vernal pools (Zedler 1987), or they are dependent on groundwater

sources. In more moist climates, they may accumulate sufficient peat to

develop a domed topographic relief, or a tertiary mire. Such wetlands necessarily receive their water from precipitation. Ombrotrophy can result in

strong seasonal fluctuations in water table because of the seasonality of

the ratio of precipitation to potential evapotranspiration (PET). Where

landscapes have undergone widespread paludification, the depressional

category no longer applies because wetland surface features become decoupled from the topography of the underlying landscape. Such wetlands

would fall under the next category, extensive peatlands.

Extensive peatlands cover large areas of land such that the peat substrate dominates the movement and storage of water, the mineral nutrition

of the plants, and patterns in the landscape itself (Moore and Bellamy

1974). Blanket bogs and tussock tundra are examples of extensive peatlands. Once tertiary mire formation radiates by paludification across the

landscape, surface patterns develop that are independent of underlying topography. The connections of bogs in higher topographic positions may

occur through surface unchannelized flow paths and subsurface pathways

to peatlands lower in the landscape or drainage basin (Ingram 1967).

Hence, there is a gradient from the truly headwater ombrotrophic wetlands

with diffuse outlets only to ones further downstream with distinct inlets

and outlets with fen-like characteristics (Siegel and Glaser 1987).

Riverine wetlands form as linear strips throughout the landscape. They

have predominately unidirectional flow. Hydroperiod ranges from short

and flashy in headwater streams to long and steady in higher order

streams. Their slope controls whether a given section of stream is predominately erosional or depositional.

Fringe wetlands occur in estuaries where tidal forces dominate or in

lakes where water moves in and out of the wetland from the effects of

wind, waves, and seiches. Consequently, bidirectional flow, largely

across the surface, dominates and the hydroperiod is long as a result of the

cumulative frequency of many flooding events, especially for wetlands

that receive semidiurnal astronomic tides. Lakes that are too small to develop frequent seiches would not support fringe wetlands; such lakeside

wetlands would fall into the depressional category described above.

The processes that maintain wetlands at this coarse level of resolution

are often so large in scale that they define the very nature of the wetland

itself. Hence, it may be problematic to classify in isolation a distinct wetland on a small scale (ha) if it is part of a larger wetland complex of tens

of hectares. This is especially true if one or several of the wetland functions is size-dependent or a result of its strategic position in the landscape

(Brinson 1988, 1991). Also, a given wetland may have characteristics of

24 Chapter 2 Description
—

more than one of the three categories. An oxbow lake, for example, may

behave more like a depressional wetland when it is hydrologically isolated

from overbank flow than the greater riverine system that was responsible

for its genesis. It is also apparent that the three categories intergrade such

that continua form among them. Column 2 on qualitative evidence (Table 3)

suggests indicators that are characteristic of the geomorphic wetland type.

Additional quantitative estimates (column 3) that would characterize these

wetlands relate to the amplitude and frequency of water turnover or flushing (Miller and McPherson 199 1), water chemistry, substrate composition,

stream power estimates (Richards 1982), and water budgets, the latter

being useful for depressional wetlands. !1

The functions in column 4 are derived from the descriptions in the first

three columns (Table 3). To the extent that functions are the result of geomorphic settings, they should vary in response to different settings. Ecological significance (column 5) is one interpretation of the functions that

goes beyond the physical and chemical aspects of wetlands and includes

the biotic portions. Ecological significance is highly variable and could

be greatly expanded to capture the special features of regionally important

wetland types. For example, high-gradient streams in Alaska could be further characterized by estimates of vegetation cover, primary production,

and food web transfer coefficients, which Duncan, Brusven, and Bjornn

(1989) has done for estimating salmonid production.

Examples of geomorphic settings

Six commonly recognized wetland types are examined-one depressional,

one extensive peatland, two riverine, and two fringe (tidal) wetlands.

This is done to illustrate a higher resolution than the four basic settings

by using subsets of these broad categories for which the functioning is

reasonably well understood. (Note: In each case in Table 3, the reader is

referred to one of four “settings” (e.g., depressional, extensive peatland,

riverine, and fringe), and then directed to a specific row by indicating its

order (first row, second row, etc.).

Groundwater slope wetland. Groundwater slope wetlands occur

where there are breaks in slopes (Table 3, Depressional Wetlands, fifth

(last) row). In terms of water source, they are the groundwater slope wetlands of Novitzki (1979), except they are not necessarily associated with

stream flow (Figure 2c). Two conditions can exist: those with a seepage

‘ The reader is reminded that this report is meant only to provide an overview of the logic of a

methodology that relates certain hydrologic and geomorphic characteristics through a process of

interpretation, and finally to the ultimate description of ecological significance. A full, field-tested

method for actual use by persons to classify would include a larger army of options. For depressional

wetlands, if there are surface inflows and no surface outflows in a moist climate, the wetland is

likely recharging a groundwater aquifer. Information on the hydraulic conductance of sediments

underlying the wetland might corroborate with this inference. Data on water table position and

piezometric surfaces could confirm or exclude this possibility.

Chapter 2 Description

25
—

—

face caused by groundwater flow intersecting a land surface (Figure 6a)

and those with seepage at the base where the upward movement of groundwater occurs in the lower slope segment of the break (Figure 6b) (Winter

1988). A third condition in glaciated regions is where till deposits are arranged with more permeable layers overlying less permeable ones, thus

creating a shallow perched aquifer. Erosion through the discontinuity can

cause exposure of the aquifer and seepage at the surface. 1

lANl) SURFACE

a. Seepage face where groundwater flow intersects the land surface

b. Seepage in the lower slope portion of the break

26

Figure 6. Interaction of breaks in slope with groundwater flow as a source

of water for wetlands. Adapted from Winter (1988)

1 Personal Communication, 1991, Frank Golet, University of Rhode Island, Kingston, RI.

Chapter 2 Description
—

—

Figure 3 shows additional examples of depressional wetlands with varying degrees of groundwater seepage. Some tend to have relatively constant water tables if the aquifer responsible for the water source is large

and quickly recharged, thus being relatively immune to seasonal demands

by evapotranspiration. Depending on the composition of the aquifer matrix, groundwater slope wetlands may receive well-buffered, nutrient-rich

water, or be relatively poor in nutrients. Sediment loads are negligible,

and most cannot flood deeply because they have a sloped surface. If the

seep is fed by a shallow and perched aquifer, the wetland soils will likely

undergo desaturation during the growing season as vegetation intercepts

groundwater supplies to the seepage area.

Ombrotrophic peat bog. The interdependency of geomorphology and

hydrology is no better expressed than in extensive peatlands where the accumulation of organic matter as peat controls topographic relief (Table 3,

Extensive Peat-land, first row). This can be perceived as a special case in

which a gas, carbon dioxide, is deposited as organic carbon through the process of photosynthesis. As such, accretion occurs in low-energy environments through biogenic processes, rather than being limited to inorganic

sediment accretion or allochthonous organic matter deposits, processes

that require hydraulic energy for sediment transport (Gosselink and Turner

1978; Brown, Brinson, and Lugo 1979).

Ombrotrophic peat bogs are usually the terminal condition of peat accumulation in depressions, subsequent radiating paludification, and finally a

domed landscape where the highest elevation receives precipitation as the

sole water source and generally is the most nutrient poor environment.

The connections of bogs with downstream ecosystems could be through

distinct outlets or unchannelized flow. Peatlands lower in the landscape

or drainage basin may also have inlets. Hence, there is a gradient from

the truly headwater ombrotrophic wetlands with outlets only, to fens further downslope with either diffuse surface inlets and outlets or primarily

groundwater discharge (Table 3, Extensive Peatland, second row). This is

clearly a shift in peatlands from ombrotrophic toward fen-like conditions

with a minerotrophic water supply. Patterned peatlands may contain both

ombrotrophic bogs and minerotrophic fens. Sites with woody vegetation

act as recharge areas, and force groundwater to pass through inorganic

soil. When this water, now enriched in minerals, discharges to the surface, the fen-like portion of the pattern is maintained (Siegel and Glaser

1987).

The terminology and literature describing hydrologic and water chemistry continua of peat-based wetlands has a long and rich history of development (Gorham 1956; Moore and Bellamy 1974; Clymo 1983, 1984). The

range of wetlands found in Europe is summarized in Figure 7. Climate

and original Iandform combine to produce both topographic and drainage

patterns in the peatlands. This interaction has received much attention,

particularly in boreal peatlands (Ingram 1967, 1983; Ivanov 1981).

Chapter 2 Description

27
—

—

Figure 7. Major types of wetlands found in Europe. Cross-sectional and plan view in

each panel: (a) mires with primary and secondary peats only (floodplains),

(b) flat tertiary mires in open basins (deltas), (c) flat tertiary mires in closed

basins, (d) plateau mires without concentric pattern of hummocks and hollows,

(e) concentric domed mires, (f) eccentric domed mires, (g) a~pamires (as “f”

but no tertiary peat), (h) palsamires (diaganol dashes represent permafrost),

and (i) arctic mires underlain by permafrost. From Moore and Bellamy (1974)

High-gradient riparian. High-gradient riverine wetlands occur in a regime of erosion and net downcutting of the stream channel (Table 3, River28

~ne Wetlands, second row). Streams that have steep gradients overall

frequently have shorter reaches that alternate between accretion on point

bars and erosion of cutbanks. The process of sediment trading (Leopold,

Wolman, and Miller 1964) maintains areas of active deposition where the

floodplain wetlands are located. In high rainfall regions, woody debris and

debris dams may be dominant structural channel and floodplain features

Chapter 2 Description
—

—

(Triska and Cromack 1980; Gregory et al. 1991; Sedell and Beschta

199 1). Lane’s (1955) simple relationship is useful for explaining observations of the effects of changing slope on grain size of bedload.

Qs = Q,D50

where

Q=

s=

Q.=

D 50 =

water discharge

slope of the channel bed

the bed material discharge

a measure of the average diameter of grain size

of the channel bed material

If all other factors remained constant, a decrease in slope (Son the left

side of the equation) would be accompanied by a decrease in diameter of

grain size (D50 on the right). Overall, the wetlands of high-gradient

streams would tend to have sediments that are coarse (cobble, boulders),

have high hydraulic conductivity, and maintain a strong coupling between

groundwater of the floodplain and surface water of the stream (Stanford

and Ward 1988). In other cases, however, large woody debris and riparian

vegetation contribute greatly to the control of channel characteristics

(Sedell and Beschta 1991).

In arid climates where precipitation is not only lower but more variable

from year to year, wetlands may be dependent on relatively large catchment areas for providing sufficient supplies of water to maintain their integrity. In other areas, spring snowmelt at higher altitudes may account

for most of the water supply to wetlands. Many of these streams and associated riparian zones in the American West (United States and Canada)

have had dramatic changes in function because of flow modification

(Rood and Mahoney 1990).

Low-gradient nonalluvial riverine. Low-gradient riverine landforms

are represented by near-headwater drainages where surface flow is strong

enough to be recognized, but not strong enough to create distinct stream

channels (Table 3, Riverine Wetlands, last (fifth) row). This condition

would be transitional between the depressional Iandforms that lack flow

or have negligible flow, and the high-gradient riparian wetlands just described. Where channelized flow is present and there is sufficient hydraulic energy to carry sediments and create streamside natural levees, the

landform may be designated as either middle-gradient or low-gradient

alluvial (Table 3, Riverine Wetlands, third and fourth rows). In unglaciated

regions where drainage patterns have had a long time to develop, lowgradient riverine wetlands are limited to regions with very low slope, such

as parts of the coastal plain of the Southeast (Kuenzler 1989) and Florida

(Duever, Meeder, and Duever 1984), where they are variously known as

cypress strands, stream-swamps, and sloughs. In glaciated areas, the inherited landscape may continue to have major control on drainage patterns,

Chapter 2 Description

29
—

thus leading to complex and variable hydrogeologic processes (Hollands

1987). Forested wetlands of the Northeast commonly have mixed riverine

and depressional characteristics for this very reason.

As indicated by the arrangement of the riverine wetlands in Table 3, decreasing slope shifts the classification from high-gradient riparian wetlands to those with reduced slopes, to the near absence of surface flow,

and finally to the depressional category. Where the drainage of a depressional wetland has both surface inlet and outlet, a large enough catchment

can sustain marginal riverine features (Table 3, Depressional Wetlands,

fourth row). As slopes become reduced, so does the capacity of water to

carry sediment and directly influence geomorphology through accretion,

erosion, and transport. In such cases, hydrology and vegetation interact

on a geomorphic surface to further modify the substrate through the process of biotic deposition of organic sediments. This modified substrate in

turn becomes the geomorphic platform for another series of feedbacks

(Gosselink and Turner 1978).

Tidal salt marsh and tidal freshwater marsh. Both tidal salt marsh

and tidal freshwater marshes have the fringe geomorphic setting as a result of their position in relationship to sea level and the influence of tides

(Table 3, Fringe Wetlands, second row). Salinity of the adjacent estuary

determines whether salt marsh or freshwater marsh prevails. Tides represent frequent and predictable hydroperiod events in these wetlands, especially for the regularly inundated portions. In the irregularly flooded

portions of tidal salt marshes, spring tides and storms are responsible for

inundation patterns. Because of the reduced frequency of estuarine forces

in the irregularly flooded zones, precipitation becomes of greater signiflcance in contributing to hydroperiod and in influencing salinity (Brinson,

Hook, and Bryant 1991 ). While the very highest of “high marshes” would

still be located in a fringe geomorphic setting, they acquire characteristics

of depressional wetlands because precipitation often becomes the most important source of water for short-term water balance.

Depending on the local conditions of land subsidence or emergence,

the relationship of these wetlands to sea level will define their landscape

role. A relative rise in sea level normally results in overland migration

with the irregularly flooded portion of the fringe wetland invading upland

ecosystems. Several combinations of sea-level change and marsh development processes are illustrated in Figure 8. Vertical accretion by deposition of allochthonous mineral sediment or in situ accumulation of organic

substrate are local manifestations of processes that allow these wetlands

to maintain their geomorphic setting. In tidal marshes that become progressively isolated from their inorganic sediment source, deterioration appears to take place within the marsh rather than preferentially at the edges

(Stevenson, Kearney, and Pendleton 1985; Hackney and Cleary 1987).

This illustrates the vulnerability of fringe marshes to sediment balance.

30 Chapter 2 Description
—

EMERGING COAST SUBMERGING COAST

Perched Peat

ESTUARINE SUBMERGED UPLAND

Levee

Backmarsh

I 1

~Dead trees

. ..

. .

..

. . . . . . .

,,. . .

. .. “.”.”: . :..”.c. .

. : .“.”:”.”...”:....... ... . #.

. #. ............... .

FLOATING

,Orgonic Mat

TIDAL - FRESH

. .

. . . . . . . . . . . . . . . . . . .

. . .

Figure 8. Six major types of fringe wetlands and their position relative to changing sea

level and vertical movement of coastal marshes. From Stevenson, Ward, and

Kearney (1986)

Water Sources

One of the more difficult phenomena to observe in wetlands is the

source of water, particularly when the observed water at the surface is derived from groundwater. In glaciated regions where landscapes are relatively “immature” and drainage patterns have not had time to fully

develop, the relationship between surface water and groundwater is often

complex.

31

Chapter 2 Description
Background

Most treatments of hydrology in wetlands begin with the components

of a water budget whereby inputs and outputs of water are defined and

itemized 1 (LaBaugh 1986). The sum of all inputs and all outputs tend to

cancel one another over periods of 1 to several years. From this standpoint,

a hydrologic budget of a wetland does not differ from any other ecosystem

except for the specific cases of tidal exchange and the prevalence of surface

inflows and outflows. Without further qualifying information, a water budget, no matter how detailed and accurate, may not provide information that is

critical in evaluating how the storage term influences ecosystem processes

and sets wetlands apart from other ecosystems. Adaptations of plants,

animals, and protists are insensitive to the water budget per se. Rather

such factors as water depth, flood duration, flow velocity, and water

source either act as selective factors for the adaptations of organisms or

are capable of performing work in the system.

For purposes of this discussion, hydrologic inputs will be simplified to

three water sources: (a) precipitation, (b) groundwater discharge (inflow,

usually into and through wetland sediments), and (c) surface or near-surface

inflow (i.e., depending on the wetland, this could include flooding from

tides, overbank flow from stream channels, and interflow or overland flow

from higher potentiometric surfaces in the wetland). The movement of

sources to a location within a wetland is illustrated in Figure 9. The quantity of water that each source contributes to the hydroperiod annually

could be expressed as an average if a detailed water budget were developed. Even if such detailed data were not available, it may be possible to

rank the relative importance of the sources to the total site water balance.

Consideration should be given to interannual variation.

A less quantitative approach could utilize the interpretation of hydrography to determine the dominant sources of water. Figure 10 illustrates

hypothetical hydrography in four wetlands on the east coast and four

stream channel hydrography on the west coast and Rocky Mountain region. For the east coast wetlands (Figure 10a-d), two are on floodplains

adjacent to streams (stream-swamp and alluvial swamp), one is precipitation driven (pocosin), and another is affected by sea level combined with

irregular flooding from wind events rather than astronomic tides (sea-level

controlled swamp or marsh). For floodplains of higher order streams and

especially streams with steep gradients (Figure 10 e-h), most water table

1 The components of a water budget can be reduced to the following:

dS=PR+Si +GWi+NCi -ET- So- GWo-NC.

32

where dS = the change in storage in the wetland, PR = net precipitation reaching the soil surface;

Si = stream inflows such as overbsnk flooding; GWi = groundwater inflow; NCi = unchannelized

surface water inflow; ET= evapotranspiration, SO= stream outflows; GWO= groundwater outflows

or discharge to another hydrologic unit; and NCO= unchannelized water outflow. When dS = zero,

inflows are equat to outflows.

Chapter 2 Description
—

—

()a

(b)

,.

, #

()c

LATERAL

,.. - -

,.,p..$, J’““. ‘“ ,:;,L

Figure 9. Principal sources of water

fluctuations above the surface are derived from overbank flows-when

stream discharge exceeds channel capacity. Under natural conditions, lateral flows tend to be negligible in precipitation-driven wetlands except

with major rainfall events when radial flow from the center may occur

from raised peatlands.

Chapter 2 Description
—

—-.,.-.

Evapotranspiration can result in extreme drawdowns because of sole dependence on precipitation as a water source. In the sea-level controlled

wetland, estuarine water is delivered to the wetland surface during infrequent wind events. Although the amount and distribution of precipitation

could be the same for all four east coast wetlands in Figure 10 (a-d), the

influence that rainfall has on the hydrographic signature would depend primarily on the relative contribution of other water sources (i.e., groundwater discharge and delivery from upstream sources), whether the sediments

were flooded or unsaturated prior to the rainfall event, and the physical

properties of the sediments (storativity of unsaturated sediments and the

hydraulic conductivity).

0.7

STREAM-SWAMP (a.]

0.5

--- + 0.3

+

(b.] 0.1

I

ALLUVIAL SWAMP

U

;

d

POCOSIN OR CAROLINA BAY

s

(n

i

> WHIAND SUlifACE n

LLJ

z

<

---- -..

m ti

w

1- 9

$

IRREGULARLY FLOODED

“’TIDAL” SWAMP OR MARSH (d.)

+

L I

JF MAMJJ A S ON D

MONTH

0.5

0.3

0.1

ON DJFMAMJJAS

MONTH

Figure 10. Hydrography of wetlands and streams associated with wetlands. Left panel:

Idealized hydrography of water levels within four wetlands in the eastern

United States. The horizontal line is the wetland surface. The first two

(a and b) represent the riverine type that has flood peaks principally because

of overbank flooding. The hydrography for the depressional wetland (c) is

dominated by precipitation and evapotranspiration. The fringe marsh hydrography (d) is subject to sea-level control, and thus does not show strong seasonal fluctuations. From Brinson (1985). Right panel: Hydrography from

streams in riverine wetlands in the western United States and Canada:

(e) N. California, Oregon, Washington, and British Columbia, (f) Oregon and

Washington, (g) Rocky Mountain states and provinces, and (h) British Columbia and Alaska. Horizontal dotted line is the discharge at which significant

bedload transport and overbank flooding are expected to occur. From Everest

et al. (1987)

34

Chapter 2 Description
—

—

Groundwater discharge from slopes or in the wetland itself may be derived from perched sources or from regional groundwater systems. A

perched source can occur where a downcutting of a stream through glacial

till intersects a stratum of relatively lower permeability than the strata

above or below it. 1 Consequently, water that would normally continue

downward seepage as gravity flow is diverted laterally by an aquitard to

the edge of a slope. The groundwater above the aquitard is described as

“perched” when sediments below the aquitard are not fully saturated. Alternatively, regional groundwater systems may discharge directly into a

wetland. The origin of such water may be a groundwater recharge area

many kilometers removed from the wetland itself. All that is required for

significant flow to occur is for the potentiometric surface of the groundwater slope toward the wetland to be steep enough and for the permeability

of the shallow aquifer to be high enough.

Regardless if groundwater is discharged into a depression (Figure 2b)

or a slope (Figure 2c), the saturated conditions are derived from water that

has been in contact with the mineral content of the aquifer or soil. Depending on the time of contact and the composition of the lithology, such

water normally has much higher plant nutrient content than water derived

directly from precipitation. Consequently, plant communities in wetlands

that receive groundwater discharge tend to be more productive than ombrotrophic bogs.

One of the challenges of comparing the relative importance of the three

water sources is finding appropriate and comparable measurements and

units that allow hydroperiod to be quantified. Several approaches to the

problem have been proposed and are mentioned briefly below. The cumulative frequency distribution of flooding duration and dry periods has been

used for tidal marshes (Swenson and Turner 1987), but the source of the

water is not revealed (Figure 11a). Twilley (1985) used cumulative tidal

amplitudes to scale the capacity of mangroves to export organic carbon

(Figure 1lb). Hook (1991) used total water table drawdown below the surface in an attempt to express the potential for sediment aeration in experimental treatments. Brinson, Hook, and Bryant (1991) explored the use of

cumulative rise in water table, which is the sum of all increases in water

table over a specified time. In this example, water table rises in coastal

marshes are separated into those responding to precipitation and those responding to flooding by surface flow from an adjacent water body. The

values of the two vary with distance from the shoreward margin of the wetland (Figure 1lc). Over sufficiently long periods of time, cumulative

rises and drawdowns cancel. Finally, the flow duration curves for streams

draining a groundwater bog (i.e., groundwater-fed fen) and a perched bog

(i.e., ombrotrophic peatland) illustrate both the effects of differing water

sources and the differences in range of water table fluctuation (Figure 11d).

I ~emon~ Cornrnutication>1991, Frank Golet, University of Rhode Island, Kingston, RI.

Chapter 2 Description

35
—

FLOODING EVENTS

100

1-

~go

0

a

Ill (1.

~ 80 –1

i=

j ;

~ 7(J +

o I

i

60 1

I I I

o 200 400 600 800

DAY TIME EVENTS

‘ (a.)

A

I I I 4

0 200400600~

EVENT LENGTHS, HOURS

-3 ~

T x

200m 800m 1600m wm

RAIN FLOOD

CUMULATIVE TIDAL AMPLITUDE, m yr ‘1

-GROUND WATER FEN

0.01 \

c1 \

$

“ \

E

g 0.001 \

cc

u

I

$

\

E k

PERCHED BOG

0.0001 \

\

0.00003

0 20 40 60 60 100

TIME DISCHARGE WAS

EQUALEDOR EXCEEDED, PERCENT

Figure 11. Several approaches for quantifying hydroperiod in wetlands that reveal the

dynamics of flooding: (a) Distribution of flooding and dry time intervals expressed as cumulative percent of occurrence for a tidal marsh (solid line)

and partially impounded marsh (broken line) (Swenson and Turner 1987),

(b) Cumulative tidal amplitude in m yr‘1 shown as the independent variable

for net export of organic carbon from mangrove forests (Twilley 1985),

(c) Cumulative drawdown (Hook 1991) or cumulative rise (Brinson, Hook,

and Bryant 1991 ) of water table, (d) Flow duration curves for ombrotrophic

peatland (perched bog) and fen (groundwater bog) (Boelter 1977)

Once the relative importance of the three sources of water is established,

the sources can be illustrated graphically as in Figure 12. Hypothesized

positions of several wetland types are shown in the triangle. The scale

in percent could represent any one of the indices of water quantity just

discussed. Wetlands that have only two sources of water could be illustrated

as easily on Cartesian coordinates. In arid climates, for example, precipitation may be negligible relative to groundwater sources and inflows from

upstream. If groundwater discharge were the principal source, the wetland

would likely be a groundwater slope or depressional wetland. If water

from a stream channel dominated flows through delivery of surface flow

(by overbank flooding or flood tides), the wetland would have riverine or

36 Chapter 2 Description
—

—

- .,

052 1002

i~

.& xx% Bogs, 67% +C

Pocosins .

100%/ /

\ 02

F m >

02 -—> 33% 67Z 1Ooz

SURFACE FLOW

Figure 12. Diagram that allows expression of the relative contribution

of three water sources—precipitation, groundwater discharge,

and lateral surface flow—to a wetland; location of major wetland types (bog, riverine, etc.) within the triangle show the

relative importance of water sources (Brinson 1987)

fringe characteristics. In contrast, ombrotrophic bogs would lack the

groundwater and upstream inflow component. A similar diagram was prepared for Wisconsin wetlands (Zimmerman 1987).

It could be argued that quantifying the fates (sinks) of water might be

more useful than quantifying the sources. Fates include evapotranspiration, groundwater infiltration to underlying aquifers, and export by surface

flow or interflow. The main difference is that emphasis would be shifted

to the behavior of water after it affects the wetland rather than before. As

such, it would not encompass potential influences of differing water quality being delivered to the wetland under evaluation. Information on the

fates of water nevertheless would be influential when assessing effects on

adjacent ecosystems. For the immediate purposes of classifying wetlands,

both inflows and outflows were treated in the geomorphic setting component of the classification (Table 3) where combinations of inlets and outlets

are considered. Information on the destination of water may, however, be

very useful in evaluating the implications of hydrologic modification.

Chapter 2 Description

37
—

—

. .

Examples of water sources

Knowledge of the source of water often reveals its chemical make-up.

The source may also reveal something about flow paths and the energy required to transport the water to the wetland surface. However, hydrodynamics, covered below, deals more explicitly with the mechanism of

delivery. Here we will examine what functional characteristics tend to

covary with water source.

Precipitation is a source of water for virtually all wetlands and, because it varies with climate, it is essential for comparisons of wetlands

among climatic regimes. Even within a given climatic regime, however,

the relative importance of precipitation is a function of presence and dominance of the other two sources. Table 4 provides examples of five combinations of water sources, discussed next, while other examples are

described later in this section. Dominant water sources and the climatic

setting in which each of the five examples occurs are listed in column 1 of

Table 4. The first one receives precipitation exclusively and is necessarily

located in a moist climate. The second wetland receives surface transport

or near-surface transport from overbank flows, but groundwater sources

cannot be excluded without the availability of appropriate data. The remaining three wetlands receive substantial groundwater discharge; the

first of these three is in a mesic climate, and the other two are in more

arid climates. Obviously, more combinations than these five examples

exist in nature. Next, a qualitative scale given in column 2 provides some

assessment of the relative dominance of the principal water sources relative to others. In column 3, these water sources are described in more

quantifiable terms (e.g., seasonality of flooding, cumulative rise or drawdown in water table, duration and frequency of flooding). The fourth ~olumn explains how the water sources function to interact with each other,

create anaerobic conditions or flush sediment pore-waters, and transport

nutrients and sediments. Further elaboration would be possible based on

site-specific studies. Finally, the ecological significance of the functions

is explained in the last column of Table 4. Ecological significance in

some cases is little more than an interpretation of function based on hydrologic common sense or professional opinion. While hydrologic measurements are time-consuming and expensive, others could be estimated by

extrapolating from studies that have been conducted in similar ecosystems.

To further illustrate the importance of climate, and, by inference, the

importance of recognizing the environmental setting in which a functional

classification is conducted, six common wetland types are examined to

see how different atmospheric moisture regimes might interact with other

sources of water to result in a characteristic, composite group of water

sources. Each wetland is examined to consider the hypothetical effects of

two climatic conditions: a warm arid climate and a climate characterized

as cool and moist. (A shorthand approach is expressed quantitatively as

PET ratio. An arid climate would have less precipitation than PET (PET

ratio <1), while a cool, moist climate would have greater precipitation

38 Chapter 2 Description
Table 4

Examples of Water Source as a Property of Hydrogeomorphic Classification

ExamplaaofWater Sourcel

(and climatic setting)

Ecoiogicai Significance or

Quaiitativa Scaie2 Quantitative Eatimate3 Functions4 Characters Maintained

Precipitation (moist climate). Precipitation dominates site-water Precipitation > PET during Rarity of water table drawdown Biogenic landscape isolates

balance and water supply to plant growing season. promotes organic matter mineral soil from access by

community.6 accumulation, which further plants; low primary production

retards drainage; paludification is eventually results.

promoted.

Lateral surface or near-surface Discharge commonly exceeds Duration and frequency of Overbank flow contributes to both Conditions maintained for high

transpori from overbank flow bankfuil-channel capacity.

(mesic climate).

overbank flow to floodplain can be flashy hydroperiod and vertical primary productivity and complex

inferred from hydrography and accretion of sediments. This habitat structure.

floodplain elevation. creates rapid biogeochemical

cycling and supplies nutrients.

Groundwater (GW) discharge to Seeps occur at bases of hillslopes Hydraulic gradient of groundwater GW supplies nutrients, renews Conditions conducive for stable

wetland (mesic climate). or below breaks in slope, and increases with distance from water, and flushes potential plant piant community of high

along edges of streams and lakes. wetland. Substrate permeabie growth inhibitors. productivity. Peat accumulation

enough to allow flows. possible.

Both GW discharge and, during Nonatmospheric sources greatly Precipitation cc PET during

flood flows, lateral surface

High water tables are maintained Water supplies support vegetative

exceed supply from precipitation. growing season. by catchment supplies from

transport from upstream (arid

complexity and habitat structure

upstream and from GW sources. not found in uplands because of

ciimate). water stress in arid climate.

All 3 sources, but precipitation is Alternate drought and wet periods Precipitation c PET. High water levels induced by High primary production occurs

minor (subhumid to semiarid). produce decade-iong cycles of precipitation; GW discharge when water is abundant;

water table fluctuations. prevents extreme drawdowns; decomposition is rapid enough

wetland may recharge GW when during dry periods to prevent peat

water table is high; conserves/ accumulation.

reduces GW discharge when

water levels are normal.

1 Five simple examples with three climatic settings are represented by dominance of one type (first three) or mixtures of several sources of water illustrated in Figure 9:

precipitation, groundwater, and lateral surface or near-surface flow. In floodplains, groundwater is normally transported subterraneanly from the upland to the wetland. Surface

water may be delivered by overbank flow from the stream channel or from channelized or nonchannelized flow by riparian transport.

2 Climate and source of water are combined to illustrate four different types of wetland. Roughly speaking, the mesic climate has precipitation approximately equivalent to PET, the

humid ciimate has precipitation> PET, and the arid climate has precipitation<< PET.

3 Climatic records allow calculation of potential evapotranspiration (PET), wNch can be determined from the empirical formula of Hoidridge et ai. (1971 ): mean annuai

biotemperature x 58.93 = PET in millimeters, where mean annual biotemperature is the average of all values >0 “C. Where precipitation fails short of PET, sources of water other

~han precipitation reduce site-water deficits. Supplies from groundwater and inflows from upstream are necessaty to maintain wetland conditions in all but moist climates.

This coiumn describes mechanisms by which hydrology contributes to and maintains wetland conditions.

5 Other ecologically significant functions may exist; those listed serve oniy as examples.

6 High amounts of orographic rainfall are necessaty to maintain waterlogged conditions on sloped landscapes (Lwo, Brinson, and Brown 1990b).

‘“l I I
than PET (PET >1 ).) The six ecosystems are prairie pothole marsh, ombrotrophic peat bog, high-gradient riverine forest, low-gradient riverine

forest, tidal freshwater marsh, and tidal saltmarsh.

“ Prairie pothole marsh. (May also apply to kettles, Carolina bays,

playa lakes, and other depressional wetlands with limited catchment

area.)

- Arid climate. Under climatic conditions of the northern prairie

(central Iowa and northward), wetlands have a negative

atmospheric water balance (precipitation minus PET) ranging

from -10 to -75 cm (Winter 1988). Depressional storage during

each year is a function of previous runoff and climatic

conditions. Groundwater interaction with wetlands occurs

through a nested set of local to regional flow systems in which

recharge occurs at water table highs and discharges at water table

lows. Both recharge and discharge can occur in depressions

containing wetlands; it is common for a wetland to receive

groundwater discharge in one part and recharge in another and

for seasonal reversals to occur. The variation of both

atmospheric and groundwater interaction induces fluctuations in

water quality (LaBaugh 1989) and wet-dry cycles (ephemeral

wetlands) characteristic of multiyear successional cycles in

prairie pothole marshes (Weller and Fredrickson 1974).

— Cool, moist climate. Conditions probably are not conducive to

maintain the depressional configuration of prairie potholes or the

marsh vegetation associated with them. Instead, peat-forming

wetlands are likely to develop in depressions, and paludification

may propagate more poorly drained conditions across the

landscape (Heinselman 1970). Where depressions may have

occurred in the landscape at one time, potentially inherited

topographic features have been obliterated by paludification and

replaced with new topographic features. If domed peatlands

develop, characteristics described below (ombrotrophic peat

bogs) apply. Deep kettle lakes that are oligotrophic may remain

as open water rather than undergoing hydrarch succession as

typified in many ecology textbooks.

● Ombrotrophic peat bogs.

– Arid climate. Conditions are antithetical to development of this

ecosystem.

40

– Cool, moist climate. This wetland type is dependent upon

precipitation exceeding PET. Where climatic conditions are

marginally conducive for peat accumulation, restricted drainage

is likely required to initiate peat accumulation.

Chapter 2 Description
—

—

“ High-gradient riverine.

– Arid climate. Small catchment areas with intermittent flow

seldom support wetlands. In such climates, wetlands are

dependent primarily on groundwater seeps, or, in larger drainage

basins, inflows from upstream. Both V-shaped and U-shaped

bedrock valleys in the Sierra Nevada region of California tend to

be barren of riparian vegetation in comparison to valleys

containing glacial till (Harris 1988).

– Cool, moist climate. As with the low-gradient floodplain

described below, water may play a major role as a geomorphic

agent for transporting sediments, nutrients, and organic matter,

In the smaller streams, woody debris can be considered a

geomorphic agent that is supplied through biologic processes

(Triska and Cromack 1980, Sedell and Beschta 199 1). Where

present, beaver result in further modification of the

geomorphology (Naiman, Johnston, and Kelley 1988).

“ Low-gradient alluvial floodplain wetland.

– Arid climate. In arid regions, wetlands and streams on alluvium

of a floodplain, when adequately saturated with water during

moist periods, may act as a source of recharge to water tables in

the vicinity. Flows from upstream are often critical to riparian

plant communities because of little local rainfall at low

elevations (Zimmerman 1969), as exemplified in regions where

flow diminution occurs with decreasing elevation (i.e., losing

streams). In such situations, riparian vegetation becomes

increasingly dependent on water supplied from upstream.

Detention of water in reservoirs on tributary streams places

additional stress on riparian vegetation downstream.

– Cool, moist climate. Unlike the situation described for arid

climates, only protracted droughts are likely to create extreme

drawdown of water tables under natural conditions. In fact,

water may be more important either as a geomorphic agent or as

a stressor because of waterlogging rather than having an effect on

water storage. Physical disturbance from ice floes may abrade

the vegetation in colder regions (Bliss and Cantlon 1957). Such

flooding may result in accretion of sediments on the floodplain in

some areas and scouring in others (Hardin and Wistendahl 1983).

It appears that as floodwaters become less limiting in

supplementing site water balance, they become more important as

a medium for transporting nutrients and sediment. For floodplain

wetlands assembled along a gradient from low to high order

streams, the ratio decreases between groundwater and run-on

sources and overbank flooding.

Chapter 2 Description

41
“ Tidal freshwater marsh.

—

—

—,

– Arid climate. Conditions would not be conducive for this type

of marsh because sustained sources of freshwater runoff are

lacking.

– Cool, moist climate. The freshwater source from stream

discharge and the opposing tidal wedge contribute to gradients of

salinity in the estuary on the large scale. Within this gradient,

tides provide the energy to flood these areas, and consequently

are the immediate regulating force on hydroperiod of the wetland

(W. E. Odum 1988, Odum et al. 1984, Simpson et al. 1983).

“ Tidal saltmarsh.

– Arid climate. In arid regions where estuaries receive minimal

dilution from freshwater inputs, tidal flushing is the principal

mechanism to mitigate hypersaline conditions in estuaries and to

prevent porewater salinities from exceeding the tolerance limits

of marsh vegetation (Beare and Zedler 1987). Fresh water, when

it is episodically delivered, becomes the driving force for species

change, sediment supply, and relief from osmotic stress created

by hypersaline pore waters. Hence, the ecosystem is controlled

by a frequently repeated (usually twice daily) signal from the

tides on one hand and infrequent freshwater pulses on the other.

– Cool, moist climate. Where PET is less than precipitation, salt

marshes develop in estuaries with ranges in salinity from

oligohaline to euhaline. Salt marshes in these moist climates

may develop a broad zone of brackish wetland, normally without

an interior zone of hypersaline pore waters like Ewing and

Kershaw (1986) describe in James Bay coastal marshes. Lower

evapotranspiration in colder climates, such as the marshes of the

Atlantic Coast of Canada (Roberts and Robertson 1986), the

Pacific Northwest of the United States (Ewing 1983), British

Columbia, and Alaska, would not subject plants to extreme water

stress and may account for the higher species diversity in the

marshes of cooler climates (Vince and Snow 1984) as well as the

declining importance with increasing latitude of species with C-4

metabolic pathways (Thompson 199 1).

42 Chapter 2 Description
Hydrodynamics

Background

The term hydrodynamics, as used here, refers to the motion of water

and the capacity of that water to do work (i e., transport sediments, flush

hypersaline water from sediments, transport nutrients to root surfaces).

For example, as water loses velocity when it moves across the surface of a

floodplain after overbank flow, it also loses its capacity to maintain sediment particles in suspension. The kinetic energy of such floodflows is recorded in the sediments as a progressive decrease in mean particle size

from the streamside levee toward the floodplain interior. Ideally the kinetic

energy of water could be quantified in terms of its capacity to do work in

the ecosystem. Additional examples of such work include surface erosion

(Leopold, Wolman, and Miller 1964) and rafting of litter (Hardin and

Wlstendahl 1983); aeration of sediments; and dispersal of seeds and other

propagules to more favorable microsites (Huenneke and Sharitz 1986;

Schneider and Sharitz 1986). Because of the stochastic nature of most of

these processes, the capacity or opportunity for performing work is seldom

quantified. Hence, we are constrained to making inferences about hydrodynamics based on velocity of flow, rate of water table fluctuations, particle size distribution of bedload sediments, and the capacity to replace soil

moisture deficits created by evapotranspiration.

Figure 13 portrays three qualitative categories of hydrodynamics:

(a) vertical fluctuations of the water table that result from evapotranspiration and subsequent replacement by precipitation or groundwater discharge into the wetland, (b) unidirectional flows that range from strong

channel-contained currents to sluggish sheet flow across a floodplain, and

(c) bidirectional, surface or near-surface flows resulting from tides or

seiches. These prevalent directions of water movement correspond, respectively, to the geomorphic setting categories (i.e., depressional, riverine, and

fringe) covered previously. The rate of water level fluctuations of the hydrography in Figure 13 are somewhat indicative of hydrodynamic conditions.

Examples of hydrodynamic settings

Table 5 provides elaboration of the three categories of hydrodynamics.

Each of these is more fully described below.

Vertical fluctuations. Although the hydrodynamics of all wetlands exhibit a vertical fluctuation component, it occurs in its simplest form in depressional wetlands. Two of the major variables affecting vertical fluctuation are

the rate of evapotranspiration and the frequent y at which water deficits are

replaced by groundwater transport and surface flow. An unlimited number

of scenarios could be developed matching duration and intensity of evapotranspiration with the frequency and intensity of water delivery to the wetland site. In arid climates where drawdown can be extreme and prolonged,

Chapter 2 Description

43
—

-

VERTICAL

.--

FLUCTUATIONS

..- r.-

,.

‘.

Figure 13. Categories of hydrodynamics based on dominant flow pattern:

(a) vertical fluctuations normally are caused by evapotranspiration and precipitation, (b) unidirectional flows are horizontal

surface and subsurface, and (c) bidirectional flows are hori44

zontal across the surface

Chapter 2 Description
Table 5

Examples of Hydrodynamic Properties of Hydrogeomorphic Classification

Examples of Hydrodynamics’ Qualitative Evidence* Quantitative Evidence3 Functions4 Ecological Significance

Vertical Fluctuation of Water Table6

Seasonal fluctuations nested Prairie pothole region. Soils Aerial photos show variable year- Landscape a mosaic of ponds Flyway and breeding sites for

within multiyear cycles. diagnost~ of dominant water to-year extent of flooding. Hydro- varying in depth at a single point

sources.

waterfowl. Retention of water

graphs of water table confirm both in time. Floodwaters retained by results in aquatic/moist habitat in

short- and long-term fluctuations. depressions. otherwise semiarid conditions.

Drawdowns of WT interspersed High ET, but supply by rain in Hydrography confirm that water Precipitation and ET dominate Fluctuating WT conducive to rapid

between frequent rain events that poorly drained landscape of table fluctuates widely.

fully saturate sediments.

site-water balance. Floodwaters biogeochemical cycling; strong

impewious sediments creates retained by depressions. atmospheric exchanges.

wetland conditions.

Drawdown extreme during course Arid climate or sources of Hydrography confirm that water Frequent deficits in site-water Support of rare plant and aquatic

of growing season; periods of recharge minimal. table is low during long periods. balance result in ephemeral

flooding brief.

communities such as vernal

aquatic ecosystems because of pools?

temporary floodwater storage.

Alternating recharge and Wetland narrow and adjacent to Water table hydrography

discharge varying with stream

Hgh exchange between channel Substrate well aerated and

channel; sediment texture coarse. proportional to and coincident with end groundwater. Temporarily flushed; hydrophytic vegetation

stage.g stream hydrography. stores floodwaters; conserves/ may occur in well-aerated soils.

reduces GW discharge.

(Sheet 1 of 3)

Note: ET= evepotranspiration; V/l= water table; GW = groundwater.

‘ Hydrodynamics is a measure of the kinetic energy of water that takes into account velocity, turbulence, and rate of change of water table position. Because velocity changes so

much temporally, sediment particle size can be useful for integrating hydrodynamics.

2 Potentially useful indices that require little or no laboratory analysis. Usually observable in field.

3 Velocities can be measured with sensitive current meters; water table fluctuations can be measured within the wetland with continuous stage recorders or frequently monitored

wells without instrumentation.

4 Processes or conditions that contribute to ecological significance.

5 Other ecologically significant functions may be present; those provided are only examples.

6 Not mutually exclusive with horizontal ubidirectional and bidirectional flows listed below.

7 Hubbard (1988).

8 Zedler (1987).

9 Stanford end Ward (1988).
Table 5 (Continued)

Examplea of Hydrodynamics Qualitative Evidence Quantitative Evidence Functions Ecological Significance

Vertical Fluctuation of Water Table (Continued)

Nearly constant water table at or Relatively constant WT position Water table hydrography have Stable WT encourages peat Landscape patterns dominated

near surface. suggests low ET because of cool, little fluctuation and are at or near accumulation. Where ET is low, by biogenie process of peat

moist climate; if ET is high, strong surface. A cool, moist climate may ombrotrophic conditions promote accumulation that is vulnerable to

groundwater discharge required. suggest low ET; otherwise, strong bog formation. Strong ground- changes in drainage and climate.

GW discharge must be assumed. water sources encourage For seepage slopes, species

development of fens or maintain composition reflects waterlogged

seepage slopes. soils that are nevertheless well

flushed and not strongly reduced.

Unidirectional Flow

Flow velocities correspond with Coarse sands and cobble Currents strong enough to export Strong currents ensure active Strong downstream transport

high-gradient Iandforms. sediments; pool and riffle bedform. fines; particle sizes confirm high

Evidence of flooding is transient

geomorphic Iandform. Wetland

fluvial energy.

processes. Well-aerated water

well flushed because of high supports coldwater fish

(e.g., debris lines, tree damage). turnover rate of water. populations.

Flow velocities correspond with Fine to coarse sediments (silts Measurements of flow velocities Interspersion between well- Interspersion of low- and highmiddle-gradient Iandforms. and sands); easily observable and sediment particle size confirm flushed and stagnant areas on energy environments supports

flow; point bars develop. Evidence middle-gradient condition. floodplain. During flooding, strong complex food webs. Possesses

of flooding is persistent (e.g., transport of particulate matter capacity to import nutrients or

sediments, disturbance-dependent occurs. export toxins.

plant communities).

Flow velocities correspond with Fine sediments (silt-clay and high- Slope, flows, and particle size Residence time of water allows

low-gradient Iandforms.

Good conditions for trapping

organic content); barely perceptible distribution all confirm low-energy long contact between water and

flow during flooding.

sediment and altering water

system. sediment; low-suspended load quality. As nutrient trap, food web

allows light penetration. support is strong. Reducing

conditions favor strongly obligate

wetland species.

(Sheet 2 of 3)

’11
Table 5 (Concluded)

Examples of Hydrodynamics Qusiitstive Evidence Qusntitstive Evidence Functions Ecoiogicsi Significance

Bidirectionsi Fiow

Astronomic tides:

Reguiar fiooding (iow marsh or Adjacent to estuary; frequent Sea-ievei controiied; daily to Very active region for Many known attributes of intertidal

fringe mangrove). flooding. twice-daiiy tides. biogeochemicai process and wetlands.

estuarine food web support.

irreguiar fiooding (high marsh Landscape position iandward of Fiooding during extreme tides and Infrequent events transport sait Leading edge of iandward wetiand

or basin mangroves). iow marsh or fringe mangroves. during storms. and maintain distinctive ecotone migration responding to rising sea

between haiophytes and upiand. Ievei. Transition establishes barrier

to saiine water and unique habitat.

Wind-generated water ievel Wetland is adjacent to iake:

fluctuations from seiches of iarge

Shailow water and vegetation Contributes to food web support

Strongiy influenced by lake if at Hydrography show strong evidence contribute to habitat complexity

iakes. iake ievei.

and habttat maintenance for

of mupling between wetiand water and food production. aquatic and amphibious species

table and wetiand surface. in region of otherwise featureless

iake bottom and shoreiine.

Likeiy GW-supported if on slope Hydrography of wetiand WT

above iake ievei; minimal iake proportional to lake ievei, but

input in most years. consistently above it (except in

extreme GW drawdown/drought).

(Sheet 3 of 3)

Ill
—

—

phreatic water must be available to sustain some species of vegetation

(Gatewood et al. 1950). At the other extreme, the short and cool growing

season of Arctic tundra wetlands or the continual water source from a

groundwater seep provides little opportunity for vertical fluctuations of

the water table. Five examples of vertical fluctuations are given in Table 5

(Vertical Fluctuation of Water Table; five rows). They range from seasonal fluctuations within multiyear cycles of wet and dry to nearly constant water table at the soil surface. As in Tables 3 and 4, the logic is

developed for deriving functions and ecological significance. These range

from the reduction of physiological stress by the flushing and displacement sediment porewaters at microenvironmental scales to the maintenance of waterfowl populations along flyways at intercontinental scales

(Table 5).

Once the sediments are saturated and gas diffusion to belowground processes is restricted, deeper flooding in itself probably has little influence

on most ecosystem processes. There are exceptions, however, such as surface flows, which are largely responsible for transport of suspended sediments. Except for small seedlings that may become completely

submerged and die, most tree species can tolerate at least temporarily

flooded conditions. However, unseasonably deep floods during the growing season may result in physiological stress to trees (Bacchus 1992).

Fluctuations below the wetland surface can have important consequences

for root aeration and microbial activity. Alternating reduced and oxidized

conditions facilitate interdependent biogeochemical processes that otherwise would become spatially isolated under static conditions. For example, it is well established that denitrification, an anaerobic process,

requires vitrification (NH$ to NO;), an aerobic process, before the sometimes high ammonium concentrations in pore waters can be depleted (Patrick and Tusneem 1972).

Examples of quantitative expressions of vertical fluctuations are presented in Figure 11 (a-c). The functions resulting from these fluctuations

can be translated into ecological significance. Whether the direction is

vertical, horizontal and unidirectional, or horizontal and bidirectional,

water movement represents an important factor in characterizing a wetland. Suboptimal plant performance and selection for only the most floodtolerant species occur under stagnant conditions when water currents are

too weak to supply nutrients, flush toxins from sediments, and transport

sediments to the wetland surface.

Unidirectional horizontal flow. Unidirectional flow of water can

range from barely perceptible surface and near-surface movement to strong,

channel-contained erosive currents. Dominance by surface transport is a feature that distinguishes most wetlands from uplands. In terms of sediment

transport, the supply for deposition and the loss by erosion governs whether

the sediment balance for a site is positive or negative. This in turn is controlled largely by the capacity of a stream to supply sediment. It is axiomatic

that wetlands undergoing erosion will have a very short life expectancy or

have a very limited surface area. While the transport of water, nutrients,

48 Chapter 2 Description
—

—

and organic matter from wetlands to downstream ecosystems may be considered a positive attribute of some wetlands, excessive export obviously

jeopardizes the self-sustaining properties of wetlands.

The three velocities of unidirectional flow listed in Table 5 span the extremes normally found in wetlands. More discrete categories could be developed to span the continuum between these extremes. The low end

member (Table 5, Unidirectional Flow, third row) might be characteristic

of the surface flow in a fen portion of a wetland complex in northern Minnesota where the spatial arrangement of groundwater recharge and discharge

alters water chemistry in such a way that bogs and fens are interspersed

(Siegel and Glaser 1987). The high energy end member (i.e., “... high

gradient landforms”) might be represented by Arctic streams subjected to

strong currents and damage to vegetation by ice floes (Bliss and CantIon

1957).

The three velocity or energy regimes represented in Table 5 under unidirectional flow are accom anied by a range in qualitative descriptors, but

Y -1 quantitative values (i.e., m s ) are not given. Presumably, quantitative

scales would best be developed for geographic regions where similar patterns of landform allow resolution of high and low energy regimes.

Within the function column, documentation is provided where available.

The ecological significance column of these functions is highly speculative, but there is some documentation for specific geographic regions. Regardless, the proposed qualitative changes that occur when water flow and

landform gradient interact create relatively sharp differences among ecosystem functions. The reasons for such changes in “state” are due to the

concomitant changes in many interdependent variables. This is why a

classification for wetlands cannot be fragmented into a series of independent variables that can be later reassembled into an unrestricted number of

combinations.

Bidirectional flow. Current velocities of bidirectional tidal flows on

the wetland surface tend to be low, on the order of 1.5 cm S-l or less for

South Carolina salt marshes (Wolaver et al. 1985). Inorganic suspended

sediments in flood tides between a tidal creek and marsh ranged between

6.0 and 68.4 mg L-l (Wolaver et al. 1988). While any given tidal cycle is

unlikely to have much influence on net sediment transport, nutrient exchange, and detritus movement, the cumulative effects of nearly two tides

per day becomes a dominant force in these ecosystems. The variation in

water transport and water chemistry among tidal cycles makes difficult

the development of reliable seasonal and annual water budgets. However,

progress has been made in dealing with the error associated with variations

in concentrations over space, the asymmetry associated with tidal events

(Reed 1987), and the seasonal changes that must be dealt with to develop

a water budget (Whiting et al. 1985). In fact, Chalmers, Wlegert, and

Wolf (1985) have suggested that precipitation on exposed sediment during

ebb tides may be responsible for most of the particulate organic matter export from salt marshes. For estuarine food webs, however, access for fish

Chapter 2 Description

49
—

—

and other nekton to the food-rich surface of the marsh during inundation

is a major factor in contributing to trophic support.

Bidirectional flows can be generated by astronomic tides and by wind.

Both regular and irregular flooding occurs within the astronomic tidal

group (Table 5, Bidirectional Flow, first row). Regular flooding ranges

from semidiurnal tides on the Atlantic and Pacific coasts, to diurnal tides

on the Gulf coast. Landward and upslope of the regularly flooded portion

of a tidal marsh is the irregularly flooding portion. Functions of regularly

flooded marshes include high rates of primary production, nurseries for

finfish and shellfish, habitat for wading birds, intense biogeochemical cycling, and others (Teal 1986). As many of these functions have been

shown to diminish with tidal amplitude, it follows that irregularly flooded

portions of the same marsh are likely to have both a reduction in these specific functions and a qualitative change in functions. As an example of a

qualitative change, the landscape role of “high marshes” may be of fundamental importance in initiating landward migration in a regime of rising

sea level (Stevenson, Ward, and Kearney 1986; Brinson 1991), in maintaining the pattern of groundwater salinity, and in providing habitat for

marsh residents not adapted to the deeper and regular flooding.

Water level fluctuations forced by winds are most apparent in lakes and

nontidal or microtidal estuaries (Table 5, Bidirectional Flow, row 2). The

latter occur in North Carolina because the narrow inlets of the Outer

Banks attenuate tides. Sustained wind “set-up” can create floods up to

and exceeding 1 m in depth on marsh surfaces (Brinson, Hook, and Bryant

199 1). In the fresher portions of these estuaries, forested wetlands occupy

the eroding shorelines (Brinson 1989). Eustatic rise in sea level forces

these wetlands to vertically accrete so that plants do not become subjected

to excessive flooding. For lakeside wetlands, seiches are the source of

water level fluctuations. These wetlands dissipate wave energy, provide

areas of refuge for small fish, and provide waterfowl habitat. Herdendorf,

Raphael, and Jaworski (1986) describes some of the fringe wetlands in the

Great Lakes region, but points out that many of these have been impounded, and thus isolated from flooding by seiches. The Black Swamp

region of the Ohio shoreline of western Lake Erie was a vast area of forested wetlands that extended far beyond the fringe marshes (Kaatz 1955).

Studies on freshwater fringe wetlands are not abundant (Prince and D’Itri

1985), especially in comparison with saltwater ones (Lugo 1990).

50 Chapter 2 Description
—

—

3 Indicators of Functioning

In the foregoing sections, Tables 3, 4, and 5 provided the logic of connecting fundamental wetland properties with ecological significance. It

was assumed that with sufficient information on geomorphic setting,

water source, and water movement, it should be possible to make reasonable judgments on how these physical properties can be translated into

wetland functions. This is an inductive approach and should remain open

to testing and further development. What is missing from this sequence

for a practitioner who classifies and determines functions is a methodology for estimating or quantifying these properties for a particular wetland.

This requires a deductive approach. In practice indicative of functions

are need rather than the functions themselves.

Indicators can be observed in the field (high-water marks, species composition, soil texture, etc.) or can be derived from other data sources

(maps, water quality data, etc.). They can be used both to aid in classification of a wetland and to predict whether a wetland has a particular function. In other words, they are surrogates for function rather than function

itself. There would be little gained in using indicators for groundwater

discharge or overbank flooding if one could depend upon observing these

directly on any given field visit. To illustrate indicators and how they can

be interpreted to reveal water source and associated habitat features, three

examples are given.

a. Prairie potholes. The salinity of water in prairie potholes provides

insight into both the water source and fate. Seepage conditions

range from potholes with outflow only (Figure 14c) to ones with

seepage inflow only (Figure 14b). These two extremes correspond

to fresh and saline end points shown in Figure 14a. A balance

between inflow and outflow results in intermediate salinity (Figure

14d). From such information, inferences can be made about the

source of water and to some extent hydrodynamics.

b. Brackish marsh salinity. If a coastal brackish marsh contains water

of 17 ppt salinity, one can normally assume that about half of the

water was derived from sea water (35 ppt) and the other half from a

combination of freshwater runoff and precipitation (<0.5 ppt).

Consequently, rather than salinity being a hydrogeomorphic factor

Chapter 3 Indicators of Functioning

51
—

—

1+Not saapaga outflo~ 1+Net seepage Inflow*

g

E!

1-

3

0

u

u

<

n

w

1#

4

Fresh Brackish SalIn{

SALINITY

Land

?-Water

Figure 14. The relationship between seepage conditions (groundwater

flow) and water salinity in prairie potholes. (a) Left-hand side

of graph is fresh with net seepage outflow. Right-hand side

of graph is saline because all the water that flows into the

wetlands is evaporated. Diagrams b, c, and d illustrate crosssectional profiles and direction of water

(1972)

52

flows. From Sloan

Chapter 3 Indicators of Functioning
—

—

itself, it is an indicator of the sources of water, the geomorphic

setting (near or at sea level), and, less reliably, hydrodynamics

(probably tidal).

c. Hydrography of flooding. Hydrography can be used as indicators to

determine water source and hydrodynamics. This was illustrated in

Figure 10. Such hydrography often can be interpreted to show when

wetlands are flooded, how deeply they flood, and the thresholds of

discharge for associated streams that are likely to transport bedload.

Duration and depth of flooding might be interpreted further to

indicate a function of supporting fish populations, for example.

Welcomme (1979) demonstrated the positive relationship between

flooding duration and fisheries yield in African floodplain-river

systems. In addition to floods providing access for fish to the

floodplain surface, the topographic complexity of floodplains that

result from hydraulic energy contributes to fish habitat (Leitman,

Darst, and Nordhaus 1991 ).

Characteristics of both the water and soils or sediments of a wetland tend

to roughly correspond with the three major hydrogeomorphic characteristics and, thereby, the functioning of the wetland itself (Table 6). Organic

Table 6

Examples of Indicators for Water and Soil Often Associated

with Hydrogeomorphic Characteristics

Hydrogeomorphic characterietice

Indicator Geomorphic setting Watersource Hydrodynamic

Water Characteristics

Suspended sediments’

Low Depressional Precipitation, groundwater Mostly vertical

High Ftiierine or fringe Lateral surface Bidirectional or unidirectional

Organically stained Depresaional or riverine Surficial groundwave? Vertical, unidirectional

Salinity Fringe Lateral surface Bidirectional

High pH in moist climate Fen or fringe Groundwater, marine origin Unidirectional, bidirectional

Soil or Sediment Characteriatice

Organic Not in fringe if strongly tidal Precipitation, groundwater Vertical, unidirectional

Finegrained mineral Not in high-gradient riverine Headwater floods Overbank flow

Cobble-sized mineral Riierine Headwater flmds Unidirectional

Note: Geographic variation of the characteristics will vary widely. Consequently, they should be developed for local

wetfand types. It is unlikely that one or two indicators would reliably characterize a wetland.

1 If standing water is subjected to wind mixing, resuspended sediments maybe high in any category.

2 Groundwater derived from sandy soils that are not wetlands often have high concentrations of humic and fulvic

compounds as indicated by water color.

Chapter 3 Indicators of Functioning

53
—

—

soils (Table 6, column 1) normally develop in environments that have low

hydrodynamic energy. Some indicators are better at eliminating several

possibilities than at choosing a specific one. For example, organic soils

do not develop in strongly tidal creeks of fringe wetlands. (However,

tidal creeks may incise previously developed organic sediments by headward erosion.) Organic soils, of course, typify ombrotrophic bogs in lowenergy environments where precipitation and evapotranspiration control

vertical fluctuations rather than lateral displacement of water. Because

the source of bog water is precipitation, both the water characteristics

(low-ion content) and the sediment (peat) are indicators of the source of

water, the hydrodynamic environment, and, by extrapolation, the geomorphic setting (i.e., poorly drained, generally flat, and elevated above the

nearby landscape).

Each of these generalizations has obvious limitations. One might find

many exceptions where indicators are not reliable, especially if their use

is not restricted to a fairly homogeneous group of wetland types or to a

uniform climatic regime. Indicators are not intended to replace general

knowledge about hydrology, biogeochemistry, and geomorphology, but,

rather, to provide shortcuts to predicting functions. Their reliability in

doing so requires verification through testing.

Table 7 proposes a list of parameters that could be used as indicators.

Indicators may be either qualitative or quantitative. Some tentative suggestions are given for their ecological significance. In contrast to “water quality,” which has overtones of societal use and values, “water characteristics”

are interpreted in terms of what they reveal about ecosystem function. For

example, low concentrations of suspended sediments allow light penetration

in the water column. Adequate light potential y could support aquatic primary production by phytoplankton and epibenthic algae (Minshall 1978).

In contrast, high-suspended sediments represent a resource of nutrients

that may contribute to the fertility of a soil, while accumulation of sediments may provide a mechanism for geomorphic adjustment of the wetland surface to a changing hydrology such as rising sea level.

Water Characteristics as Indicators

Salinity is a strong selective agent of plant life form in nontropical

zones because it determines whether the wetland will be dominated by

graminoids and herbs (i.e., marshes) rather than woody plants (i.e., shrub

wetlands and swamps). Salinities below that of sea water indicate that terrigenous or atmospheric water sources are present; hypersaline conditions

provide additional information on local water balance. Where saline soils

develop inland, some of the same halophytic genera (Distichlis, Salicm-nia,

Salkornia, Suaeda) proliferate as those found in coastal marshes (Stewart

and Kantrud 1972). Halophytes can be roughly ranked according to their

salinity tolerance range, although many factors such as hydroperiod often

vary in parallel with salinity and thus make it difficult to independently

54 Chapter 3 Indicators of Functioning
Table 7

Interpretation of Potentially Useful Indicators of Ecological Significance

Parameter (units) Range Ecological aigniflcsncs

Water Characteristics

Suspended sediments, mg/L

Low ‘4?0 Low suspended sediment indicates either weak sediment

source or weak hydrodynamics. Light transmission may

facilitate establishment of SAVS and mamo algae.

High >50 Abundant sediment source reduces light transmission;

hydrodynamics sufficiently strong to transport sediment.

Salinity, parts par thousand

Slightly brackish 4-3 Terrigenous sources dominate water source; sediments

potentially abundant.

Estuarine 3-30 Favorable for salt marsh and mangrove ecosystems.

Sea water Ca. 35 Lacking terngenous source% likely nutrient-limited growth.

Hyparsaline *O Arid climate; wetland processes limited by salinity stress.

Inland salinities (i.e., Variable Potential evapotranspiration exceeds precipitation.

prairie potholes)

Color (platinum units)

Clear Low absorbance Source predominantly groundwater in contact with soils finer

than sand. Allows light penetration if suspended sediments

are low (see above).

Black High color Source in contact with organic sediments or derivdd from sandy

soils. Complex organics may chelate and mobilize metals.

PH

Acid <5 Favors Sphagnum domination, restricts denitrification.

Circumneutral >5 Water sources other than precipitation are likely (i.e., grourxtwster

and estuarine sources).

Nutrient Status

Low Oligotrophic Lack of contact with mineral soil.

Medium Mesotrophic Not distinctive.

High Eutrophic Potentially high primary and secondary production.

Soil or Sediment Characterietica

Mineral Sand, silts, and Thorough flushing; water table dries down during growing season

days to facilitate decomposition of soil organic matter.

Organic sediments (peat) High percent IOS Long hydroperiod; low decomposition rates.

on ignition

Note: Indicators may not be useful in all wetland types (e.g., salinity).

evaluate the influence of either. Animal communities are often distributed

within relatively narrow ranges of salinity (Remane and !.khlieper 1971).

In freshwater wetlands, adaptations to the range of conditions are a bit

more subtle than adaptations to salinity. However, the often autocorrelated

factors of pH, conductivity, hardness, and alkalinity have been useful in

the classification of peatlands (Moore and Bellamy 1974). Concentrations

of these factors and availability of plant nutrients commonly correspond

Chapter 3 Indicators of Functioning

55
—

—

to whether the wetland is classified as a bog, a rich fen, or a poor fen, for

example. On a broader scale of freshwater wetland types, water chemistry

has been used only marginally. Turbidity and darkly stained waters tend

to be mutually exclusive in the coastal plain of the Southeast. “Backwater streams” originate in the coastal plain and tend to be sediment and nutrient poor while “alluvial rivers” are sediment rich and originate in the

more highly erodible mountains and piedmont.

Water color is sometimes diagnostic of the history of groundwater before it discharges to the surface. For example, streams flowing through

clayey soils may be either clear or turbid, but their water is not normally

stained by humic and fulvic compounds. In contrast, water discharging

from sandy aquifers often retain whatever organic content they may have

acquired before becoming part of the surflcial groundwater. Similarly,

acidic pH may be reflective of a low-ionic content, and hence weak buffering

capacity, or may have sufficient concentration of organic acids to maintain

low pH. Circumneutral conditions are favored where groundwater has

had extensive contact with mineral soils, particularly carbonate lithology.

Nutrient status is seldom a definitive indicator of function, but may

have important implications for rates of primary production and food web

support. For example, water chemistry and algal species composition

have been used extensively as an indicator of the trophic states of lakes

(Rawson 1956). Similarly, the water chemistry of peatlands has been related to site productivity and species composition of vegetation (Moore

and Bellamy 1974). In fact, when the rate of nutrient delivery to oligotrophic wetlands is increased substantially, changes in species composition

usually occur. The most common species replacement is by Typha spp.,

which can invade shrub bogs when wastewater is discharged into them

(Guntenspergen and Stearns 1985). Extended hydroperiod and deeper

flooding, presumably without enhanced nutrient supplies, also favor dominance by cattails (Wilcox, Apfelbaum, and Hiebert 1985).

In contrast to some of the gross physical features discussed in preceding sections, water characteristics, such as elemental compositicm, provide

quantifiable tests that can be used to compare one water source with another. Because of the overlap that may occur between water chemistry

and geomorphic features, the characteristics in Table 7 are meant to provide additional dimensions to the classification rather than proliferate the

number of wetland types.

Other Indicators of Functioning

56

Indicators have been used in other classifications. For example, the

Fish and Wildlife Service classification (Cowardin et al. 1979) uses several “modifiers” of water, pH, soil material, and salinity regimes. Their

usefulness for resolving wetland types is due, in part, to the fact that they

are a consequence of ecosystem processes and function. Indicators have

Chapter 3 Indicators of Functioning
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—

also been used extensively in assessing the opportunity of a wetland to fulfill a particular function (Adamus et al. 1987).

Topographic maps and remote sensing information can yield indicators

that are useful in interpreting geomorphic settings of a wetlands. Topographic maps can provide clues on the likelihood of groundwater discharge,

overbank flooding, and other similar processes. Wetlands bordering estuaries in tidal regimes will have characteristics of fringe wetlands. The

hydroperiod of those located next to a large lake will be determined by the

larger body of water and perhaps intermittent flushing from storms.

Small, isolated water bodies (i.e., no inlets or outlets) are depressional,

while linear features that are wetlands and are located adjacent to streams,

undoubted y have characteristics of riverine wetlands. The use of remote

sensing and maps, however, should not be substituted for field visits when

classifying wetlands.

Classification of wetlands into functional groups is no more precise

than the ability of the observer to record and interpret indicators and other

surrogates of ecosystem function. Indicators can help not only in determining the hydrogeomorphic type of wetland, but their interpretation can

lead directly to ecological significance (Table 7). Society often places

value on the ecological significance (Figure 1), but the relationship between the two is beyond the scope of the classification or this report.

Finally, it has been argued that developing a species list of an ecosystem

is the cheapest and most useful approach for answering questions about

function (Slobodkin et al. 1980). The rationale is that the life-history requirements of a species provide information on environmental conditions

(i.e., ecosystem type) to which the species is adapted. The emphasis on

physical factors rather than organisms in the hydrogeomorphic approach

is partially a choice of the scale around which one wishes to focus. By

choosing major forcing functions, the classification moves beyond the details of biogeographic variation in species composition and beyond the

boundaries of the wetland where the forcing functions generally originate.

However, once the classification is applied to a specific geographic region

of the country, it is appropriate to depend upon the adaptation of species

as indicators of environmental conditions. Wetland indicator status of

plants is an excellent example of this application (Tiner 1991).

Chapter 3 Indicators of Functioning

57
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4 Profile Development and

Reference Wetlands

Recognizing that wetlands represent a continuum between aquatic and

terrestrial ecosystems, it could be argued that there are no discrete categories of wetlands from the standpoint of hydrology and associated properties (Figure 12). The process of interpreting ecological significance from

the geomorphic setting, water source, and hydrodynamics illustrates how

one can arrive at a description of a wetland’s function without actually

placing it into a discrete category. This contrasts with most classifications

where a finite number of categories is chosen a priori, and each wetland is

evaluated to determine which category is the best match. (This open-ended

approach used here is intentional because different “classes” of wetlands

will emerge as the procedure is developed for different physiographic

regions of the country.)

The ecological significance column of Tables 3-5 and 7 can be used to

develop a “profile” for a wetland. Profiles are the hydrogeomorphic descriptions of wetland ecosystems resulting from the information developed during a site visit or more detailed study. They can be narrative

descriptions or appear in tabular form as quantitative data. Profiles are

the end point of the classification as presented in this document. For a

particular region of the country, one could develop an array of profiles to

describe the existing wetland types. (This process of profile development

is addressed below.) Gebhardt et al. (1989) suggests the use of narrative

descriptions to describe the “state” of riparian systems to reveal dominant

hydrologic processes such as substrates and soils, discharge and channel

capacity, and flow regime. By preparing such narratives, one is forced to

recognize the processes that are responsible when an ecosystem changes

from one state to another.

Closely tied to profile development is the establishment of reference

wetlands. These wetlands represent benchmarks against which other wetlands can be compared for various purposes including assessment, training, and mitigation. The process of developing profiles is intended to

result in a population of reference wetlands. Those involved in assessment can utilize this documentation as standards against which to compare

wetlands being assessed in the future.

58 Chapter 4 Profile Development and Reference Wetlands
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.,

Profile Development

A narrative form has been used below to illustrate the development of

profiles for three very different wetlands. (These are not actual sites, but

are prepared to illustrate the process.) To do this, information from the

ecological significance columns are abstracted and modified from Tables

3, 4, and 5. Within each table, the categories and row numbers are identified (first, second, etc.). (“Table 5, second row” under Unidirectional

Flow indicates that the information was taken from within the Unidirectional Flow heading in Table 5, and represents the second entry below the

heading.)

Examples of profiles are developed below for an ombrotrophic bog, an

arid riparian ecosystem, and a groundwater seepage wetland. Each of the

items listed can be more thoroughly documented by research results from

similar wetlands. The Community Profiles of the U.S. Fish and Wildlife

Service are essentially extensive, composite profiles developed from

many studies and numerous localities. Other synthesis efforts and original research from the primary literature can be assembled to develop composite profiles.

Ombrotrophic bog profile

Table 3, first row under Extensive Peatland. The wetland site and

much of the surrounding landscape is occupied by peatlands similar to

that described by Glaser et al. (1981) in northern Minnesota. Upland

areas are scarce. Consequently, wetland-upland interactions are minor relative to atmospheric exchanges of gases such as carbon dioxide

(Armentano and Menges 1986) and methane (Matthews 1987) and interception of precipitation and dry fall.

Table 4, first row (Precipitation). As a consequence of peat accumulation, plants are dependent exclusively on “new” nutrient sources from

precipitation and recycling from organic matter. Low primary productivity is a consequence of these conditions as well as the lack of access to

nutrients from weathering in mineral soils.

Table 5, last row under Vertical Fluctuation of Water Table. Because of nearly constant water table and low-flow velocities, accumulation

of peat is encouraged; weak lateral flows minimize export of particulate

organic matter. Of the dissolved organic matter that is exported, most consists of humic and fulvic compounds capable of transporting heavy metals

(Saar and Weber 1982). Existing landscape is vulnerable to changes in

drainage because of strong coupling between landform and hydrology and

to changes in climate because of climatic control of water balance.

Chapter 4 Profile Development and Reference Wetlands

59
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Arid riparian profile (middle-gradient stream floodplain

in an arid region.)

Table 3, third row under Riverine Wetlands. Wetland in this middlegradient landform differs from surrounding landscape by displaying a preponderance of woody vegetation and high structural complexity (Brinson

et al. 1981; Brinson 1990). Such corridors of forest provide habitat for

many songbirds and other wildlife (Johnson and Haight, In preparation;

Knopf 1986). Active geomorphology assures interspersion of plant communities, thus contributing to beta diversity.

Table 4, fourth row. Water supply supports vegetative complexity and

habitat structure in contrast to poorly developed vegetation in arid uplands. Floodplain topographic features are varied and complex, providing

the template for interspersion of several plant communities ranging from

eariy successional, shade-intolerant species to those occupying more stable sites (Campbell and Dick-Peddie 1964). Sediments consist of allochthonous mineral sediments.

Table 5, second row under Unidirectional Flow. Interspersion of

low- and high-energy environments supports complex food webs. Because they are relatively well flushed during flood events and aerated near

surface, accumulation of organic matter is prevented. Consequently, they

possess a high capacity to import nutrients and export toxins.

Groundwater seep profile

Table 3, last row under Depressional Wetlands. Groundwater flow

provides abundant water when regional water tables are sufficiently high.

When these drop, evapotranspiration in the wetland may deplete storage

in pore waters and desaturate sediments. In cases where groundwater discharge occurs at the face of a slope (Figure 6), flows may maintain saturated conditions year-round, resulting in shallow but predictably stable

water-table flooding. Availability of this water source during dry periods

may contribute to beta diversity of the landscape.

Table 4, third row. Seepage wetlands tend to have high nutrient supplies from groundwater sources. Resources are abundant and the environment is predictable, making conditions conducive to relatively high

primary production and biomass accumulation.

Table 5, fifth row under Vertical Fluctuations. The water table is stabilized by groundwater flow. Such water replacement may maintain high

redox levels relative to stagnant saturated soils, thus allowing the establishment of plant species that are not restricted to strongly reducing environments.

60 Chapter 4 Profile Development and Reference Wetlands
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Reference Wetlands

Reference wetlands are a product of profile development. They can

represent a particular site for which a profile has been developed, or they

can represent a population of sites that exhibit a range of variation within

a particular functional type. Reference wetlands should be sites where indicators have been tested, measured, and related to corresponding ecosystem functions. Depending on the state of knowledge, it may be necessary

to conduct research in these areas to facilitate the development of better

or new indicators, and even unforeseen ecosystem functions. The most

crucial criterion for reference wetlands is that they include representatives

of natural or quasi-natural wetlands that either occur presently in the region or occurred there at one time. This array of different wetlands needs

to be established and protected so they can represent “types” similar to

type specimens in herbaria, type localities for geologic formations, and

type series of soils.

The array of reference wetlands should include those that have been degraded or disturbed. Profiles of degraded wetlands, however, should document the nature and history of the disturbance. The extent to which they

differ from relatively undisturbed references may provide insight into the

relationship between function, structure, and the nature of the disturbance.

Disturbed wetlands may actually perform some functions better than their

natural counterparts. They may be useful in training because they force

one to identify the source of the disturbance and its effect on function, positive or negative. When dealing with disturbed wetlands, one should keep

in mind that there are both qualitative and intensity components to disturbance. For example, harvest of timber may have the effect of altering

wildlife usage (a specific quality), but the effect may be short-lived if the

forest regenerates adequately (low intensity). Irreversible disturbances

“should be distinguished from reversible ones.

Reference wetlands could also be used to standardize EMAP sites

(Liebowitz, Squires, and Baker 1991) and should serve as the reference

for goal-oriented wetland construction (Kentula et al. 1992). Areas in

which there is some level of protection and in which research is normally

conducted include the National Estuarine Research Reserves, National Research Parks, and the National Science Foundation’s (NSF) Long Term

Ecological Research (LTER) sites. The NSF LTER sites could serve as a

model for reference wetland sites. In fact, LTER sites that have wetlands

should be reviewed first as potential sites. However, it is unlikely that

LTER sites offer the full range of wetland types that are needed for a comprehensive coverage, especially ones that have been disturbed.

Chapter 4 Profile Development and Ref6rence Wetlands 61
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5 Toward Functional

Assessments

This manuscript is not an assessment tool, but rather a tool that can be

used to develop an assessment methodology. For a particular group of

wetlands, the development of profiles will begin to reveal both the major

functions of the wetland types and the hydrogeomorphic basis for the functions. The evaluation of the functioning of wetlands is envisioned as a

major outgrowth of this classification scheme (Smith 1992). By studying

in detail the functioning of various reference wetland types, one should be

able to extrapolate to other similar wetlands on the assumption that wetlands with similar landscape position and shape (i.e., geomorphic setting),

similar location with respect to water sources, and similar slope and catchment area (i.e., hydrodynamics) will also have similar functions. The confidence that one can place in such comparisons depends on how well the

reference wetlands span the range of conditions being assessed and how

sensitive the methodology and indicators are for resolving differences.

In the 404 review process,l an applicant for a dredge and fill permit

could be required to identify the functional type of wetland in the application, and within reason, identify which reference wetland it most resembles

in the same physiographic region. Presumably, the reference wetland (or

a population of them) will have had their functions assessed previously

and will be supported by a fairly comprehensive profile (or group of profiles), possibly including data from research and monitoring efforts. The

regulatory team, having been trained locally or regionally in the functioning of each wetland class, will be prepared to assess the similarity between

the one described in the permit application and the reference wetland population for which a body of knowledge exists. It would be incumbent

upon the applicant, if conditions warrant, to illustrate which functions do

or do not exist relative to the reference wetland both before and after the

‘ The details of the regulatory process are not discussed because they are beyond the scope of

this report and because they become obsolete as regulations change. The permit review sequence is

outlined in the Corps of Engineers Regulatory Program Regulations (33 CFR Parts 320-330),

Environmenttd Protection Agency 404(b)( 1) Guidelines (40 CFR Part 230), and several

Memoranda of Agreement between the Corps and EPA.

62 Chapter 5 Toward Functional Assessments
project. The burden of proof would then rest with the applicant to

demonstrate the presence or lack of similarity.

Each population of wetland types or classes would be identified with a

unique list of functions and associated indicators. (If the list is not unique

in some way for the wetland type, then it is probably not adequate for assessment. In other words, the classification was not specific enough and

the functional profile was not adequately prepared.) For example, a workshop in early 1992 identified the functions listed in Table 8 for alluvial

floodplain swamps of the Georgia Piedmont. Note that discharge of a

Table 8

Four Major Groups of Functions Identified in the Assessment of Alluvial

Floodplain Swamps of the Georgia Piedmont

Hydrology Biogeochemistry Plent Communfty Food Web/Hsbitet

Flood-peak attenuation Nutrient and contaminant Beesl area and stocking Balanced faunal species

Base-flow augmentation retention from: Balanced species

Wave celerity

composition

precipitation compos”hion Interspersion of plant

Sediment retention overbank transport Reproductive and gap

Surface water storage

communities and of plant

riparian transport processes immunities and open

detention storage Biogeochemical water

retention storage maintenance of water Complex vertical

stratification

Detritus stocks intact

Note: All characteristics were present in the relatively unmodified reference wetland. Indicators for each characteristic

have not been fully developed. A glossary of terms is needed to define critical terms. The list was developed by L. C.

Lee, M. M. Brinson, W. Nutter, R. D. Smith, end D. Whigham.

deep groundwater aquifer to the alluvium does not appear on the list. This

is because the alluvium of Piedmont floodplains is bedrock confined and,

consequently, insufficiently permeable to provide water flow to the alluvium. Also, the functions based on vegetative structure would need to be

different if one were assessing the habitat variables in depressional wetlands such as prairie pothole marshes. This is in stark contrast with the

Wetland Evaluation Technique (Adamus et al. 1987), which is used as a

tool to determine the probability that a wetland will perform a particular

function. In the hydrogeomorphic approach, the functions for the wetland

category are established first in the process of developing profiles and accumulating a population of reference wetlands. It is not until one or several reference wetlands have been assembled that an assessment is

actually attempted. The reason for this is that reference wetlands provide

scalars against which a particular “project” wetland can be assessed.

Just as reference wetlands will serve as examples of types that are identified in the classification, the classification system itself must undergo

change as new data become available and the hydrogeomorphic controls

become better understood. As structure and species composition of vegetation become part of the description, regional classifications should be

tailored to fit important quality, hydrologic, and habitat features that are

Chapter 5 Toward Functional Assessments

63
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representative of a particular region. For example, certain fringe wetlands

contribute little toward maintaining the water quality of estuaries as indicated by the fact that exports and imports of sediments and nutrients are

often in balance, or at least similar enough so they do not consistently represent either a large net import or export (Wolaver et al. 1988). Such tidal

marshes would be better assessed for other functions, such as their contribution to maintaining estuarine fish populations.

The example just given illustrates a distinction that should be made between the assessment of wetland functions and the estimation of wetland

values. In a functional assessment, tidal marshes should be evaluated for

their capacity to maintain fish populations and not fisheries. The distinction is that the maintenance of fish populations is a property of the ecosystem, not the human population that may or may not happen to use it

(Figure 1). The manner in which fish populations are utilized by a given

human society changes with the economy, among cultures, and throughout

history. Likewise, floodplains in Ohio and in Louisiana may function similarly in terms of biogeochemical cycling and hydrology. Such attributes

would be “useful” or “valuable” for wetland rice farming in Louisiana but

not in Ohio. Similarly, prairie potholes in the upper plains states may be

important habitat for waterfowl (a function) in the Mississippi Flyway, but

also have societal value (i.e., be useful and valuable to hunters). Depressional wetlands in the Carolinas (i.e., Carolina bays) may function similarly

by providing habitat for endangered species of vascular plants or amphibians (i.e., a function of the wetland and a biodiversity value for humans).

No assessment technique on wetland function is likely to be robust

enough to first evaluate the level of a particular function and then further

distinguish whether the function is part of a human-based value system.

Functional assessment should be grounded squarely in the natural sciences. Resource use and values should be handled by the appropriate disciplines of economics, sociology/anthropology, and resource management.

Each of these disciplines has its own techniques, but they usually focus on

one or several commodities of wetlands and are not sensitive to the forces

that are responsible for the self-maintaining properties of the ecosystem itself. One of the main differences between a functional assessment and an

economic assessment is that they use totally different indicators. A potential pitfall of any assessment system, whether functional or otherwise, is

the confusion that develo s when trying to handle too many disciplines with

a single set of indicators. P One of reasons for presenting the functional clas-

‘ Energy analysis is an example that uses a fundamental, thermodynamically grounded set of

techniques drrdindicators. However, energy analysis, as it is now developed, is too

time-consuming and conceptually rigorous to be appropriate for the applications discussed here for

wetlands. See H. T. Odum (1988) and Costanza (1980) for explanations and examples. More

recently, attention has been focused on the shortcomings of standardized economic assessment

procedures. Specifically, the United Nations System of National Accounts (SNA) does not

depreciate natural resources such as forests and soils when they are removed or irreversibly

damaged. Instead, the SNA ignores assets such as forests and soils, but further treats their

destruction as an increase in income rather than a loss of wealth (Solorzano et al. 1991).

64 Chapter 5 Toward Functional Assessments
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classification in this manuscript, without an assessment component, is that

the classification is the logical first step that is needed to clear the path

for a manageable assessment procedure.

The array of key wetland types that emerge as reference wetlands can

be used not only for the purposes of characterizing and quantifying various aspects of wetland function, but also as standards to evaluate wetland

construction and restoration projects. In this sense they become the standards of success in contrast to relying on endless lists of design criteria

and performance standards. One of the most valuable uses may be in the

training of wetland scientists who will be involved in work on permit review, assessment of functions, construction of new wetlands, and restoration of degraded ones.

Highly modified wetlands may require an additional level of classification. Because the hydrogeomorphic approach is to be driven by function,

it may be useful to classify according to existing, modified functions,

rather than attempting to reconstruct whatever original functions might

have existed. Because there is a finite number of functions that can be

supported by a given wetland type, and this constraint limits the intensity

at which functions can operate, management toward a specific function

(like agricultural crop production) will necessarily compromise other functions (e.g., the dictim that only one thing at a time can be maximized).

Nevertheless, examples abound where management is oriented toward multiple uses. In river floodplains of some regions of Africa, dry-season cattle grazing on floodplains is compatible with wet season fish utilization

(Welcomme 1979). Usually when a wetland is managed for a particular

commodity or value, the capacity to maintain competing uses is reduced.

In the cases of the floodplain systems described by Welcomme, there may

be complementary or even synergistic uses. If future options are to remain open, however, modification of the wetland should not be irreversible. In this regard, it is worthwhile to remember such long-term goals as

the sustainable utilization of species and ecosystems.

For a classification to undergo change and improvement, its structure

must not be so rigid or complex that it cannot be modified by its users. It

should be clear at this point that the classification, as presented above, is

not intended for a user to take it to the field for the purpose of matching

indicators with functions. Rather, the report is intended to show how

some fundamental knowledge about water flows and sources can be interpreted to illustrate ecological functioning. This purpose is lost if one bypasses the development of profiles and the establishment of reference

wetlands. It is considered essential to understand the basic underpinnings

of wetland function. Much of the hyperbole surrounding the protection of

wetlands attributes multiple functions to wetlands that have not been rigorously documented. Until such carelessness is corrected, little progress

can be made beyond the current state of understanding.

65

Chapter 5 Toward Functional Assessments
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66

Local adaptations of this classification, and ultimately assessments, are

anticipated to incorporate the use of either locally recognized wetland

names or a modification of cover types with modifiers from the Fish and

Wildlife Service’s classification (Cowardin et al. 1979). A greater number of specific categories may be advantageous to some geographic regions (Table 9). However, unless the logic is followed that reveals the

functions and ecological significance of those functions,, then the basis for

function has not been well articulated and probably not understood. Conducting rigorous assessments of poorly understood functions or doing a

poor job assessing well-understood functions is not in the best interest of

wetland science and resource management.

Table 9

Categories of Geomorphic Setting, Water Source,

and Generalized Functional Types

Geomorphic Setting Functlonel Typss

Blanket bog/raised Blanket bog

Depressional Raised bog

neither inlet nor outlet Surface-water depression

surface inlet only Groundwater depression

surface outiet only Groundwater slopa

surface inlet and outlet High-gradient channel

Slope Low-gradient channel

Channel Stream floodplain

Floodplain Lake floodplain

Fringe Channel fringe

Lake fringe

Watar Source

Precipitation

Upslope runoff

Groundwater (regional or perched)

Channel flow (perennial and intermittent)

Overbank flow (stream or lake)

Tides (astronomical and wind)

Note: Presentad by Frank Golet, University of New Hampshire, at the Stone Mountain Workshop.

It could be argued that one of the problems with classification and assessment procedures is the shortage of qualifled people to do them. Resources usually are not availabk? to support assessments by professionally

trained teams of hydrologists, ecologists, and geomorphologists. Likewise,

it is probably futile to expect a classification or assessment technique to

substitute for such expert opinion. Most routine needs for assessment

probably do not justify what might be construed as “overkill.” Consequently, what is needed is a reliable and defensible approach that utilizes

as much as possible the body of information that is currently available on

wetlands. Thus, professionals who are conducting the assessments must

either be trained or train themselves to learn the fundamentals of hydrology, geomorphology, plant and animal ecology, biogeochemistry, etc., and

apply these disciplines to interpreting information that is available on wetlands. There is no substitute for applying knowledge about wetlands in

reaching valid assessment conclusions. The use of reference wetlands as

Chapter 5 Toward Functional Assessments
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training vehicles should facilitate the learning process for the following

reasons: (a) The number of wetland types that need to be mastered in a

given locality is probably less than five or six. Thus, one does not have to

deal with an infinite array of wetlands to adequate] y assess a few types.

(b) Reference wetlands create mental images that can be carried from one

site to another. Although observable structure alone might be misleading

in terms of functional interpretation, 1 visual acuity may be as reliable as

lists of individual indicators. Mental images, although powerful, are not

defensible scientifically and should not substitute for narrative descriptions in functional profiles and, if possible, quantitative data. (c) The environmental setting of reference sites in agricultural, urban, and other

disturbed locations makes one aware of the effects of modifications on

wetlands, especially alterations of hydrology and how the history of

human activity may account for their current functional condition.

While there is little new in the hydrogeomorphic classification that has

not been demonstrated previously, it does require a synthesis of information and a fairly rigorous interpretation of information to reveal functions.

There are essentially no limits to the amount of information that could be

brought to bear in functional classification. Consequently, there is much

to be done in constructing profiles, generating and testing indicator lists,

and establishing reference wetland populations. The process is completely

open to new information and modification. However, we are still poorly

positioned to have definitive answers in all aspects of wetland functioning. One of the challenges that makes this an exciting endeavor is the potential to discover new functions and reveal their ecological significance.

‘ Personal Communication, 1991, Dennis Whigham, Plant Ecologist, Smithsonian

Environmental Research Center, Edgewater, MD.

Chapter 5 Toward Functional Assessments

67
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Appendix

Glossary

Some of the terms are defined specifically for wetlands or this classification. Consequently, definitions may differ from those more broadly applied. If so, they are marked with an asterisk (*). Definitions marked

with a ~ were taken from Fetter (1988 ).1

Abiotic—Not living. Deposition of suspended sediments on floodplains

is an abiotic process.

Accretion—Vertical accumulation of sediments or organic matter. If organic matter is accumulating as a result of photosynthesis, the process

is biotic and may result in biogenic landscapes such as peat bogs.

Aerobic—Occurring in the presence of free molecular oxygen. Obligate

aerobic bacteria cannot be active in the absence of oxygen.

Alkalinity—The capacity of water to buffer changes in pH. The carbonate buffering system is the most common. See buffered water and hardness.

Allochthonous organic matter—Organic matter that is transported to a

site rather than being produced by photosynthesis at the site.

Alluvial—Pertaining to alluvium, or material transported by flowing

water.

Alluvial swamp—A forested floodplain wetland with soils consisting generally of fine-grained sediments that have been deposited by overbank

transport of sediments from a stream.

Ammonium—A reduced inorganic form of nitrogen in a monovalent cation form (NI@. Ammonia (NH3) is a gas.

] References cited in this appendix are included in the References at the end of the main text.

Appandix A Glossary

Al
—

—

Aquifer, confined-An aquifer that is overlain by an aquiclude or

aquitard, and thus does not have a water surface in direct contact with

the atmosphere.

Aquifer, perchedT—A region in the unsaturated zone where the soil may

be locally saturated because it overlies a low-permeability unit.

Aquifer, phreatic—The zone that contains unfrozen fresh water.

Aquifer, surficial-The uppermost region of the aquifer that is near the

land surface.

Aquifer, unconfined-An aquifer that is in direct vertical contact with

the atmosphere through open pores. Synonymous with water-table aquifer.

Artesian—The condition of water flowing freely from uncapped wells because it originates from a confined or semiconflned aquifer for which

the potentiometric surface is above the ground surface.

Basin wetland—See depressional wetland.

Bedload, sediment—Sediments that move along the bottom of a stream

channel.

Bedrock-confined channel—A stream channel that has as its bottom the

bedrock that is normally undergoing erosive downcutting.

Beta diversity—Variety of organisms occurring in different habitats of a

region in”contrast to alpha diversity, the variety of organisms within a

small, homogeneous area.

Biogenic—Derived or originating from living material, as peat.

Biogeochemical—The interaction and integration of biological and geochemical cycles.

Biogeographic—Refers to the distribution of species on the surface of

Earth.

Biotic—Refers to living processes or entities.

Backwater streams—Streams common in the southeastern United States

that have high concentrations of dissolved organic carbon and humic

compounds, resulting in a darkly stained water.

Bog—A peatland that is nutrient poor because it lacks access to substantial quantities of mineral-rich water.

A2

Bottomland—General term that refers to floodplain wetlands.

Appendix A Glossary
Brackish—Water containing salt. Normally a mixture of fresh water and

sea water.

Buffered water—Water that is resistant to changes in pH. See alkalinity

and hardness.

C-4 plants—Vascular spermatophytes that are usually monocots, and possess anatomy and physiology that allow high water use efficiency and

rates of photosynthesis that tend not to saturate in full sunlight.

CapiIIarity—The phenomenon of adhesive forces between water and solids that results in matrix potential in soil-water systems.

Capillary fringe—The saturated zone in soils above the water table as a

result of capillarity.

Carolina bays—Depressional wetlands of the Carolinas and nearby states

to the north and the south. They normally have a sandy rim, the long

axis is orien’ted northwest to southeast, and they may be filled with

open water or completely vegetated. Sediments range from sands to

clays which can be overlain by peat or organic-rich soils.

Catchment—A term similar tq watershed, which consists of all of the

land upstream from a point where rainfall may potentially flow.

Channel capacity—The discharge of a stream just prior to overbank flow.

Channelized flow-Flow that is confined to a channel in contrast to unchannelized (nonchannelized) flow or overland flow.

Chelate—A reversible complexation of an organic molecule normally

with an ionic form of a metal.

Circumneutral—Water with a pH of around 7.

Conductivity—The capacity of water to conduct electrical current. Conductivity is proportional to the ionic content of the water. Units are

normally micromhos per centimeter, which is equivalent to the S1 units

of microSiemens.

Cumulative rise in water table*—The sum of increases in water table

over a specified period of time, normally as a consequence of influxes

of water because of precipitation and lateral surface transport.

Cutbank—The outside meander of a stream channel that is undergoing

erosion by lateral migration of the channel.

Cypress strands—Shallow drainages dominated by cypress trees. Channels are poorly defined such that overbank flow is quickly exceeded

with minor increases in discharge.

Appendix A Glossary A3
—

—

Denitrification-The microbially mediated heterotrophic process of converting nitrate or nitrite to either nitrous oxide or dinitrogen gas.

Depressional wetland*—A wetland located in a depression in the landscape so that the catchment area for surface runoff is generally small.

Depressional*—A wetland geomorphic setting that occurs in depressions,

but usually at the headwaters of a local drainage. Consequently, surface flows are restricted.

Detritus—Organic matter undergoing decomposition, with the attendant

protists, protozoans, and other organisms that serve as food for detritus

feeders.

Discharge—The volume of flow per unit time, such as m3/sec.

Distichlis spicata—Salt grass.

Drawdown of water table*—The phenomenon of a natural decrease in

water table usually as a result of evapotranspiration, or unnatural decrease as a result of consumptive withdrawal of groundwater.

Edaphic (control)—The controls on plant-species distribution or function

as a result of conditions in the soil in contrast to atmospheric controls.

EMAP—Environmental Monitoring and Assessment Program is an Environmental Protection Agency program to assess long-term changes in

ecosystem performance or “health. ” EMAP-Wetlands is the component

limited to wetland ecosystems.

Epibenthic algae—Algae that live on the bottom or benthos of an aquatic

ecosystem.

Euhaline—Approximately the salinity of sea water, or about 35 ppt.

Evapotranspiration—The combination of evaporation and transpiration

expressed in the same units as precipitation.

Fen—A peatland that is fed by groundwater. Poor fen—A peatland that receives groundwater flow and achieves productivity intermediate between that of a rich fen and an ombrotrophic bog. Rich fen—A highly

productive peatland often dominated by grasses or trees in contrast

with shrubs and mosses.

Floodplain—The land beside a river that receives overbank flooding

when discharge exceeds channel capacity.

Flow, groundwater—Water that flows below the land surface through a

porous medium normally under saturated conditions.

Appendix A Glossary
—

—

Flow, near-surface* —Flow that occurs just below the surface of a wetland in a layer that is often more permeable than the more consolidated

sediments just below.

Flow, surface—Nonchannelized flow (unchannelized) that occurs above

the surface. Overland flow.

Fringe wetland*—A wetland that is located near a large body of water,

most typically the ocean, and receives frequent and regular two-way

flow from astronomic tides or wind-driven water-level fluctuations.

Function (ecosystem) *—Processes that are necessary for the self-maintenance of an ecosystem such as primary production, nutrient cycling, decomposition, etc. The term is used primarily as a distinction from

values. The term “values” is associated with society’s perception of

ecosystem functions. Functions occur in ecosystems regardless of

whether or not they have values.

Functional profile* -Narrative or quantitative information on a wetland

being assessed that describes the ecological significance of properties

of water source, hydrodynamics, etc.

Geomorphic—A term that refers to the shape of the land surface.

Geomorphic setting*—The location in a landscape, such as stream headwater locations, valley bottom depression, and coastal position.

Geomorphology—The study of Earth’s surface and its development.

Graminoid—A grass or grass-like plant, such as sedges and rushes.

Gravity flow-Flow of water controlled by gravity instead of strictly

piezometric head differences.

Groundwater discharge—Flow originating from an aquifer that flows to

the surface.

Groundwater inflows*-Flow of water received by a wetland or some

other area as a result of groundwater discharge via lateral seepage or

upward movement.

Groundwater recharge—Flow of water from an area that contributes to

an aquifer. Most upland areas contribute to groundwater recharge.

Halophytes—Plants that are tolerant of salty water.

Hardness—A property of water that is roughly proportional to the ion conAppendix A

centration. ‘Water from calcareous aquifers is often hard because of the

calcium carbonate content. Such waters are very resistant to fluctuations in pH.

Glossary

A5
Hydrarch succession—The sequence of community changes that occurs

as an aquatic ecosystem fills with sediment and eventually, through

mostly extrinsic factors, develops into a terrestrial ecosystem.

Hydraulic conductivity~—A coefficient describing the rate at which

water can move through a permeable medium.

Hydraulic gradientt—The change in total head with a change in distance

in a given direction. The direction is that which yields a maximum rate

of decrease in head.

Hydrodynamics*—The motion of water that generally corresponds to its

capacity to do work such as transport sediments, erode soils, flush pore

waters in sediments, fluctuate vertically, etc. Velocities can vary

within each of three flow types: primarily vertical, primarily bidirectional and horizontal, and primarily unidirectional and horizontal. Vertical fluxes are driven by evapotranspiration and precipitation.

Bidirectional flows are driven by astronomic tides and wind-driven

seiches. Unidirectional flows are downslope movement that occurs

from seepage slopes and on floodplains.

Hydrologic—Dealing with the field of hydrology or the distribution and

movement of water.

Hydroperiod*—The depth, duration, seasonality, and frequency of flooding.

Hydrostatic head—Piezometric head. A position of higher water-table

stand relative to a lower one. Water flows toward decreasing hydrostatic heads, and not necessarily to lower elevations. See hydraulic

gradient.

Hypersaline—Generally values well above sea water, i.e., greater than

40 ppt.

Ice floes-Floating masses of ice usually associated with spring breakup

on lakes and rivers.

Igap6—Refers to the portion of the Amazon Basin not derived from the

foothills of the Andes where suspended sediments arise. In contrast,

these waters tend to be clear albeit somewhat stained.

Indicators (of function) *—Water chemistry, species composition, soil

characteristics, or some other feature that allows one to infer or predict

certain ecosystem functions or other conditions.

A6

Inundation—The condition of water occurring above the surface, i.e.,

flooding.

Appendix A Glossary
—

Juncus gerardi—A species of Juncus (a true rush) that occupies high or irregularly flooded portions of salt marshes from the mid-Atlantic to the

north.

Juncus roemerianus—The ecological equivalent of J. gerardi that is distributed in warmer climates from the mid-Atlantic southward including

Florida and the Gulf coast.

Kettles—Deep depressions in glaciated areas that resulted from the melting of an ice block that had been buried previously by glacial outwash.

These small lakes may undergo hydrarch succession and fill with peat

and become forested wetlands.

Kinetic energy—Energy of motion in contrast to stored or potential energy.

Landscape—Gross features of the land surface, including but not limited

to slope, aspect, topographic variation, and position relative to other

land forms.

Life form, plant—The general morphologic category of plants, such as

tree, shrub, herb, etc.

Lithology—Refers to the composition of Earth’s crust. The consequences

of weathering of this parent material may carry over to properties of

the soils that develop.

Mangrove—A general term for several of halophytic woody species that

occupy fringe wetlands. They tend to be restricted to climates that

have little or no frost.

Marsh—A wetland with emergent, herbaceous vegetation.

National Wetland Inventory—A program of the Fish and Wildlife Service that maps and categorizes wetlands of the United States. The categories used are those developed in the “Classification of Wetlands and

Deep Water Habitats of the United States.”

Near-surface flow-Flow that is not visible just below the surface. It

often occurs in the rhizosphere where hydraulic permeability is high.

See Seepage.

Nitrate—The most oxidized form of nitrogen that can be used as an alternate electron acceptor in anaerobic respiration.

Vitrification-The microbial transformation from ammonium to nitrite

and from nitrite to nitrate. It is an energy-yielding aerobic process.

Nonchannelized flow-Normally reserved for surface flow that is diffuse

and thus not confined to a channel. Also unchannelized and overland

flow.

Appendix A Glossary

A7
—

—

Nonpoint source—Diffuse sources of nutrients or contaminants, often

from agricultural and urbanized landscapes. They are in contrast to

point sources, which are discharged from a pipe.

Nurseries, fin fish and shellfish-Normally used to designate habitat critical for young stages of fish or shellfish.

Overbank flooding* -Refers to excess flow to a floodplain when discharge of a stream exceeds channel capacity.

Overbank, transport*—Movement of water from the channel to the

floodplain surface.

Ombrotrophic bog—A peatland that receives precipitation as the sole

source of water. Generally peat has accumulated enough to isolate the

plants from acquiring nutrients from the underlying mineral strata. The

elevated surface is indicative of tertiary mires.

Overland flow-Water movement parallel with the soil surface.

Paludification-The landscape phenomenon of organic-matter accumulation on mineral soil thus forming a histosol.

Palustrine—Nontidal wetlands where the salinity from ocean-derived

salts is less than 5 ppt. Further modifiers are used by the National Wetland Inventory.

Parts per thousand—See ppt.

Perched—Describes an aquifer that is underlain by an unsaturated zone.

Permeability—See hydraulic conductivity.

pH—The negative log of the hydrogen (hydronium) ion concentration.

Phreatic flow-The movement of water that occurs in an unconfined aquifer.

Physiognomy—The gross structure of a plant community resulting from

the dominance of life forms such as trees, shrubs, graminoids, etc.

Phytoplankton—Algae that are carried with water currents in contrast to

the relatively immobile and attached epibenthic algae.

Piedmont—The steeper, rolling physiographic province formed at the

base of mountains. Locally it is west of the Atlantic coastal plain and

east of the mountains.

A8

Pipe flow-Flow of groundwater that results from secondary porosity

(macropores) often formed by decayed root channels or animal burrows.

Appendix A Glossary
—

—

Plant-life form—The general morphologic category of plants, such as

tree, shrub, herb, etc.

Playa lakes—Shallow depressions similar to prairie potholes, but abundant on the Southern High Plains on a tableland south of the Canadian

River in Texas and New Mexico. They undergo annual and multiyear

cycles of drydown and filling.

Playa wetlands—See playa lakes.

Pocosin—Evergreen shrub bogs and fens of the southeastern United

States that frequently burn.

Pore water—Water that fills the interstices of soil or sediment.

Potential evapotranspiration (PET)—The amount of water that would

be lost by evapotranspiration from natural vegetation in a particular climate if water were never limiting during the year.

Potential evapotranspiration ratio (PET ratio) *—The ratio between

evapotranspiration and actual precipitation. Values greater than 1.0 represent water deficits. Wetlands in such climates must be supplemented

by other sources of water.

PET—See potential evapotranspiration.

Potentiometric surface—The elevation to which the water table of an

aquifer would raise if there were no confining layer. The potentiometric surface of an unconfined aquifer is the water table.

ppt—Parts per thousand, units generally used for expressing salinity.

Prairie pothole—Depressional wetlands in the upper Midwestern States

and the plains provinces of Canada.

Primary production—The conversion of solar energy into organic matter

by photosynthesis.

Principal-components analysis—A multivariate statistical analysis that

distributes data in two-dimensional space along gradients that represent

variables that appear to be independent.

Profile, wetland*—A qualitative or quantitative descriptive depiction of a

wetland that, in the case of the hydrogeomorphic classification, emphasizes the physical characteristics such as geomorphic setting, water

source, and hydrodynamics. Profiles also may include the biotic components.

Propagules—Reproductive structures, as the seeds or cuttings from plants.

Appendix A Glossary

A9
Recharge, groundwater—Addition to the storage component of an aquifer.

Redox—The potential difference, usually expressed in millivolts, between

a platinum electrode and a reference electrode in a solution. The scale

is especially useful for sediments that are devoid of oxygen because it

allows an expression of reducing conditions beyond the scale of oxygen.

Reference wetIand*—A wetland or one of a group of wetlands within a

relatively homogeneous biogeographicaI region that represents typical,

representative, or common examples of a particular hydrogeomorphic

wetland type, or examples of altered states.

Reference wetland population*—A group of wetlands of the same

hydrogeomorphic type that represents the variation that occurs within

the type because of natural or society-influenced causes.

Riparian*—Pertaining to the boundary between water and land. Normally represents the streamside zone and the zone of influence of the

stream toward the upland.

Riparian transport*—Movement of water from upland regions to floodplains either by groundwater discharge at the slope face or toe, and by

direct precipitation and overland flow.

Salicornia—Pickle weed, a salt-marsh plant,

Saturated—In reference to soils, the condition in which all pore spaces

are filled with water to the exclusion of a gaseous phase.

Seasonal programming—The genetically controlled behavior or activities of organisms.

SeepageT—A site where groundwater of a surficial aquifer discharges to

the surface, often at the toe of a slope.

Seiche—Harmonic water level fluctuations in large lakes resulting from

wind relaxation after a period of set-up.

Setting, geomorphic*—See geomorphic setting.

Set-up—The increase in water surface elevation downwind of a large

body of water because of sustained winds.

Shade intolerant—Normally refers to tree species that require full sunlight to survive the early stages of growth.

Source, nonpoint—See nonpoint source.

Spartina alternij70ra— Salt-marsh cordgrass. A true grass that dominates

regularly flooded portions of salt marshes.

Appendix A Glossary
—

-

Spartina cynosuroides—Giant cordgrass. A true grass that is most common in the fresher portions of salt marshes, especially in the high

marsh or the marsh-upland transition zone.

Spartina patens—Salt-meadow hay. A true grass that is common in the irregularly flooded zones of tidal salt marshes.

Stratigraphy—The vertical layering of sediments or other materials often

as a consequence of the chronological sequence in which they were deposited.

Stochastic—A phenomenon that varies over time, usually in an irregular

or unpredictable pattern.

Storage, specific~-In hydrology, it is the amount of water released from

or taken into storage per unit volume of porous medium per unit

change in head.

Storativity (storage coefflcient)~-The volume of water released from or

taken into storage per unit surface area of aquifer per unit change in

head. In an unconfined aquifer, the storativity is equivalent to the specific yield.

Stress—The condition of diverting potentially useful energy from an ecosystem or an organism.

Stressor—The factor or group of conditions that cause the stress.

Succession—The predictable and orderly change in species composition

over time at a particular location. Succession is sometimes called ecosystem development which places additional emphasis on abiotic components of change.

Swamp—An emergent wetland in which the uppermost stratum of vegetation is composed primarily of trees.

Swamp-stream *—Tentative: A headwater swamp containing a shallowly

incised and intermittently braided channel. As a consequence of lowchannel capacity, increases in discharge tend to be dissipated across the

entire floodplain. In Florida, they are called strands or sloughs.

Terrigenous—Of or deriving from terrestrial or land.

Tidal amplitude—The elevational difference between high and low tide.

Tidal marshes, irregularly flooded-Marshes located in a tidal region,

but too isolated to be inundated by all tides.

Tidal marshes, regularly flooded-Marshes located in a tidal regime

with elevations low enough to be flooded by nearly all tides.

Appendix A Glossary

Al 1
Topographic—A term referring to the slope and elevation of land.

Al 2

Transmissivity —The capacity of a porous medium to conduct water. It is

a function of properties of the liquid, the porous media, and the thickness of the porous media.

Transport, overbank*—Movement of water from the channel to the

floodplain surface.

Riparian transport* —Movement of water from upland regions to floodplains either by groundwater discharge at the slope face or toe or by

surface-water (overland) transport.

Turbidity-Low water clarity principally because of suspended

sediments.

Ty@a-The genus of cattail.

Unchannelized flow-Normally reserved for surface flow that is diffuse

and thus not confined to a channel. Also nonchannelized flow.

Upland—The land upslope from a wetland that lacks wetland characteristics.

Valuation—The process of ascribing values.

Values—The rules that determine what people consider important. It can

be measured by what motivates people into activity.

Vtfrzea-Lakes in the floodplain of the main Amazon basin where water

high in suspended sediments occurs.

Water quality—Descriptive or quantitative conditions of water, usually

in reference to the physical, chemical, and biological properties, and

usually from the perspective of society’s use.

Water stress—A water-deficit condition of plants that develops because

plants are losing water by transpiration faster than they can take up

water through their roots.

Water table—The surface of an unconfined water mass where the piezometric head equals atmospheric pressure.

Water table, rebound*—The night-time phenomenon of an increase in

water table after evapotranspirational drawdown during the day. The

cause may be due to redistribution of water among pore spaces toward

an equilibrium state or the adjustment toward hydrostatic equilibrium

with controlling piezometric gradients.

Appendix A Glossary
—

Wetland—Those areas that are inundated or saturated at a frequency to

support, and which normally do support, plants adapted to saturated

and/or inundated conditions. They normally include swamps, bogs,

marshes, and peatlands.

Appendix A Glossary Al 3
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L TITLE AND SUBTITLE 5. FUNDING NUMBERS

A Hydrogeomorphic Classification for Wetlands

5. AUTHOR(S)

Mark M. Brinson

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Biology Department

East Carolina University

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3. ABSTRACT (Maxtmum 200 words)

Wetlands can be classified by methods that range from the use of commonly recognized vegetation or

cover types, to systems based on hydrology, geomorphology, or some combination of the two, The classification presented here is based on the hydrogeomorphic functions of wetlands. There are three basic properties

that are used to provide insight into wetland functions:

1. Geomorphic setting—The three categories are depressional, riverine, and fringe. Extensive peatlands

constitute a separate category because of their unique topographic and hydrologic conditions. Depressional

wetlands can be open or closed to surface flows, and can be tightly or loosely connected to groundwater

flows, Riverine wetlands range from those associated with steep to low gradient streams and are represented

by floodplains. Fringe wetlands are sea level or lake level controlled. Peat lands normally initiate their development in depressions. If peat lands develop beyond the original depression, they can create their own

unique geomorphic settings. Each of these four types roughly corresponds with limited combinations of

water sources and hydrodynamic conditions.

(Continued)

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13. (Concluded).

2. Water source—The three sources are precipitation, lateral flows from upstream or upslope, and

ground water. Respectively these correspond to transport from the atmosphere, transport by surface or

near-surface flows, and subsurface transport by flow of groundwater from unconfined aquifers. Each of

these sources tends to have different water chemistry. This influences how the wetland functions. If precipitation is the sole source, evapotranspiration must necessarily be low enough to maintain water storage.

Transport by surface flows allows sediment to be delivered to the wetland surface, as in floodplains.

Groundwater sources are often rich in minerals. The flushing common in groundwater flows often counterbalances stressful conditions otherwise expected under waterlogged conditions.

3. Hydrodynamics-Velocities can vary within each of three flow types: primarily vertical, primarily

unidirectional and horizontal, and primarily bidirectional and horizontal. Vertical movements are due to

evapotranspiration and precipitation, unidirectional flows are down slope movement that occurs from seeps

and on floodplains, and bidirectional are astronomic tides or wind driven seiches. Where the vertical flow

type dominates, the wetland has characteristicly low hydraulic energy. Sediment accretion in such low energy environments is necessarily restricted to peat accumulation. Where flows are primarily unidirectional

and horizontal, they may range from erosive, as occurs during the cutoff process in meandering streams, to

depositional, as occurs in most floodplain environments. The bidirectional movement of tidal regimes creates predictable flooding and cumulatively long hydroperiods which are conducive habitat conditions for

many estuarine organisms.

Indicators of function are discussed also, but they are considered derivatives of the three basic properties. Indicators range between short-term and ephemeral to long-term and stable. Short term indicators are

high water marks, the annual plants, debris piles, etc. Long term indicators are geomorphic structure, forest

canopy species composition, and geomorphic features that can be determined from topographic maps and aerial photographs.

The ecological significance of each of the properties is quantified, if possible, from published studies on

similar ecosystems, or the significance is developed through logic. For the properties possessed by a particu- ‘

lar wetland, a “profile” is developed that reveals probable functions carried out by the wetland, both within

the wetland and as a landscape entity. Profiles are the end point of this classification. It is recommended

that a number of wetland profiles be developed in a geographic region which will constitute a reference wetland population. Reference wetlands should typify the functions of a given type as well as the probable variations of these functions within a wetland type. Such reference wetlands represent the basis of comparison

for any assessment procedures. Reference sites or types may also be established to recursively validate or

correct the classification and to serve as benchmarks for additional functional studies.

The classification can be adapted to any geographic region and is easy to modify as additional information becomes available. The present manuscript is not appropriate for direct use, but can serve as the basis

for developing a classification within a distinct biogeographic region.

14. (Concluded).

Classifications of wetlands Indicators of ecosystem functioning

Geomorphic setting Reference wetlands

Hydrodynamics Water source

Hydrogeomorphics