MARSHES AND MARSH DRAINAGE
(FIGS. 19 TO 27)
Mention has been made of the objects seen better from the air than from
any viewpoint on the ground; but there are some objects which as a whole
can be seen only from above. Swamps, parts of everglades, peaks in the
midst of difficult country, precipitous canyon walls, and many volcanic
craters cannot be seen from the ground without undue effort. Photographs
of bluffs, terraces, and other slopes facing bodies of water have
hitherto been adequately obtainable only from the water. All of these
can be readily viewed and photographed from the airplane. Pictorial
representations of drainage systems were rare until photographs such as
Figure 19 were taken from airplanes. The intricate drainage of marshes
like those along the Pamunkey River in Virginia pictured in Figure 20
was never accurately shown until photographed from the air.
Of frequent occurrence on the Atlantic Coastal Plain of the United
States are swamps and marshes inaccessible from the ground. Much of the
surface material is so soft that they cannot be easily traversed; and,
even where firm enough to support a man’s weight, few of the details are
deemed of sufficient importance to warrant the trouble and expense of
mapping by ordinary methods. Yet the trapper would scarcely admit that
these details are unimportant, and, to the student, they are an
interesting feature of marsh topography that has thus far received
little attention.
Figure 25 is part of the excellent New Kent, Va., sheet of the
topographic map and is probably as detailed as a map of this character
should be when made from ground surveys only. However, on comparison of
the map with a photograph of the same area (Fig. 24), there is no
difficulty in detecting errors; and it is probable that, had the
photograph been available when the map was made, the marshes would have
been represented differently.
[Illustration: FIG. 19--Stream development in a tidal marsh, showing, at
the right, the northern end of Ludlam Beach, about 6 miles south of
Ocean City, N.J., and the mouth of Corsons Inlet leading to Ludlam Bay,
and, at the left, the marsh just west of the inlet, with streams rising
close to the bank of the larger stream at the extreme left and flowing
in meandering courses across the marsh. The great variety of types of
vegetation probably is one cause of the remarkable meandering of these
drainage lines by reason of the fact that the accumulated remains as
well as the annual growth of different weeds and grasses offer varying
resistance to the current of the streams. Scale, about 1:10,000.]
MARSH DRAINAGE
One of the most striking characteristics of marsh topography illustrated
by the photographs presented here is the great wealth of drainage lines
and the resemblance of the drainage patterns to those of river systems
developed on higher ground. The dendritic patterns, the meanders, and
the sharply outlined divides are surprising in areas which have
altitudes varying from only a few inches to a little more than a foot at
times of ordinary high tide and which are wholly submerged at times of
maximum tide. Some of the streams have gently winding courses suggestive
of normal stream development. Others, particularly the smaller, have a
conspicuous angularity of course. It is possible that the latter may
have originated as the trails of animals. Some of the lines are observed
to cross the larger streams and are probably tracks made by muskrats.
Some of the streams rise close to the river’s brink and lead to
through-going waterways near the center of the marsh. This suggests the
deposition of silt on the brink of the river at times of high water. The
notched appearance of the shore in Figure 20 seems to be due to
overhanging bunches of sedge grass and, in some instances, to the
breaking away of the surface mat or crust of the marsh formed by the
interlacing roots of grass. The mottled appearance of the marsh in this
picture may be partly due to shadow of clouds, but to some extent, at
least, the difference in shade is caused by differences in the character
of the plants.
The marshes used for illustration here are typical of many along the
Atlantic Coast. They are situated near West Point, Va. The Pamunkey and
the Mattaponi Rivers both rise in the Piedmont Plateau, flow
southeastward through the tidewater portion of Virginia, and join about
midway of the Coastal Plain to form the York River[3] (see Fig. 58).
[Illustration: FIG. 20--Details of marshland. A part of Lee Marsh near
West Point, Va. (cf. Fig. 26), as photographed from a height of 2,000
feet, June, 1920. Local observers report that this marsh has been
submerged only twice in nineteen years. The drainage systems are well
entrenched. The larger stream channels are cut 1 to 5 feet or more below
low tide (the tidal variation at West Point is about 3.4 feet), and
their form is made stable by the tough surface crust of the marsh,
consisting of the matted roots of the luxuriant sedge grass. The
intricate, veinlike appearance of the drainage lines and the furry
appearance of the edges of all the waterways, showing overhanging
vegetation, are of interest. Drainage systems flowing in opposite
directions slow connecting tributaries apparently silted up. Scale,
about 1:4,000.]
[Illustration: FIG. 21--Details of frequently submerged marshland. A
part of Cousaic Marsh on the Pamunkey River, near Sweet Hall, Va. (cf.
Fig. 24), as photographed from a height of 2,000 feet, June, 1920. The
surface of the marsh is covered with water several times each year,
according to local report. It is relatively soft, and a comparison with
Fig. 20 shows an apparently different, less dense vegetation than that
of Lee Marsh, which is rarely submerged. The stream channels are less
definitely fixed and lack the evidence of overhanging vegetation. Scale,
about 1:4,000.]
[Illustration: FIG. 22--Atlantic City and Ocean City, N.J. Strips of
photographs taken from an airplane, March, 1920, from a height of 10,000
feet, showing, in order from east to west: the ocean water, which
appears dark-colored; the surf, white where it breaks into foam; the
light-colored beach sand; the cities laid out on the sand of the barrier
beach; and the marshes, channels, and drainage systems west of the
barrier. West of Peck Beach in the strip of photograph at the right many
features characteristic of salt marsh areas of the Coastal Plain are
shown back of the barrier beach. The right strip forms the southern
continuation of the left strip. Scale, about 1:75,000.]
[Illustration: FIG. 23--A river system in miniature. A small stream near
Hampton, Va., showing flood plain, meanders, an ox-bow lake and cut-off,
abandoned channels and a delta partly under water. Scale, not known.]
[Illustration: FIG. 24--Sweet Hall Marsh on the lower Pamunkey River,
near West Point, Va., as photographed from a height of 10,000 feet at 11
A.M., December 11, 1920. Cousaic Marsh lies to the left and Hill Marsh
to the right of the central meander. Some of the watercourses in these
marshes are thoroughfares, or channels opening to the river at both
ends, that can be traversed by boat at high tide. But many of them are
quite different in nature, beginning as minute rills and broadening
toward the mouth in a manner suggesting typical drainage channels on
higher land. Scale, about 1:31,000.]
[Illustration: FIG. 25--The same area as shown in Fig. 24, enlarged from
the New Kent, Va., topographic sheet, 1:62,500, published by the U.S.
Geological Survey. The cross section at the left lies along the line
indicated on the map and extends somewhat beyond its borders. The
somewhat greater height of the map than of the photograph, although both
cover exactly the same area, is due to the unavoidable slight difference
in tilt of each of the exposures of which the photographic mosaic is
made up. This illustrates the fact that airplane photographs cannot be
directly used as equivalent to maps, until the necessary adjustments
have been made. Experiments in camera construction are under way to
overcome these difficulties by automatic devices. Scale, 1:31,000.]
[Illustration: FIG. 26--Eltham Marsh on the lower Pamunkey River, as
photographed from an altitude of about 10,000 feet at 11 A.M., December
11, 1920. At the right lies the town of West Point, Va., at the junction
of the Mattaponi and Pamunkey Rivers, and at the left appears a part of
Lee Marsh. Eltham Marsh, in the center of the illustration, is traversed
by a so-called thoroughfare, through which boats of light draft make
their way at high tide. At one point in the middle of the marsh the
thoroughfare is perceptibly broader than elsewhere, and the tidal
currents entering from opposite ends of the thoroughfare meet there and
cause slack water in which silt is deposited, forming mud flats exposed
at low tide. The cultivated fields south of the marsh are on a bench
about 10 feet higher than the marsh. Scale, about 1:31,000.]
[Illustration: FIG. 27--The same area as shown in Fig. 26, enlarged from
the New Kent, Va., topographic sheet, 1:62,500, published by the U.S.
Geological Survey. It is obvious that many interesting details shown by
the photograph are missed or neglected as unimportant in the most
careful mapping. The cross section at the left lies along the line
indicated on the map and extends somewhat beyond its borders. Scale,
1:31,000.]
The York is one of the estuaries of the tidewater portion of Virginia,
and the water level at West Point, the junction of the two tributaries,
rises and falls about 3½ feet under tidal action. The Pamunkey is
affected by the tide 53 miles by channel above West Point, and the
Mattaponi 42 miles. Much of the broad lowland along these rivers is
marshy, but the largest marshes are found near West Point, where the
river current in swinging from side to side has formed great meanders.
For some reason the valleys eroded long ago by these streams have filled
with sediment here to a greater extent than farther downstream; perhaps
because this is essentially the head of sea water, so that the checking
of the current of the river causes it to deposit much of its load. Sea
water regularly mingles with the river water as far upstream as West
Point, but above this point the water is chiefly fresh. The marshes
consist of soft mud and muck to a considerable depth. A well driven in
Hill Marsh to an underlying artesian horizon penetrated 50 feet of this
soft material before entering rock such as is exposed in the river bank.
The thickness of the mud is comparable to the maximum depth of the York
farther downstream and suggests that the old valley which there is
filled with water is here filled to a depth of 50 feet or more with
sediment brought down by the river. Only a small part of the marsh near
the landward margin has surface material firm enough to support the
weight of large animals except when the surface is frozen.
Many kinds of marsh plants grow here, among which is sedge grass
(_Spartina cynosuroides_ (L.) Willd.), which grows to a height of 10
feet or more and forms dense thickets. Its roots interlace to form a
tough mat which in some places will support the weight of a man. In
other places the soft muck reaches to the surface.
“THOROUGHFARES”
These marshes are cut by a few waterways open at both ends, known as
thoroughfares, or tidal runs, which also serve as the trunk streams
through which the marsh is drained. Some of the thoroughfares may be
trunk streams modified by tides, or they may be silted remnants of
abandoned river channels. Some seem to be channels in the last stages of
silting. The incoming tide enters the down-river end but ascends the
thoroughfare more slowly than it ascends the river. The tide in the
river reaches the upper end of the thoroughfare, enters it, and meets
the opposing tide within the marsh near the upstream end of the
passageway. Where the tides meet, thus causing slack water, silt is
deposited and mud flats are formed. In Eltham Marsh (Fig. 26) these
flats are well within the marsh. In the larger thoroughfares of Sweet
Hall Marsh (Fig. 24) the tide passes entirely through while the tide in
the river is making its long way around, so that slack water and the
deposition of silt occur at the extreme upper end of the passage. In all
of the thoroughfares the silting has reached the stage that precludes
their use by boat, except at times of high water. Even at high tide some
are navigable only by small skiffs, although throughout much of the
course the water is many feet deep.
Some of the thoroughfares become narrow and shallow upstream in a manner
that suggests that they originate as two normal streams flowing in
opposite directions from a common point and that they were later united
by the breaking down of the divide between their headwaters. Such a
junction might be affected by an unusually high tide breaking through a
divide and cutting a channel. Such a divide, be it noted, consists of
soft mud only a few inches above the general level and might readily be
broken down. In some instances the connection may have originated as an
animal trail, as we have seen. Muskrats, otters, and other marsh
animals use the waterways as lines of travel and make paths in between
them from one to another. Apparently many of the small drainage lines
originated in this way, but in some instances stream systems of
considerable size and complexity are independent of all others and
possess all the characteristics of normally developed river systems.