CHAPTER VI

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.

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).

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.