Geology Chapter 17

bar 626

A bar in a river is an elevated region of sediment (such as sand or gravel) that has been deposited by the flow.

braided stream 630

A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with high slope and/or large sediment load.[1] Braided channels are also typical of environments that dramatically decrease channel depth, and consequently channel velocity, such as river deltas, alluvial fans and peneplains.

longitudinal profile 626

A cross section of a stream from its mouth to its head, showing elevation versus distance to the mouth.

distributary 635

A distributary, or a distributary channel, is a stream that branches off and flows away from a main stream channel. They are a common feature of river deltas. The phenomenon is known as river bifurcation. The opposite of a distributary is a tributary. Distributaries usually occur as a stream nears a lake or the ocean, but they can occur inland as well, such as on alluvial fans or when a tributary stream bifurcates as it nears its confluence with a larger stream. In some cases, a minor distributary can "steal" so much water from the main channel that it can become the main route.

flash flood 644

A flash flood is a rapid flooding of geomorphic low-lying areas: washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields. Flash floods may occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam, as occurred before the Johnstown Flood of 1889. Flash floods are distinguished from a regular flood by a timescale of less than six hours.[1] The temporary availability of water is often utilized by foliage with rapid germination and short growth cycle, and by specially adapted animal life.

oxbow lake 634

An oxbow lake is a U-shaped body of water that forms when a wide meander from the main stem of a river is cut off, creating a free-standing body of water. This landform is so named for its distinctive curved shape, resembling the bow pin of an oxbow.

dissolved load 624

Dissolved load is material, especially ions from chemical weathering, that are carried in solution by a stream. The dissolved load contributes to the total amount of material removed from a catchment. The amount of material carried as dissolved load is typically much smaller than the suspended load, though this is not always the case. Dissolved load comprises a significant portion of the total material flux out of a landscape, and its composition is important in regulating the chemistry and biology of the stream.

headward erosion 617

Headward erosion is erosion at the origin of a stream channel, which causes the origin to move back away from the direction of the stream flow, and so causes the stream channel to lengthen.

turbulence 623

In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and flow velocity in space and time.

competence 625

In geology competence refers to the degree of resistance of rocks to either erosion or deformation in terms of relative mechanical strength.[1] In mining 'competent rocks' are those in which an unsupported opening can be made.[2] Competent rocks are more commonly exposed at outcrop as they tend to form upland areas and high cliffs or headlands, where present on a coastline. Incompetent rocks tend to form lowlands and are often poorly exposed at the surface. During deformation competent beds tend to deform elastically by either buckling or faulting/fracturing. Incompetent beds tend to deform more plastically, although it is the 'competence contrast' between different rocks that is most important in determining the types of structure that are formed. The relative competence of rocks may change with temperature, such as in metamorphosed limestones, which are relatively competent at low metamorphic grade but become highly incompetent at high metamorphic grade.

saltation 624

In geology, saltation (from Latin, saltus, "leap") is a specific type of particle transport by fluids such as wind or water. It occurs when loose material is removed from a bed and carried by the fluid, before being transported back to the surface.

stream rejuvenation 637

In geomorphology a river is said to be rejuvenated when the base level that it is flowing down to is lowered. This can happen for various reasons.

drainage network 618

In geomorphology, a drainage system are the patterns formed by the streams, rivers, and lakes in a particular drainage basin. They are governed by the topography of the land, whether a particular region is dominated by hard or soft rocks, and the gradient of the land. Geomorphologists and hydrologists often view streams as being part of drainage basins. A drainage basin is the topographic region from which a stream receives runoff, throughflow, and groundwater flow. Drainage basins are divided from each other by topographic barriers called a watershed. A watershed represents all of the stream tributaries that flow to some location along the stream channel. The number, size, and shape of the drainage basins found in an area varies and the larger the topographic map, the more information on the drainage basin is available.

thalweg 623

In hydrological and fluvial landforms, the thalweg is a line drawn to join the lowest points along the entire length of a stream bed or valley in its downward slope, defining its deepest channel.

discharge 621

In hydrology, discharge is the volume rate of water flow, including any suspended solids (e.g. sediment), dissolved chemicals (e.g. CaCO3(aq)), or biologic material (e.g. diatoms), which is transported through a given cross-sectional area.

channel 615

In physical geography, a channel is a type of landform consisting of the outline of a path of relatively shallow and narrow body of fluid, most commonly the confine of a river, river delta or strait. Channels can be either natural or human-made. A channel is typically outlined in terms of its bed and banks.

meandering stream 630

Rivers flowing over gently sloping ground begin to curve back and forth across the landscape. These are called meandering rivers.

stream gradient 626

Stream gradient is the grade measured by the ratio of drop in elevation of a stream per unit horizontal distance, usually expressed as feet per mile or metres per kilometre.

runoff 617

Surface runoff (also known as overland flow) is the flow of water that occurs when excess stormwater, meltwater, or other sources flows over the earth's surface

suspended load 624

Suspended load is the portion of the sediment that is carried by a fluid flow which settle slowly enough such that it almost never touches the bed. It is maintained in suspension by the turbulence in the flowing water and consists of particles generally of the fine sand, silt and clay size.

stream piracy 636

headward erosion by one stream causes the stream to intersect the course of another stream

stream terrace 629

n geology, a terrace is a step-like landform. A terrace consists of a flat or gently sloping geomorphic surface, called a tread, that is typically bounded one side by a steeper ascending slope, which is called a "riser" or "scarp." The tread and the steeper descending slope (riser or scarp) together constitute the terrace. Terraces can also consist of a tread bounded on all sides by a descending riser or scarp. A narrow terrace is often called a bench.[1][2] The sediments underlying the tread and riser of a terrace are also commonly, but incorrectly, called terraces, leading to confusion. Terraces are formed in various ways.

alluvial fan 630

over-time, deposition episodes build a broad gently sloping wedge-shaped apron of sediment called an alluvial fan.

floodway 647

regions likely to be flooded and thus off limits for building homes or businesses

abrasion 623

sand-laden water acts like sandpaper and grinds or rasps away at the channel floor and walls, a process called abrasion.

sheetwash 616

sheet-wash A geomorphological process in which a thin, mobile sheet of water flows over the surface of a hill-slope and may transport the surface regolith. It is important in semi-arid regions, and may also be significant in temperate zones if the vegetation cover has been removed. See rain-wash.

headwaters 615

source of stream

alluvium 626

stream or river deposists

trunk stream 618

the main trunk stream, which functions as a channelway through which water and sediment move from the collecting area toward the ocean. (Erosion and deposition also occur in a river's transporting system)

watershed 619

an area or ridge of land that separates waters flowing to different rivers, basins, or seas.

drainage divide 619

area from which all precipitation flows to a single stream or set of streams. For example, the total area drained by the Mississippi River constitutes its drainage basin, whereas that part of the Mississippi River drained by the Ohio River is the Ohio's drainage basin. The boundary between drainage basins is a drainage divide: all the precipitation on opposite sides of a drainage divide will flow into different drainage basins. A drainage basin provides a limited surface area within which physical processes pertinent to the general hydrology may be considered. The climatic variables and the water and sediment discharge, water storage, and evapotranspiration may be measured; from these measurements, denudation rates and moisture and energy balances may be derived, each of which is useful in the consideration and understanding of landscape formation.

superposed stream 638

because the stream pattern was superposed on the underlying rocks.

pothole 623

a deep natural underground cavity formed by the erosion of rock, especially by the action of water.

flood 615

A flood is an overflow of water that submerges land which is usually dry.[1] The European Union (EU) Floods Directive defines a flood as a covering by water of land not normally covered by water.[2] In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may occur as an overflow of water from water bodies, such as a river or lake, in which the water overtops or breaks levees, resulting in some of that water escaping its usual boundaries,[3] or it may occur due to an accumulation of rainwater on saturated ground in an areal flood. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, these changes in size are unlikely to be considered significant unless they flood property or drown domestic animals. Floods can also occur in rivers when the flow rate exceeds the capacity of the river channel, particularly at bends or meanders in the waterway. Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers. While riverine flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat and fertile and because rivers provide easy travel and access to commerce and industry. Some floods develop slowly, while others such as flash floods, can develop in just a few minutes and without visible signs of rain. Additionally, floods can be local, impacting a neighborhood or community, or very large, affecting entire river basins.

flooplain 634

A flood plain is an area of land that is prone to flooding. People realize it is prone to flooding because it has flooded in the past due to a river or stream overflowing its banks. A flood plain usually is a flat area with areas of higher elevation on both sides.

graded stream 628

A graded stream is one in which, over a period of years, slope is delicately adjusted to provide, with available discharge and the prevailing channel characteristics, just the velocity required for transportation of all of the load supplied from above.

permanent stream 620

A perennial stream or perennial river is a stream or river (channel) that has continuous flow in parts of its stream bed all year round during years of normal rainfall.[1] "Perennial" streams are contrasted with "intermittent" streams which normally cease flowing for weeks or months each year, and with "ephemeral" channels that flow only for hours or days following rainfall. During unusually dry years, a normally perennial stream may cease flowing, becoming intermittent for days, weeks, or months depending on severity of the drought. The boundaries between perennial, intermittent, and ephemeral channels are indefinite, and subject to a variety of identification methods adopted by local governments, academics, and others with a need to classify stream-flow permanence. As stream flow decreases in dry weather, visible flow above the stream bed may not be readily evident, especially in streams with coarse substrate (gravel and rocks), where water is flowing beneath and between these particles (hyporheic flow). From a biological perspective, a stream may be considered flowing if sufficient water is available to support flow-dependent aquatic life, including fish and gill-breathing amphibians, benthic insects, crustaceans, and mollusks, many of which survive in shallow hyporheic flow beneath rocks or logs. This extreme low flow may not be detectable on typical USGS stream-flow gages, but is vital to stream ecology

point bar 626

A point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope. Point bars are found in abundance in mature or meandering streams.

recurrence interval 648

A return period, also known as a recurrence interval (sometimes repeat interval) is an estimate of the likelihood of an event, such as an earthquake, flood or a river discharge flow to occur.

delta 624

A river delta is a landform that forms at the mouth of a river, where the river flows into an ocean, sea, estuary, lake, or reservoir.[1] Deltas form from deposition of sediment carried by a river as the flow leaves its mouth. Over long periods, this deposition builds the characteristic geographic pattern of a river delta. Despite a popular legend, this use of the word delta was not coined by Herodotus.

ephemeral stream 620

A stream drainage that is usually dry and fills with water only during brief episodes of rainfall. Many desert streams ephemeral

downcutting 617

Downcutting, also called erosional downcutting, downward erosion or vertical erosion is a geological process that deepens the channel of a stream or valley by removing material from the stream's bed or the valley's floor. The speed of downcutting depends on the stream's base level, the lowest point to which the stream can erode. Sea level is the ultimate base level, but many streams have a higher "temporary" base level because they empty into another body of water that is above sea level or encounter bedrock that resists erosion. A concurrent process called lateral erosion refers to the widening of a stream channel or valley. When a stream is high above its base level, downcutting will take place faster than lateral erosion; but as the level of the stream approaches its base level, the rate of lateral erosion increases. This is why streams in mountainous areas tend to be narrow and swift, forming V-shaped valleys, while streams in lowland areas tend to be wide and slow-moving, with valleys that are correspondingly wide and flat-bottomed. The term gradient refers to the elevation of a stream relative to its base level. The steeper the gradient, the faster the stream flows. Sometimes geological uplift will increase the gradient of a stream even while the stream downcuts toward its base level, a process called "rejuvenation." This happened in the case of the Colorado River in the western United States, resulting in the process that created the Grand Canyon.

continental divide 619

The Continental Divide of the Americas (also known as the Continental Gulf of Division, the Great Divide, or merely the Continental Divide) is the principal, and largely mountainous, hydrological divide of the Americas. The Continental Divide extends from the Bering Strait to the Strait of Magellan, and separates the watersheds that drain into the Pacific Ocean from (1) those river systems that drain into the Atlantic Ocean (including those that drain into the Gulf of Mexico and the Caribbean Sea), and (2) along the northernmost reaches of the Divide, those river systems that drain into the Arctic Ocean. Though there are many other hydrological divides in the Americas, the Great Divide is by far the most prominent of these because it tends to follow a line of high peaks along the main ranges of the Rocky Mountains and Andes, at a generally much higher elevation than the other hydrological divisions. A continental divide is a drainage divide on a continent such that the drainage basin on one side of the divide feeds into one ocean or sea, and the basin on the other side either feeds into a different ocean or sea, or else is endorheic, not connected to the open sea. The endpoints where a continental divide meets the coast are not always definite, because the exact border between adjacent bodies of water is usually not clearly defined. The International Hydrographic Organization's publication Limits of Oceans and Seas defines exact boundaries of oceans, but it is not universally recognized. Where a continental divide meets an endorheic basin, such as the Great Divide Basin of Wyoming, the continental divide splits and encircles the basin.

base level 627

The base level of a river or stream is the lowest point to which it can flow, often referred to as the 'mouth of the river'.

bed load 624

The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the bed. Bed load is complementary to suspended load and wash load. Bed load moves by rolling, sliding, and/or saltating (hopping).

scouring 623

Tidal scour is an erosion process which is carried out by the tidal movement of water. Examples of this hydrological process can be found in many areas of the world. Two locations in the United States where tidal scour is the predominant shaping force is the San Francisco Bay and the Elkhorn Slough. Many other coastal areas around the world are shaped by this process.

canyon 629

a deep ravine between pairs of escarpments or cliffs and is the most often carved landscape by the erosive activity of a river over geologic timescales. Rivers have a natural tendency to cut through underlying surfaces so will eventually wear away rock layers to lessen their own pitch slowing their waters; given enough time, their bottoms will gradually reach a baseline elevation—which is the same elevation as the body of water it will eventually drain into. This action, when the river source and mouth are at much different base elevations will form a canyon,[1] particularly through regions where softer rock layers are intermingled with harder layers more resistant to weathering. A canyon may also refer to a rift between two mountain peaks, such as those in ranges including the Rocky Mountains, the Alps, the Himalayas or the Andes. Usually a river or stream and erosion carve out such splits between mountains. Examples of mountain-type canyons are Provo Canyon in Utah or Yosemite National Park in California's Sierra Nevada. Canyons within mountains, or gorges that have an opening on only one side are called box canyons. Slot canyons are very narrow canyons, often with smooth walls.

natural levee 634

a deposit of sand or mud built up along, and sloping away from, either side of the flood plain of a river or stream.

drainage reversal 636

a drainage network reorganizes so that water flows, overall, in the opposite direction.

tributary 618

a river or stream flowing into a larger river or lake.

antecedent stream 639

a stream that maintains its original course and pattern despite the changes in underlying rock topography

mouth 615

a winding curve or bend of a river or road