The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- The hydrologic cycle describes the continuous interchange of water
among the oceans, atmosphere, and continents. Powered by energy from the Sun, it is a global
system in which the atmosphere provides the link between the oceans and continents. The
processes involved in the hydrologic cycle include precipitation, evaporation,
infiltration (the movement of water into rocks or soil through cracks and pore
spaces), runoff (water that flows over the land rather than infiltrating into
the ground), and transpiration (the release of water vapor to the atmosphere
by plants). Running water is the single most important agent sculpturing Earth's land
surface.
- The land area that contributes water to a stream is its drainage
basin. Drainage basins are separated by imaginary lines called
divides.
- River systems consist of three main parts: the zones of erosion, transportation, and
deposition.
- The factors that determine a stream's velocity are gradient (slope of
the stream channel), shape, size, and roughness of the channel,
and the stream's discharge (amount of water passing a given point per unit of
time, frequently measured in cubic feet per second). Most often, the gradient and roughness
of a stream decrease downstream, while width, depth, discharge, and velocity increase.
- Streams transport their load of sediment in solution (dissolved load),
in suspension (suspended load), and along the bottom of the channel (bed
load). Much of the dissolved load is contributed by groundwater. Most streams carry
the greatest part of their load in suspension. The bed load moves only intermittently and is
usually the smallest portion of a stream's load.
- A stream's ability to transport solid particles is described using two criteria:
capacity (the maximum load of solid particles a stream can carry) and
competence (the maximum particle size a stream can transport). Competence
increases as the square of stream velocity, so if velocity doubles, water's force increases
fourfold.
- Streams deposit sediment when velocity slows and competence is reduced. This results
in sorting, the process by which like-sized particles are deposited together.
Stream deposits are called alluvium and may occur as channel deposits called
bars, as floodplain deposits, which include natural levees, and as
deltas or alluvial fans at the mouths of streams.
- Stream channels are of two basic types: bedrock channels and
alluvial channels. Bedrock channels are most common in headwaters regions
where gradients are steep. Rapids and waterfalls are common features. Two types of alluvial
channels are meandering channels and braided channels.
- The two general types of base level (the lowest point to which a
stream may erode its channel) are (1) ultimate base level and (2)
temporary, or local base level. Any change in base level will cause a stream
to adjust and establish a new balance. Lowering base level will cause a stream to downcut,
whereas raising base level results in deposition of material in the channel.
- When a stream has cut its channel closer to base level, its energy is directed from
side to side, and erosion produces a flat valley floor, or floodplain. Streams
that flow upon floodplains often move in sweeping bends called meanders. Widespread
meandering may result in shorter channel segments, called cutoffs, and/or
abandoned bends, called oxbow lakes.
- Floods are triggered by heavy rains and/or snowmelt. Sometimes human
interference can worsen or even cause floods. Flood-control measures include the building of
artificial levees and dams, as well as channelization, which
could involve creating artificial cutoffs. Many scientists and engineers
advocate a nonstructural approach to flood control that involves more appropriate land use.
- Common drainage patterns produced by streams include (1)
dendritic, (2) radial, (3) rectangular, and (4)
trellis.
- As a resource, groundwater represents the largest reservoir of
freshwater that is readily available to humans. Geologically, the dissolving action of
groundwater produces caves and sinkholes. Groundwater is also an
equalizer of streamflow.
- Groundwater is water that occupies the pore spaces in sediment and rock in a zone
beneath the surface called the zone of saturation. The upper limit of this
zone is the water table. The zone of aeration is above the water
table where the soil, sediment, and rock are not saturated.
- The quantity of water that can be stored depends on the porosity (the
volume of open spaces) of the material. The permeability (the ability to
transmit a fluid through interconnected pore spaces) of a material is a very important
factor controlling the movement of groundwater.
- Materials with very small pore spaces (such as clay) hinder or prevent groundwater
movement and are called aquitards. Aquifers consist of
materials with larger pore spaces (such as sand) that are permeable and transmit groundwater
freely.
- Springs occur whenever the water table intersects the land surface and
a natural flow of groundwater results. Wells, openings drilled into the zone
of saturation, withdraw groundwater and create roughly conical depressions in the water
table known as cones of depression. Artesian wells occur when
water rises above the level at which it was initially encountered.
- When groundwater circulates at great depths, it becomes heated. If it rises, the
water may emerge as a hot spring. Geysers occur when groundwater
is heated in underground chambers, expands, and some water quickly changes to steam, causing
the geyser to erupt. The source of heat for most hot springs and geysers is hot igneous
rock.
- Some of the current environmental problems involving groundwater include (1)
overuse by intense irrigation, (2) land subsidence caused by
groundwater withdrawal, and (3) contamination by pollutants.
- Most caverns form in limestone at or below the water table when acidic
groundwater dissolves rock along lines of weakness, such as joints and bedding planes.
Karst topography exhibits an irregular terrain punctuated with many
depressions, called sinkholes.
|
|
