Essentials of Geology: Chapter 9

longitudinal profile

A cross section of a stream channel along its descending course from the head to the mouth.

rectangular drainage pattern

A drainage pattern characterized by numerous right angle bends that develops on jointed or fractured bedrock.

Knickpoint Consider what would happen if displacement along a fault raised a layer of resistant rock along the course of a graded stream. The resistant rock forms a waterfall and serves as a temporary base level for the stream. Eventually, the river erases the knickpoint from its path and reestablishes a smooth profile.

A sharp irregularity (such as a waterfall, rapid, or cascade) in a stream-channel profile

Capacity and Competence The increase in discharge results in greater capacity; the increased velocity produces greater competency.

A stream's ability to carry solid particles is described using two criteria:

radial drainage pattern

A system of streams running in all directions away from a central elevated structure, such as a volcano.

1.36 billion cubic kilometers (326 million cubic miles) The vast majority of it, about 97 percent, is stored in the global ocean. Ice sheets and glaciers account for slightly more than 2 percent, leaving less than 1 percent to be divided among lakes, streams, subsurface water, and the atmosphere.

Amount of water in Earth's hydrosphere

wetted perimeter The most efficient channel is one with the least wetted perimeter for its cross-sectional area.

As water in a stream channel moves downslope it encounters a significant amount of frictional resistance. The cross-sectional shape (a slice taken across the channel) determines, to a large extent, the amount of flow in contact with the banks and bed of the channel. This measure is referred to as the _______________________.

Levees Meandering rivers that occupy valleys with broad floodplains tend to build natural levees that parallel their channels on both banks.

Barriers composed of sediments made on either side of a river due to flooding.

Evapotranspiration More water falls on land as precipitation than is lost by evapotranspiration. The excess is carried back to the ocean mainly by streams—less than 1 percent returns as groundwater.

Because both evaporation and transpiration involve the transfer of water from the surface directly to the atmosphere, they are often considered together as the combined process of _________________________.

Divide Divide is something that is clearly visible as a sharp ridge in some mountainous areas but can be rather difficult to discern in more subdued topographies. The outlet, where the stream exits the drainage basin, is at a lower elevation than the rest of the basin.

Each drainage basin is bounded by an imaginary line called a ______________.

Transpiration

Evaporation of water from the leaves of a plant.

Drainage Basin

Every stream drains an area of land called a

(1) channel slope or gradient; (2) channel size and cross-sectional shape; (3) channel roughness; and (4) the amount of water flowing in the channel.

Factors influencing flow velocities in streams/rivers?

potholes The rotational motion of sand and pebbles acts like a drill to bore the holes. As the particles wear down to nothing, they are replaced by new ones that continue to drill the stream bed. Eventually, smooth depressions several meters across and deep may result.

Features common to some bedrock channels are circular depressions known as _________________ created by the abrasive action of particles swirling in fast-moving eddies.

amount and intensity of rainfall

Flow velocity can vary significantly from time to time due to ____________.

bankfull At this stage, the channel's ratio of the cross-sectional area to wetted perimeter is highest and stream flow is most efficient. Similarly, an increase in channel size increases the ratio of cross-sectional area to wetted perimeter and therefore increases channel efficiency. All other factors being equal, flow velocities are higher in large channels than in small channels.

Maximum flow velocity occurs when a stream is ____________, before water starts to inundate the floodplain.

longitudinal profile

One useful way of studying a stream is to examine its __________________________ . Such a profile is simply a cross-sectional view of a stream from its source area (called the head or HEADWATERS ) to its mouth , the point downstream where it empties into another water body—a river, lake, or ocean.

grand base level/ ultimate base level

Powell postulated that the level of the sea to be ________________.

Transpiration

Some of the water that soaks into the ground during rainfall is absorbed by plants, which later release it into the atmosphere. This process is called _______________.

quarrying and abrasion Quarrying involves the removal of blocks from the bed of the channel. This process is aided by fracturing and weathering that loosen the blocks sufficiently so they are moveable during times of high flow rates. Quarrying is mainly the result of the impact forces exerted by flowing water. Abrasion is the process by which the bed and banks of a bedrock channel are ceaselessly bombarded by particles carried into the flow. In addition, the individual sediment grains are also abraded by their many impacts with the channel and with one another.

Streams cut channels into bedrock through ______________ and _______________.

Meandering Channel Streams that transport much of their load in suspension generally move in sweeping bends called meanders. Outside of a meander is a zone of active erosion, it is often referred to as the cut bank. Debris acquired by the stream at the CUT BANK moves downstream where the coarser material is generally deposited as POINT BARS on the insides of bends. In this manner, meanders migrate laterally by eroding the outside of the bends and depositing sediment on the inside without appreciably changing their shape.

Streams that transport much of their load in suspension generally move in sweeping bends.

1. Dissolved Load - in solution 2. Suspended Load - in suspension 3. Bed Load - sliding or rolling along the bottom (1) Most of the dissolved load is brought to a stream by groundwater and is dispersed throughout the flow. (2) Most streams carry the largest part of their load in suspension. Indeed, the muddy appearance created by suspended sediment is the most obvious portion of a stream's load. (3) Coarse material, including coarse sands, gravels, and even boulders typically move along the bed of the channel as bed load. The particles that make up the bed load move by rolling, sliding, and saltation.

Streams transport their load of sediment in three ways:

Flow Velocity

The ability of a stream to erode and transport material is directly related to its ______________.

(1) intensity and duration of rainfall; (2) amount of water already in the soil; (3) nature of the surface material; (4) slope of the land, and; (5) the extent and type of vegetation

The amount of water that runs off rather than soaking into the ground depends on several factors:

discharge

The measure most often used to compare the size of streams is ______________ —the volume of water flowing past a certain point in a given unit of time.

laminar flow

The movement of water particles in straight-line paths that are parallel to the channel. The water particles move downstream without mixing. Flows in parallel lines in a smooth progression.

back swamps When a tributary stream enters a river valley having a substantial natural levee, it often flows for many kilometers through the back swamp before finding an opening where it enters the main river. Such streams are called YAZOO TRIBUTARIES, after the Yazoo River, which parallels the lower Mississippi River for more than 300 kilometers.

The natural levees of the lower Mississippi River rise 6 meters (20 feet) above the adjacent valley floor. The area behind the levee is characteristically poorly drained for the obvious reason that water cannot flow over the levee and into the river. Marshes called _______________ often result.

terraces

The remnants of a former floodplain are sometimes present as relatively flat surfaces called ______________.

Gradient Gradient also varies along the length of a particular channel. When the gradient is steeper, more gravitational energy is available to drive channel flow.

The slope of a stream channel, expressed as the vertical drop of a stream over a specified distance, is called ____________.

flow velocity and settling velocity

The type and amount of material carried in suspension are controlled by two factors:

True

True or False: As the channel slope decreases toward a stream's mouth, the flow velocity generally increases.

braided and meandering channel

Two common types of alluvial channel:

(1) laminar flow (2) turbulent flow

Two types of streamflow:

Infiltration, runoff

When precipitation falls on land, it either soaks into the ground, a process called ______________, flows over the surface as __________, or immediately evaporates.

Deltas

________ are areas where rivers enter oceans or lakes, the water slows and sediment is deposited. As the stream's forward motion is slowed, sediment is deposited by the dying current.

stream Although the terms river and stream are often used interchangeably, geologists define stream as water that flows in a channel, regardless of size. River, on the other hand, is a general term for streams that carry substantial amounts of water and have numerous tributaries

a channel along which water is continually flowing down a slope

graded stream On average, a graded system is neither eroding nor depositing material but is simply transporting it.

a stream that has the necessary slope and other channel characteristics to maintain the minimum velocity required to transport the material supplied to it.

trellis drainage pattern

a system of streams in which nearly parallel tributaries occupy valleys cut in folded strata.

artificial levees

are earthen mounds built on river banks to increase the volume of water the channel can hold.

alluvial fan The fan shape is produced because the main flow swings back and forth between the distributaries from a fixed point where the stream exits the mountain.

are fan-shaped deposits that accumulate along steep mountain fronts back and forth between the distributaries from a fixed point where the stream exits the mountain.

drainage system

are interconnected networks of streams that form a variety of patterns

Stream Channel A basic characteristic that distinguishes streamflow from overland flow is that it is confined in a channel.

can be thought of as an open conduit consisting of the streambed and banks that act to confine flow except, of course, during floods.

Alluvial Channel Alluvial channels form in sediment that was previously deposited in the valley. When the valley floor reaches sufficient width, material deposited by the stream can form a floodplain that borders the channel. Alluvial channels often flow in valleys that have wide valley floors consisting of sand and gravel deposited in the channel, and clay and silt deposited by floods.

channels where the bed and banks are composed mainly of unconsolidated sediment or alluvium

stream valley It includes the valley floor, which is the lower, flatter area that is partially or totally occupied by the stream channel, and the sloping valley walls that rise above the valley floor on both sides.

consists of a channel and the surrounding terrain that directs water to the stream.

base level

defined as the lowest elevation to which a stream can erode its channel.

Dendritic In fact, the word dendritic means "treelike." This drainage pattern develops whenever underlying bedrock is relatively uniform, such as flat-lying sedimentary strata or massive igneous rocks. Because the underlying material is essentially uniform in its resistance to erosion, it does not control the pattern of streamflow. Rather, the pattern is primarily determined by the slope of land.

drainage pattern resembling the branches of a (deciduous) tree.

Depositional floodplain

floodplain produced by major fluctuations in base level or climate conditions

Chanelization

involves altering a stream channel in order to make the flow more efficient. This may simply involve clearing a channel of obstructions or dredging a channel to make it wider and deeper.

Competence Flow velocity is key—swift streams have greater competencies than slow streams, regardless of channel size. A stream's competence increases proportionately to the square of its velocity.

is a measure of a stream's ability to transport particles based on size rather than quantity.

Settling velocity

is defined as the speed at which a particle falls through a still fluid. The larger the particle, the more rapidly it settles toward the stream bed.

Running Water

is the dominant agent of landscape alteration, eroding more terrain and transporting more sediment than any other process.

Hillslope erosion

is the main source of fine particles (clays and fine sand) carried in stream channels.

capacity The greater the discharge, the greater the stream's capacity for hauling sediment. Consequently, large rivers with high flow velocities have large capacities.

is the maximum load of solid particles a stream can transport per unit time.

outlet

it is where the stream exits the drainage basin, is at a lower elevation than the rest of the basin.

Deposition occurs whenever a stream slows, causing a reduction in competence. As its velocity decreases, sediment begins to settle, largest particles first. Thus, stream transport provides a mechanism by which solid particles of various sizes are separated. This process, called sorting , explains why particles of similar size are deposited together.

occurs whenever a stream slows, causing a reduction in competence.

incised meanders

some rivers have meandering channels that flow in steep, narrow, bedrock valleys. Such meanders are called ______________.

bars Such features, however, are only temporary, as the material will be picked up again and even- tually carried to the ocean.

streams continually pick up sed- iment in one part of their channel and deposit it downstream. These small-scale channel deposits are called _______.

John Wesley Powell

the pioneering geologist who first explored the Grand Canyon and later headed the U.S. Geologi- cal Survey, introduced the concept of a downward limit to stream erosion, which he called base level .

turbulent flow whirlpools and eddies, as well as in rolling whitewater rapids. Even streams that appear smooth on the surface often exhibit turbulent flow near the bottom and sides of the channel where flow resistance is greatest.

the water moves in an erratic fashion characterized by a series of horizontal and vertical swirling motions.

Bedrock Channels Bedrock channels are cut into the underlying strata and typically form in the headwaters of river systems where streams have steep slopes. The energetic flow tends to transport coarse particles that actively abrade the bedrock channel. Potholes are often visible evidence of the erosional forces at work. Bedrock channels tend to be located in narrow V-shaped valleys. In some arid regions, where weathering is slow and rock is particularly resistant, narrow valleys having nearly vertical walls are also found.

those channels in which the streams are actively cutting into solid rock.

erosional floodplain

when a river erodes laterally and creates a floodplain

If you have ever waded into a stream, you know that velocity increases as you move into DEEPER PARTS OF THE CHANNEL. This is the result of frictional resistance, which is greatest near the banks and beds of stream channels.

which part of a stream/river has the fastest flow velocities?

Mississippi River

River in North America that is collecting and carrying 40 percent of the flow in the United States

(1) sediment production—where erosion dominates, (2) sediment transport, and (3) sediment deposition The zone of sediment production, where most of the water and sediment is derived, is located in the headwater region of the river system.

River systems can be divided into three zones:

Amazon River, next is Congo River

River with the largest discharge

sheet flow, rills, gullies, and rivers

Runoff is high in urban areas because large areas are covered by impermeable buildings, roads, and parking lots. Runoff initially flows in broad, thin sheets across hillslopes by a process called _____________. This thin, unconfined flow eventually develops threads of current that form tiny channels called ____________ that meet to form ___________, which join to form brooks, creeks, or streams—then, when they reach an undefined size, they are called __________.

saltation

Sediment moving by _______________ appears to jump or skip along the stream bed. This occurs as particles are propelled upward by collisions or lifted by the current and then carried downstream a short distance until gravity pulls them back to the bed of the stream.