Wetland and Water 

Supply

GEOLOGICAL SURVEY 

CIRCULAR 431
Wetland and Water Supply

By John A. Baker

GEOLOGICAL SURVEY CIRCULAR 431

Washington, D. C, I960
United States Department of the Interior

FRED A. SEATON, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

application to the U.S. Geological Surrey, Washington 25, 0. C
Wetland and Water Supply

By John A. Baker

Statement at Nortiieast Field Committee Meeting Eastiiam, 

Mass., Aug. 5, 1959.

The Geological Survey has received numerous inquiries about the effects of proposed 

changes in the wetland environment. The 

nature of the inquiries suggests a general 

confusion in the public mind as to wetland 

values and an increasing concern by the public with the need for facts as a basis for 

sound decisions when public action is required.

Perhaps the largest gap in our knowledge 

is in regard to the role played by the wetland 

in the natural water scheme. Specialists in 

such fields as agriculture and conservation 

have studied the wetland in relation to its 

special uses and values for farming and as a 

habitat for fish and wildlife. However, except as studied incidentally by these specialists, the role of the wetland with respect to 

water has been largely neglected. This facet 

of the wetland problem is of direct concern 

to the Geological Survey.

We commonly speak of water in terms of 

its place in the hydrologic environment as, 

for example, surface water or ground water. 

These terms imply that water can be neatly 

pigeonholed. With respect to the wetland environment nothing can be further from the 

truth. In fact, one objective of this discussion 

is to demonstrate that for the wetland environment surface water, ground water, and 

soil water cannot be separated realistically, 

but are closely interrelated and must be studied together. It should be noted that this 

statement holds true for the hydrologic environment in general, and that the wetland environment is by no means unique in this respect. Our second and principal objective is 

to identify some of the problems that must be 

studied in order to clarify the role of the 

wetland in relation to water supply.

We have chosen to approach these objectives 

by briefly describing one area for which we 

have some information, and by using this example to point out some of the problems that 

need study. First, however, let us define 

what we, as geohydrologists, mean by wetland 

and briefly consider wetland classifications.

For our purpose wetlands are land areas 

that are covered with shallow water or subjected to intermittent flooding and subsequent 

slow drainage, and which gene rally are characterized by an accumulation of organic matter hereafter termed "swamp deposits." 

These wetlands may be classified in a number of different ways depending on the purpose of the classifier. For example, the Fish 

and Wildlife Service classifies wetlands into 

20 different types based on water quality 

(fresh or salty), drainage, and vegetation. At 

the risk of some oversimplification we might 

consider 4 types of wetland from the standpoint of hydrology: (1) fresh-water swamps 

in which the swamp deposits are underlain 

by glacial till or bedrock; (2) fresh-water 

swamps in which the swamp deposits are underlain by marine or lacustrine clay and silt; 

(3) fresh-water swamps in which the swamp 

deposits are underlain by glacial outwash or 

alluvium consisting mostly of sand or sand 

and gravel; and (4) salt .marshes and salt 

meadows. The three fresh-water types of 

wetland are of interest with respect to water 

supply, and of these, the type in which swamp 

deposits are underlain by glacial outwash is 

of particular interest in New England.

In the Ipswich River basin above the Geological Survey gaging station at South Middleton, Mass., is an area of 44 square miles 

which forms the headwaters section of the 

basin. The relief of the area is low. About 

half the area consists of hills mostly underlain by bedrock but mantled by a thin layer of
glacial till. The other half consists of lowlands including swamps, low terraces and 

plains underlain by glacial outwash. Swampland, used hereafter as a synonym for wetland, forms about a fourth of the area. Some 

of the swamps occupy depressions in the till 

blanket and are situated at somewhat higher 

levels than the lowlands. The largest swamps, 

however, border the Ipswich River and its 

tributaries. Here the swamp deposits, which 

consist of muck and peat mixed with some 

silt and sand, overlie glacial outwash deposits, which consist principally of sand with 

subordinate amounts of silt, clay, and gravel. 

The swamp deposits are thin at most places 

but locally they may be as much as 55 feet 

thick. The outwash, which occupies the preglacial valley of the Ipswich, is more than 

100 feet thick at places over the old buried 

channel.

The outwash forms an extensive semicontinuous ground-water reservoir. The body of 

outwash is rather narrow along the main 

stem of the Ipswich but in the headwaters 

area it is broad and mantles the lower parts 

of the preglacial divides between the headwater streams. In general, the more permeable materials in the outwash occur along 

the margins of the valley where they form 

terraces and more or less isolated mounds, 

plains, and ridges. Less permeable materials occupy the deeper parts of the preglacial 

valleys and overlap or interfinger with the 

more permeable materials toward the sides 

of the valleys. These less permeable materials are overlain nearly everywhere by 

swamp deposits, which form a discontinuous 

semipermeable cover over about half the 

area of the ground-water reservoir. Bedrock 

together with a thin layer of glacial till forms 

a relatively impermeable floor beneath the 

outwash. During most of the year the reservoir materials are saturated with water nearly to the land surface.

In this part of the Ipswich basin, the towns 

of Reading, North Reading, and Wilmington 

draw their water supplies from wells. Several towns in the lower part of the Ipswich 

River basin draw water from wells, from 

ponds, or from the Ipswich River or its tributaries. Also, a number of towns outside the 

drainage basin draw water from streams or 

lakes within the basin.

Some wells draw water principally from 

ground-water storage in their immediate

vicinity. Others draw water not only from 

ground-water storage locally but also induce 

recharge from nearby streams during part 

of the year at least. During periods of no direct runoff the flow of these streams consists 

principally of ground-water runoff and, in 

effect, wells that induce recharge import 

water by way of the streams from upstream 

parts of the ground-water reservoir.

The ground-water reservoir, the swamps, 

and the streams together form a major watersupply system. To illustrate how this system 

operates consider atypical water year beginning in October. At this time of the year the 

surfaces of the swamplands are dry, and the 

water table stands in the swamp deposits or 

possibly below them at some places. Streamflow is low and is derived principally from the 

ground-water reservoir. Vegetation is discharging water from the swamp deposits either from soil-moisture storage or from the 

ground-water reservoir. With the first killing frost the discharge of water by vegetation 

is turned off. The first appreciable precipitation thereafter is accompanied by recharge of 

the ground-water reservoir, a rise in the water table, and an increase in runoff, including 

an increase in the base flow of the streams. 

From late fall throughout the winter the water 

table remains at or near land surface in the 

swamps. Ponded water is present during 

most of this time on the swamp surfaces. 

Perhaps some of this ponded water is a result 

of precipitation catch, some comes from, 

streams by overbank flooding, and some may 

come from upward seepage of ground water 

derived from the outwash at the periphery of 

the reservoir. In the spring, the melting snow 

and the spring rains assure a high water table 

and a fully recharged ground-water reservoir. Water remains ponded on the swamps. 

The rate of runoff of the Ipswich and its tributaries is high. The growing season begins 

and progresses. Water is discharged from 

the swamps to the atmosphere by evaporation 

and transpiration.

The ground-water reservoir continues to 

discharge to the streams. There is very little, if any, recharge to the ground-water 

body. For a time runoff is sustained by 

swamp storage that is water temporarily 

stored on the swamp surfaces as well as by 

ground-water runoff. Finally the swamp sur*- 

faces dry up and streamflow is sustained 

principally by ground-water runoff. If the 

summer is unusually dry the antecedent
moisture conditions in the swamp soils may 

generally be unfavorable to direct runoff except after heavy extended rains and so on to 

the end of the water year.

This picture is, of course, highly generalized. Nevertheless it shows us that the swamp 

deposits and the swamp surfaces have a role 

in the operation of the water system. Swamp 

surfaces store flood waters. Swamp deposits 

store water. With respect to the ground-water 

reservoir, the swamps serve as discharge 

areas for much of the year, but they may serve 

as recharge areas occasionally. When the 

swamp surfaces are dry the moisture conditions in the swamp soils affect the infiltration 

rate directly and the rate of direct runoff indirectly. Perhaps also the soil-moisture 

conditions affect the rate of recharge to the 

ground-water reservoir and the rate of discharge of ground water by evapotranspiration.

We know in general how the water system 

operates, but the details of operation are 

vague and poorly defined. Knowledge of these 

details is essential if people are to manage 

the water system effectively, and to protect 

and improve it. Such knowledge also is essential if the effects of changes in the hydrologic environment on the system are to be 

predicted with assurance.

In considering the details needed it is apparent that we have a problem in which surface water, ground water, and soil water are 

so intimately associated that they must be

studied together. The no man's land of this 

problem is the swamp environment. With 

respect to this environment we specifically 

need to know:

1. The hydrologic properties and physical 

properties of the swamp deposits under 

various conditions for example: when saturated, when dry, at various degrees of 

wetting, and when intermittently wetted.

2. The infiltration capacity of swamp soils 

under given sets of conditions.

3. The operation of the swamps and the 

swamp deposits as a discharge element 

under given sets of conditions.

4. The operation of the swamps and the 

swamp deposits as a recharge element under given sets of conditions.

5. The operation of the swamps and the 

swamp deposits as a storage element under given sets of conditions.

6. The relative contribution of swamp storage and ground water to runoff.

7. The effect of the swamp environment on 

the chemical quality of the water.

In the foregoing discussion we have given 

an example of one area in which a wetland 

plays a role with respect to water supply. 

We have listed some of the facets of wetland 

hydrology that need study if this role is to be 

properly evaluated. In conclusion it is clear 

that we have much to learn, if we are to fill 

the gap in knowledge that presently exists 

with respect to the wetland and water supply.

THT.DUP..P.C.60-TJ»»1