Down in the Mouth
Where the River Meets the Roadstead
By Tamia Nelson
February 11, 2003
Edges have always intrigued me. Horizons,
hedgerows, shorelines, riverbanks, waterfallsthey're all guaranteed
to draw my eye and hold my interest. Why? That's easy. Edges are "zones of
transition," places where change happens. Wherever there's change, there's
action. And who doesn't like being in on the action?
The fluid margin between land and sea is a big edge. I've
written about it before. But the wide sweep of the coastline embraces
an infinity of smaller zones of transition. One of these can be found
wherever fresh water meets salt. That place is the river mouth.
Rivers have complex
personalities. At source, rivers are gurgling, stumbling infants. As
they proceed inexorably downhill, however, they grow larger and more
mature. In response to gravity's ceaseless pull, the lusty infant becomes
a ponderous, big-bellied elder statesman. And like a lot of elder
statesman, what the river wants most of all is somewhere to put his head
down. For many rivers, that "somewhere" is the sea.
As these rivers cut through the land on their way to the ocean, they pick up
clay, silt, sand, and gravel and carry it downstream. But that's not
all that goes along for the ride. Rivers aren't fussy. They'll give a lift
to just about anything: trees, plastic bags full of garbage, drowned
livestock, chemical waste, raw sewage, even bits of broken boat. Rivers
don't care what they pick up. It all gets swept along by the current. Not
every hitchhiker makes it to the sea, of course. I've seen dead cows in
trees on the bank of the Schoharie, for example, and pulled shopping carts
out of trout pools in the Battenkill. A great deal gets left behind. But a
lot more goes all the way.
River-borne sediment is responsible for building
and maintaining beaches. Without regular drafts of new material,
beaches retreat and ultimately disappear. The process is (usually) too
slow for us to follow, but if you were able to soar like an osprey above a
river mouth, you could see the discolored plume of sediment-rich fresh
water join the sea. Once the waters mix completely, the color difference
The sediment burden is dropped as the water slows. And it adds up. The
greater the area of land drained by a river, the more material it carries
seaward. Small rivers pouring off high, steep peaks and plunging directly
into the sealike some of those draining the Hawaiian Islands or the
rocky coast of Labradorrun almost clear. On the other end of the
scale, rivers fed by melting glaciers are milky with rock dust quarried by
the grinding ice. And big rivers draining agricultural areas are stained
yellow, brown, or red by eroded topsoil.
But rivers do more than transport sediments. Every river that empties
into the sea brings fresh water into contact with salt. They don't mix
immediately. Salt water is the denser of the two, and fresh water tends to
slide over its heavier cousin on first meeting. At many river
mouthsparticularly those restricted between natural cliffs or
artificial seawallsthe fresh water forms a fluid lens floating on
top of a wedge of salt water. This lens will vary in size and extent.
During spring run-off, for instance, a greater quantity of fresh water
spills into the sea, and the saltwater wedge is pushed back. Later, as the
seasonal flood waters subside, the lens of fresh water retreats and the
salt water advances upriver.
And then there are the tides. The sun and moon tug on the earth's
oceans. Curiously, the moon's by far the stronger partner here. Though the
sun is much bigger, it's also much farther away, and attraction is largely
a matter of proximity. So the moon's pull creates a pair of humps in the
sea, one just beneath it and the other on the opposite side of the globe.
As the earth rotates on its axis, these humpsyou can call them
"tidal waves," if you want, but please don't confuse them with
tsunamiscircle the world in a little more than 24 hours.
The result? High and low tides, corresponding to the crest and trough
of each tidal wave. Along every marine shore, the sea rises and falls,
sometimes twice a day, sometimes only once. Of course, it's not really as
simple as I've suggested. Topography enters into the equation, as do winds
and complex harmonics. The gravitational influence of the sun can't be
neglected, either. But these are all subjects for another time. For now,
it's enough to know that while some places experience extreme tides, with
the difference between high and low water measured in tens of feet, in
others the change is measured in inches. And even the largest fresh water
lakes have no tides at all. Tides are an ocean phenomenon.
But what happens where rivers meet the sea? As the sea level rises with
the tide, ocean waters flood into river mouths and bays. The flood tide
pushes its way upriver. How far it gets depends upon many things,
including the shape of the river mouth. But whenever a river butts heads
with a flooding tide, waves grow taller and become steeper. Toss in a
stiff upriver breeze, and the stage is set for some rough, breaking seas.
It's a good time to be someplace else.
Once the tide begins to ebb (or fall), its opposition to the river's
flow diminishes. At low water, when the tide has fallen as far as it's
going to, the river runs unhindered out to sea, scouring the foreshore
channel and carrying its burden of sediment farther away from shore.
Season, too, affects the pace of change at the river mouth. Beaches
sometimes disappear altogether in winter. Sediments accumulate around
river mouths during the sultry summer months, but winter storms quickly
undo the summer's work. Only when the storms subside and the rivers
replenish the lost sediments will the beach be rebuilt.
But what if the winter storms don't wash everything away? What if more
sediment is deposited at a river mouth than is carried out to sea? Then a
delta forms. The name tells you what to look for on the map.
Generally triangular or broadly fan-shaped (just like the Greek letter
), and with its apex pointing upstream, a delta is a gently
sloping wedge of sediment, thicker near land and dissected by many
channels. Deltas aren't exclusively marine, either. They can also be found
where rivers flow into lakes.
To be sure, not every delta is as perfectly shaped as this. Currents
and rates of deposition, even compaction and subsequent depression of the
earth's crusteach of these forces helps to sculpt a delta. But the
basic formula remains the same everywhere. A delta's a sort of savings
account. When deposits exceed losses, the delta grows. On the other hand,
if losses outstrip deposits, it shrinks. The classic delta is the one
found at the mouth of the Nile. It's seen better days, however. Since the
construction of the Aswan High Dam, much of the sediment that once
nourished the Nile delta has been trapped in a huge impoundment. Now the
delta is wasting away, spending down capital accumulated over thousands of
years. Someday it will cease to exist altogether.
Fortunately, other marine deltas have fared better. As the tide rises
and falls, the slosh of water back and forth cuts so-called "distributary
channels." The resulting interfingering of water and earth creates areas
of extraordinary biological productivity. The Mississippi delta is just
such a place. It's described by geologists as a "bird's foot" delta, and
until quite recently it was expanding seaward. Now, though, the
Mississippi is also spending down its deposit account, imperiling both the
delta and the wildlife it supports.
Not all rivers form deltas. Some are simply too sediment-poor to set up
a "savings account" in the first place. Others empty into the ocean
through deep, long-drowned river valleys. These are called
estuaries, relatively narrow, funnel-like incursions of the sea,
creating a near-shore environment that extends well inland. Estuaries are
special places. Marine and fresh waters vie for dominance in an estuary,
creating a highly variegated physical and biological environment. With
every flood of the tide, salt water invades the river valley and pushes
upstream. How far? New York's Hudson River is an estuary as far north as
Albany, nearly 150 miles upriver from New York City. The Chesapeake Bay is
an estuary, too, as is the River Thames in England. And these are only a
few of many examples.
Throughout much of recent geologic time, however, estuaries have been
comparatively rare. Blame the Ice Age. Continental glaciation locked water
up as snow and ice, and the planet has only so much water to go round. Sea
level dropped, exposing the continental margins and forcing rivers to
carve new valleys on their journey toward the ocean. But then the great
continental glaciers melted and the sea rose again, drowning the
newly-created river valleys. Today, you'll find estuaries wherever
topography constrains rivers to these ancient channels.
They can be lively places. As high tides flood through narrows, they
sometimes take the form of a wave front, or tidal bore. Some bores
are so low that they're barely noticeable, but others are impressive,
indeed. The famous bore of the Amazonthis great river has both an
"imperfect" delta and an estuarine mouthreaches a height of
twenty-five feet and travels upriver at speeds approaching fifteen miles
per hour. The roar it makes in passing can be heard for fifteen miles.
Of course, surfing tidal bores isn't most folk's idea of a good time,
and not all estuaries can boast one, anyway. But there's certainly no
reason to be bored if you're down in the mouth of any river, large
or small. Everything happens on the edge. That's where the action is,
after all. So why not check it out?
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