Geology Reference
In-Depth Information
Backswamps: Beyond the levees, backswamps are composed of the smaller sedi-
ment particles such as clay and silt that are carried farther along as the floodwa-
ter slows down. Backswamps are usually flat areas and may have standing water
after the flood waters disappear.
Alluvial fans: Occasionally, a stream flowing through a mountainous region sud-
denly flows out onto the relatively flat surface of a valley floor. The speed of the
streamflow slows down very quickly and deposits a large amount of sediment, cre-
ating an alluvial fan. An alluvial fan is a sloping, fan-shaped landform that is cre-
ated as the stream spreads out and slows down quickly, depositing sediments of
all different sizes.
Reaching the sea
Eventually, a stream reaches the ocean, and any sediments it is still carrying are depos-
ited. By this time only the smaller particles of silt or clay are left in the slow-moving flow
of water. As these smaller particles are deposited, they create a triangular or fan-shaped
landform called a delta.
In a delta, the larger particles are deposited first, and as the delta grows out into the sea,
smaller particles are deposited in order of size (largest to smallest). A delta continues to
grow as long as the stream is flowing, bringing sediments from the land to the ocean.
The delta is the outlet of the stream; it represents the stream's goal right from the begin-
ning when it first started carrying sediments downslope, across the continent.
Flowing Beneath Your Feet: Groundwater
While much of the water that falls as rain flows to the ocean in streams, a portion of it
sinks through the soil and becomes groundwater. Groundwater makes up a small per-
centage of the earth's total water, but it supplies almost all the water that humans drink.
Anytime someone digs a well, the water he pumps to the surface (or draws up with a
bucket) comes from the hidden supply of groundwater.
Groundwater doesn't flow as quickly as surface streams (because it has to move
through sediments and rocks), but it does flow. In this section, I describe how groundwa-
ter flows underground, and I introduce the geologic features that result from the move-
ment of groundwater, including springs, caves, and geysers.
Infiltrating tiny spaces underground
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