Yakima River, no longer downcutting its channel. Excess
energy generated during flooding is used by the river to cut
sideways instead creating a flat-bottomed valley called a flood
plain. Within its flood plain, the river shifts constantly,
abandoning channels and creating new ones thus making the river
unpredictable. This stretch of the river is diked, method of
protecting property by confining the river to its present blanks,
but a large flood can still cause dramatic changes in the river's
location. The river is not diked below Valley Mall Boulevard and
flows freely within the flood plain.
Floodplain:
View of the flood plain following a 1 in 10 year flood on the Isle
of Wight Gravel floodplain of a glacial river near the Snow
Mountains in Alaska, 1902 Entrenched river: The Virgin River at the
upper end of Zion Canyon, Zion National Park, Utah, has almost no
floodplain at all.
Aggradation and plantation: The Laramie River meanders across its
floodplain in Albany County, Wyoming, 1949.
Aggradational floodplain: This floodplain of a small meandering
stream in La Plata County, Colorado, is underlain by silt deposited
above a dam formed by a terminal moraine left by the Wisconsin
Glacier.
Riparian vegetation on the floodplain of the Lynches River near
Johnsonville, South Carolina. These tupelo and cypress trees show
the high water mark of flooding. A floodplain, or flood plain, is a
flat or nearly flat land adjacent to a stream or river that
stretches from the banks of its channel to the base of the
enclosing valley walls and experiences flooding during periods of
high discharge. It includes the floodway, which consists of the
stream channel and adjacent areas that carry flood flows, and the
flood fringe, which are areas covered by the flood, but which do
not experience a strong current. In other words, a floodplain is an
area near a river or a stream which floods easily.
Physical geography:
Flood plains are made by a meander eroding sideways as it travels
downstream. When a river breaks its banks and floods, it leaves
behind layers of rock and mud. These gradually build up to create
the floor of the flood plain. Floodplains generally contain
unconsolidated sediments, often extending below the bed of the
stream. These are accumulations of sand, gravel, loam, silt, and/or
clay, and are often important aquifers, the water drawn from them
being pre-filtered compared to the water in the stream.
Geologically ancient floodplains are often represented in the
landscape by fluvial terraces. These are old floodplains that
remain relatively high above the present floodplain and indicate
former courses of a stream.
Sections of the Missouri River floodplain taken by the United
States Geological Survey show a great variety of material of
varying coarseness, the stream bed having been scoured at one place
and filled at another by currents and floods of varying swiftness,
so that sometimes the deposits are of coarse gravel, sometimes of
fine sand or of fine silt. It is probable that any section of such
an alluvial plain would show deposits of a similar character.
The floodplain during its formation is marked by meandering or
anastomotic streams, ox-bow lakes and bayous, marshes or stagnant
pools, and is occasionally completely covered with water. When the
drainage system has ceased to act or is entirely diverted for any
reason, the floodplain may become a level area of great fertility,
similar in appearance to the floor of an old lake. The floodplain
differs, however, because it is not altogether flat. It has a
gentle slope down-stream, and often, for a distance, from the side
towards the center.
Ecology:
Floodplains can support particularly rich ecosystems, both in
quantity and diversity. They are a category of riparian zones or
systems. A floodplain can contain 100 or even 1000 times as many
species as a river. Wetting of the floodplain soil releases an
immediate surge of nutrients: those left over from the last flood,
and those that result from the rapid decomposition of organic
matter that has accumulated since then. Microscopic organisms
thrive and larger species enter a rapid breeding cycle.
Opportunistic feeders (particularly birds) move in to take
advantage. The production of nutrients peaks and falls away
quickly; however the surge of new growth endures for some time.
This makes floodplains particularly valuable for agriculture.
Markedly different species grow in floodplains than grow outside of
floodplains. For instance, riparian trees (that grow in
floodplains) tend to be very tolerant of root disturbance and tend
to be very quick-growing, compared to non-riparian trees.
Interaction with society:
Historically, many towns have been built on floodplain, where they
are highly susceptible to flooding, for a number of reasons:
1. access to fresh water;
2. the fertility of floodplain land for farming;
3. cheap transportation, via rivers and railroads, which often
followed rivers;
4. ease of development of flat land
In the United States the National Flood Insurance Program regulates
development in mapped floodplains based on the 100-year flood (1%
annual chance of a flood of this magnitude). The Flood Insurance
Rate Maps, typically depict both the 100-year floodplain and the
500-year floodplains. Where a detailed study of a waterway has been
done, the 100-year floodplain will also include the floodway, the
critical portion of the floodplain which includes the stream
channel and any adjacent areas that must be kept free of
encroachments that might block flood flows or restrict storage of
flood waters. When a floodway is shown on the Flood Insurance Rate
Maps, the portion of the 100-year floodplain outside of the
floodway is known as the flood fringe. Another commonly-encountered
term is the Special Flood Hazard Area, which is any area subject to
inundation by the 100-year flood. A problem is that any alteration
of the watershed upstream of the point in question can potentially
affect the ability of the watershed to handle water, and thus
potentially affects the levels of the periodic floods. A large
shopping center and parking lot, for example, may raise the levels
of the 5-year, 100-year, and other floods, but the maps are rarely
adjusted, and are frequently rendered obsolete by subsequent
development.
In order for flood-prone property to qualify for
government-subsidized insurance, a local community must adopt an
ordinance that protects the floodway and requires that new
residential structures built in Special Flood Hazard Areas be
elevated to at least the level of the 100-year flood. Commercial
structures can be elevated or floodproofed to or above this level.
In some areas without detailed study information, structures may be
required to be elevated to at least two feet above the surrounding
grade. Many State and local governments have, in addition, adopted
floodplain construction regulations which are more restrictive than
those mandated by the NFIP. The U.S. government also sponsors flood
hazard mitigation efforts to reduce flood impacts. The Hazard
Mitigation Program is one funding source for mitigation projects. A
number of whole towns such as English, Indiana, have been
completely relocated to remove them from the floodplain. Other
smaller-scale mitigation efforts include acquiring and demolishing
flood-prone buildings or flood-proofing them.
In some tropical floodplain areas such as the Inner Niger Delta of
Mali, annual flooding events are a natural part of the local
ecology and rural economy, allowing for the raising of crops
through recessional agriculture. But in Bangladesh, which occupies
the Ganges Delta, the advantages provided by the richness of the
alluvial soil of floodplains are severely offset by frequent floods
brought on by cyclones and annual monsoon rains, which cause severe
economic disruption and loss of human life in this
densely-populated region.,
There are two types of river flood plains:
A mature river stream has a flat bottom and a formed flood
plain.
An inmature river stream has a youthful stream. It has V-shaped
bottom
To log this earthcache, please email me (through my gc.com
profile) the answers to the following questions. You will be able
to find the information at the roadside marker. Once you've sent
the email, go ahead and log your find without waiting for a reply
from me. I will contact you if there are any problems with your log
entry.
1. Because the river can be so unpredictable, What is the best used
of the flood plain?
2. What kind of river stream is Yakima River at the Greenway?
3. Where the river is not diked below Valley Mall Boulevard, What
kind of effect is created?
4. Optional (but appreciated): Post photos of yourself with the
view, being sure to not reveal any answers to the questions in your
photographs.