Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Raging Waters

Raging Water

Raging waters causes lots of damages that cannot be avoided.

The Inevitable Catastrophe 

Up to now, in recent posts we have focused on the process of drainage formation and evolution and on the variety of landscape features formed by streams. Now we turn our attention to the havoc that a stream can cause when flooding takes place. Floods can be catastrophic they can strip land of forests and buildings, they can bury land in clay and silt, and they can submerge cities. A flood occurs when the volume of water flowing down a stream exceeds the volume of the stream channel, so water rises out of the channel and spreads out over a floodplain or delta plain, or fills a canyon to a greater depth than normal. 
Floods happen 

  1. during abrupt, heavy rains, when water falls on the ground faster than it can infiltrate and thus becomes surface runoff; 
  2. after a long period of continuous rain, when the ground has become saturated with water and can hold no more; 
  3. when heavy snows from the previous winter melt rapidly in response to a sudden hot spell; or 
  4. when a dam holding back a lake or reservoir, or a levee or retaining wall holding back a river or canal, suddenly collapses and releases the water that it held back. 
Examples of seasonal floodplain flooding.
Geologists find it convenient to divide floods into two general categories. Floods that occur during a “wet season,” when rainfall is heavy or when winter snows start to melt, are called seasonal floods. Floods of this type typically take place in tropical regions during monsoons, and in temperate regions during the spring when storms drench the land frequently and or a heavy winter snow pack melts. When seasonal floods submerge floodplains, they produce floodplain floods, and when they submerge delta plains they produce delta-plain floods (figure above a–c). 

Flash floods can occur after torrential rains.

Events during which the flood waters rise so fast that it may be impossible to escape from the path of the water are called flash floods (figure above a, b). These happen during unusually intense rainfall or as a result of a dam collapse (as in the 1889 Johnstown flood) or levee failure. During a flash flood, a canyon or valley may fill to a level many meters above normal. In some cases, a wall of water may slam downstream with great force, leaving devastation in its wake, but the flood waters subside after a short time. Flash floods can be particularly unexpected in arid or semiarid climates, where isolated thundershowers may suddenly fill the channel of an otherwise dry wash, whose unvegetated ground allows runoff to reach the channel faster. Such a flood may even affect areas downstream that had not received a drop of rain.

Case Study: A Seasonal Flood 

In the spring of 1993, the jet stream, the high-altitude (10–15 km high) wind current that strongly affects weather systems, drifted southward. For weeks, the jet stream’s cool, dry air formed an invisible wall that trapped warm, moist air from the Gulf of Mexico over the central United States. When this air rose to higher elevations, it cooled, and the water it held condensed and fell as rain, rain, and more rain. In fact, almost a whole year’s supply of rain fell in just that spring some regions received 400% more than usual. Because the rain fell over such a short period, the ground became saturated and could no longer absorb additional water, so the excess entered the region’s streams, which carried it into the Missouri and Mississippi rivers. Eventually, the water in these rivers rose above the height of levees or broke through levees, and spread out over the floodplain. By July, parts of nine states were underwater (see a in first figure). 
The roiling, muddy flood uprooted trees, cars, and even coffins (which floated up from inundated graveyards). All barge traffic along the Mississippi came to a halt, bridges and roads were undermined and washed away, and towns along the river were submerged. For example, in Davenport, Iowa, the river front district and baseball stadium were covered with 4 m (14 ft) of water. In Des Moines, Iowa, 250,000 residents lost their supply of drinking water when flood waters contaminated the municipal water supply with raw sewage and chemical fertilizers. Row boats replaced cars as the favoured mode of transportation in towns where only the rooftops remained visible. In St. Louis, Missouri, the river crested 14 m (47 ft) above flood stage. 
For 79 days, the flooding continued. When the water finally subsided, it left behind a thick layer of sediment, filling living rooms and kitchens in floodplain towns and burying crops in floodplain fields. In the end, more than 40,000 square km of the floodplain had been submerged, 50 people died, at least 55,000 homes were destroyed, and countless acres of crops were buried. Officials estimated that the flood caused over $12 billion in damage. Comparable flooding happened again in the spring of 2011, in the Mississippi and Missouri drainage basins. 

Case Study: A Flash Flood 

On a typical sunny day in the Front Range of the Rocky Mountains, north of Denver, Colorado, the Big Thompson River seems quite harmless. Clear water, dripping from melting ice and snow higher in the mountains, flows down its course through a narrow canyon, frothing over and around boulders. In places, vacation cabins, camp grounds, and motels line the river. The landscape seems immutable, but as is the case with  so many geologic features, permanence is an illusion.
On July 31, 1976, easterly winds blew warm, moist air from the Great Plains toward the Rocky Mountain front. As this air rose over the mountains, towering thunder heads built up, and at 7:00 P.M. rain began to fall. It poured, in quantities that even old-timers couldn't recall. In a little over an hour, 19 cm (7.5 inches) of rain drenched the watershed of the Big Thompson River. The river’s discharge grew to more than four times the maximum recorded at any time during the previous century. The river rose quickly, in places reaching depths several meters above normal. Turbulent water swirled down the canyon at up to 8 m per second and churned up so much sand and mud that it became a viscous slurry. Slides of rock and soil tumbled down the steep slopes bordering the river and fed the torrent with even more sediment. The water undercut house foundations and washed the houses away, along with their inhabitants (see above figure b). Roads and bridges disappeared, and boulders that had stood like landmarks for generations bounced along in the torrent like beach balls, striking and shattering other rocks along the way. Cars drifted downstream until they finally wrapped like foil around obstacles. When the flood subsided, the canyon had changed forever, and 144 people had lost their lives.

Living with Floods 

Flood Control  

Holding back rivers to prevent floods.
Mark Twain once wrote of the Mississippi that we “cannot tame that lawless stream, cannot curb it or confine it, cannot say to it, ‘go here or go there,’ and make it obey.” Was Twain right? Since ancient times, people have attempted to control courses of rivers so as  to prevent undesired flooding. In the 20th century, flood-control efforts intensified as the population living along rivers increased. For example, since the passage of the 1927 Mississippi River Flood Control Act (drafted after a disastrous flood took place that year), the U.S. Army Corps of Engineers has laboured to control the Mississippi. First, engineers built about 300 dams along the river’s tributaries so that excess run-off could be stored in the reservoirs and later be released slowly. Second, they built artificial levees of sand and mud, and built concrete flood walls to increase the channel’s volume. Artificial levees and flood walls isolate a discrete area of the floodplain (figure above a–c). 
Although the Corps’ strategy worked for floods up to a certain size, it was insufficient to handle the 1993 and 2011 floods when reservoirs filled to capacity, and additional run-off headed downstream. The river rose until it spilled over the tops of some levees and undermined others. “Undermining” occurs when rising water levels increase the water pressure on the river side of the levee, forcing water through sand under the levee. In susceptible areas, water begins to spurt out of the ground on the dry side of the levee, thereby washing away the levee’s support. The levee finally becomes so weak that it collapses, and water fills in the area behind it. In some cases, the Corps of Engineers intentionally dynamites levees along a relatively unpopulated reach of the river upstream of a vulnerable town. This diverts water out onto a portion of the floodplain where the water will do less damage, and prevents the flood waters from over topping levees close to the town.
Another solution to flooding in some localities may involve restoration of wetland areas along rivers, for wetlands can absorb significant quantities of flood water. Also, where appropriate, planners may prohibit construction within designated land areas adjacent to the channel, so that flood water can fill these areas without causing expensive damage. The existence of such areas, which are known as flood ways, effectively increases the volume of water that the river can carry and thus helps prevent the water level from rising too high.

Evaluating Flooding Hazard  

When making decisions about investing in flood-control measures, mortgages, or insurance, planners need a basis for defining the hazard or risk posed by flooding. If flood waters submerge a locality every year, a bank officer would be ill advised to approve a loan that would promote building there. But if flood waters submerge the locality very rarely, then the loan may be worth the risk. Geologists characterize the risk of flooding in two ways. The annual probability of flooding indicates the likelihood that a flood of a given size or larger will happen at a specified locality during any given year. For example, if we say that a flood of a given size has an annual probability of 1%, then we mean there is a 1 in 100 chance that a flood of at least this size will happen in any given year. The recurrence interval of a flood of a given size is defined as the average number of years between successive floods of at least this size. For example, if a flood of a given  size happens once in 100 years, on average, then it is assigned a recurrence interval of 100 years and is called a “100-year-flood.” Note that annual probability and recurrence interval are related:
    annual probability = 1/recurrence interval 
For example, the annual probability of a 50-year flood is 1/50, which can also be written as 0.02 or 2%. 
Unfortunately, some people are misled by the meaning of recurrence interval, and think that they do not face future flooding hazard if they buy a home within an area just after a 100-year flood has occurred. Their confidence comes from making the incorrect assumption that because such flooding just happened, it can’t happen again until “long after I'm gone.” They may regret their decision because two 100-year floods can occur in consecutive years or even in the same year (alternatively, the interval between such floods could be, say, 210 years).

The conceptual relationship between flood size and probability.
The recurrence interval for a flood along a particular river reflects the size of a flood. For example, the discharge of a 100-year flood is larger than that of a 2-year flood, because the 100-year event happens less frequently (figure above a). To define this relationship, geologists construct graphs that plot flood discharge on the vertical axis against recurrence interval on the horizontal axis (figure above b).
Knowing the discharge during a flood of a specified annual probability, and knowing the shape of the river channel and the elevation of the land bordering the river, hydrologists can predict the extent of land that will be submerged by such a flood. Such data, in turn, permit hydrologists to produce flood-hazard maps. In the United States, the Federal Emergency Management Agency (FEMA) produces maps that show the 1% annual probability (100-year) flood area and the 0.2% annual probability (500-year) flood risk zones (figure above c).
Figures credited to Stephen Marshak.

The Evolution of Drainage

Drainage Evolution

Beveling Topography 

Fluvial landscapes evolve over time.
Imagine a place where continental collision uplifts a region; the landscape will evolve (figure above a). At first, rivers have steep gradients, flow over many rapids and waterfalls, and cut deep valleys. But with time, rugged mountains become low, rounded hills; once-deep, narrow valleys broaden into wide floodplains with more gradual gradients. As more time passes, even the low hills are beveled down, becoming small mounds or even disappearing altogether. (Some geologists have referred to the resulting landscape as a peneplain, from the Latin paene, which means almost; it lies at an elevation close to that of a stream’s base level.) 
Though the above model makes intuitive sense, it is an oversimplification. Plate tectonics can uplift the land again, and/or global sea-level rise or fall can change the base level, so in reality peneplains rarely develop before downcutting begins again. Stream rejuvenation occurs when a stream starts to downcut into a land surface whose elevation lies near the stream’s base level. Rejuvenation can be triggered by several phenomena, such as: a drop in base level, as happens when sea level falls; an uplift event that causes the land to rise relative to the base level; or an increase in stream discharge that makes the stream more able to erode and transport sediment. As we've seen, rejuvenation can lead to formation of stream terraces in alluvium. In cases where rejuvenation causes a stream to erode deeply into the land, a new canyon or valley will develop. If the rejuvenated stream had a meandering course, downcutting produces incised meanders (figure above b). 

Stream Piracy and Drainage Reversal 

The concept of stream capture or “piracy.”
Stream piracy sounds like pretty violent stuff. In reality, it’s just a natural process that happens when headward erosion by one stream causes the stream to intersect the course of another stream. When this happens, the pirate stream “captures” the water of the stream that it has intersected, so that the water of the captured stream starts to flow down the pirate stream. Because of piracy, the channel of the captured stream, downstream of the point of capture, dries up (figure above a, b). In some cases, stream capture changes a “water gap” (a stream carved notch through a ridge) into a dry “wind gap.” In 1775, Daniel Boone, the legendary pioneer, led settlers through the Cumberland Gap, a wind gap in the Appalachians, to new homesteads in western Kentucky. 
The pattern of stream flow in an area can also be altered, over time, on a continental scale. For example, when South America and Africa were adjacent to each other in Pangaea, a highland existed along the boundary between the two continents, and the main drainage network of northern South America flowed westward. Later, when South America rifted away from Africa, a convergent plate boundary developed along the western margin of South America, causing the Andes Mountains to rise. The uplift of the Andes caused a drainage reversal, in that the overall slope direction of the drainage network became the opposite of what it once had been. As a consequence, westward flow became impossible, and the eastward-flowing Amazon drainage network developed.

Superposed and Antecedent Streams 

The structure and topography of the landscape do not always appear to control the path, or course, of a stream. For example, imagine a stream that carves a deep canyon straight across a strong mountain ridge why didn't the stream find a way around the ridge? We distinguish two types of streams that cut across resistant topographic highs: 

 Formation of superposed drainage
  • Superposed streams: Imagine a region in which streams start to flow over horizontal beds of strata that unconformably overlie folded strata. When the streams eventually erode down through the unconformity and start to downcut into the folded strata, they maintain their earlier course, ignoring the structure of the folded strata. Geologists call such streams superposed streams, because their pre-existing geometry has been laid down on underlying rock structure (figure above a, b). 
  • Antecedent streams: In some cases, tectonic activity (such as subduction or collision) causes a mountain range to rise up beneath an already established stream. If the stream downcuts as fast as the range rises, it can maintain its course and will cut right across the range. Geologists call such streams antecedent streams (from the Greek ante, meaning before) to emphasize that they existed before the range uplifted. Note that if the range rises faster than the stream downcuts, the new highlands divert (change) the stream’s course so that it flows parallel to the range face (figure below a–c).
 Development of antecedent and diverted streams.
Credits: Stephen Marshak (Essentials of Geology)

deposition in science (river)

Deposition in science (Streams and Their Deposits in the Landscape)

Deposition by river are in different places where slumps, alluvial fans, valleys, canyons, delta, waterfalls, floodplains. 

Valleys and Canyons

About 17 million years ago, a large block of crust, the region now known as the Colorado Plateau (located in Arizona, Utah, Colorado, and New Mexico), began to rise. Before the rise, the Colorado River had been flowing over a plain not far above sea level, causing little erosion. But as the land uplifted, the river began to downcut. Eventually, its channel lay as much as 1.6 km below the surface of the plateau at the base of a steep walled gash now known as the Grand Canyon. The formation of the Grand Canyon illustrates a general phenomenon. In regions where the land surface lies well above the base level, a stream can carve a deep trough, much deeper than the channel itself. If the walls of the trough slope gently, the landforms is a valley, but if they slope steeply, the landforms is a canyon. 

The shape of a canyon or valley depends on the resistance of its walls to erosion slumping.
Whether stream erosion produces a valley or a canyon depends on the rate at which down cutting occurs relative to the rate at which mass wasting causes the walls on either side of the stream to collapse. In places where a stream downcuts through its substrate faster than the walls of the stream collapse, erosion creates a slot (steep-walled) canyon. Such canyons typically form in hard rock, which can hold up steep cliffs for a long time (figure above a). In places where the walls collapse as fast as the stream downcuts, landslides and slumps gradually cause the slope of the walls to approach the angle of repose. When this happens, the stream channel lies at the floor of a valley whose cross-sectional shape resembles the letter V (figure above b); this landforms is called a V-shaped valley. Where the walls of the stream consist of alternating layers of hard and soft rock, the walls develop a stair-step shape such as that of the Grand Canyon (figure above c).

 The evolution of alluvium-filled stream valleys and the development of terraces.
In places where active down cutting occurs, the valley floor remains relatively clear of sediment, for the stream especially when it floods carries away sediment that has fallen or slumped into the channel from the stream walls. But if the stream’s base level rises, its discharge decreases, or its sediment load increases, the valley floor fills with sediment, creating an alluvium-filled valley (figure above a). The surface of the alluvium becomes a broad floodplain. If the stream’s base level later drops again and/or the discharge increases, the stream will start to cut down into its own alluvium, a process that generates stream terraces bordering the present floodplain (figure above b).

Rapids and Waterfalls 

Examples of rapids and waterfalls.
When Lewis and Clark forged a path up the Missouri River, they came to reaches that could not be navigated by boat because of rapids, particularly turbulent water with a rough surface (figure above a). Rapids form where water flows over steps or large clasts in the channel floor, where the channel abruptly narrows, or where its gradient abruptly changes. The turbulence in rapids produces eddies, waves, and whirlpools that roil and churn the water surface, in the process creating white water, a mixture of bubbles and water. Modern-day white water rafters thrill to the unpredictable movement of rapids. 
A waterfall forms where the gradient of a stream becomes so steep that some or all of the water literally free falls above the stream bed (figure above b). The energy of falling water may scour a depression, called a plunge pool, at the base of the waterfall. Though a waterfall may appear to be a permanent feature of the landscape, all waterfalls eventually disappear because headward erosion slowly eats back the resistant ledge. We can see a classic example of headward erosion at Niagara Falls. As water flows from Lake Erie to Lake Ontario, it drops over a 55-m-high ledge of resistant Silurian dolostone, which overlies a weak shale. Erosion of the shale leads to undercutting of the dolostone. Gradually, the overhang of dolostone becomes unstable and collapses, with the result that the waterfall migrates upstream. Before the industrial age, the edge of Niagara Falls cut upstream at an average rate of 1 m per year; but since then, the diversion of water from the Niagara River into a hydroelectric power station has decreased the rate of headward erosion to half that (figure below a, b).

The formation of Niagara Falls, at the border between Ontario, Canada, and New York State. The falls tumble over the Lockport Dolomite, a relatively strong rock layer.

Alluvial Fans and Braided Streams 

 Examples of depositional landforms produced from stream sediment.
Where a fast-moving stream abruptly emerges from a mountain canyon into an open plain at the range front, the water that was once confined to a narrow channel spreads out over a broad surface. As a consequence, the water slows and drops its sedimentary load, forming a sloping apron of sediment (sand, gravel, and cobbles) called an alluvial fan (figure above a). The stream then divides into a series of small channels that spread out over the fan. During particularly strong floods, the water contains so much sediment that it becomes a debris flow that spreads over and smooths out the fan’s surface. 
In some localities, streams carry abundant coarse sediment during floods but cannot carry this sediment during normal flow. Thus, during normal flow, the sediment settles out and chokes the channel. As a consequence, the stream divides into numerous strands weaving back and forth between elongate bars of gravel and sand. The result is a braided stream the name emphasizes that the streams entwine like strands of hair in a braid (figure above b).

Meandering Streams  and Their Floodplains 

The character and evolution of meandering streams and floodplains.
A riverboat cruising along the lower reaches of the Mississippi River cannot sail in a straight line, for the river channel winds back and forth in a series of snake-like curves called meanders (figure above a). In fact, the boat has to go 500 km along the river channel to travel 100 km as the crow flies.
How do meanders evolve? Even if a stream starts out with a straight channel, natural variations in the water depth and associated friction cause the fastest-moving current to swing back and forth. The water erodes the side of the stream more effectively where it flows faster, so it begins to cut away faster on the outer arc of the curve. Thus, each curve begins to migrate sideways and grow more pronounced until it becomes a meander (figure above b). On the outside edge of a meander, erosion continues to eat away at the channel wall, forming a cut bank. On the inside edge, water slows down so that its competence decreases and sediment accumulates, forming a wedge-shaped deposit called a point bar, as noted earlier. 
With continued erosion, a meander may curve through more than 180 degree, so that the cut bank at the meander’s entrance approaches the cut bank at its end, leaving a meander neck, a narrow isthmus of land separating the portions of the meander. When erosion eats through a meander neck, a straight reach called a cut off develops. The meander that has been cut off is called an oxbow lake if it remains filled with water, or an abandoned meander if it dries out (figure above c). Streams that develop many meanders are known, not surprisingly, as meandering streams. The course of a meandering stream naturally changes over time, on a time scale of years to centuries, as new meanders grow and old ones are cut off and abandoned.
Most meandering stream channels cover only a relatively small portion of a broad, gently sloping floodplain (figure above d). Floodplains, as we noted earlier, are so named because during a flood, water over-tops the edge of the stream channel and spreads out over the floodplain. In many cases, a floodplain terminates at its sides along a bluff, or escarpment; large floods may cover the entire floodplain from bluff to bluff. As the water rises above the channel walls and starts to spread out, over the floodplain, friction slows down the flow. This slowdown decreases the competence of the running water, so sediment settles out along the edge of the channel. Over time, the accumulation of this sediment creates a pair of low ridges, called natural levees, on either side of the stream. Natural levees may grow so large that the floor of the channel may become higher than the surface of the floodplain.

Deltas: Deposition at the Mouth of a Stream 

Along most of its length, only a narrow floodplain covered by green, irrigated farm fields borders the Nile River in Egypt. But at its mouth, the trunk stream of the Nile divides into a fan of smaller streams, called distributaries, and the area of green agricultural lands broadens into a triangular patch. The Greek historian Herodotus noted that this triangular patch resembles the shape of the Greek letter delta, and so the region became known as the Nile Delta. Deltas develop where the running water of a stream enters standing water, the current slows, the stream loses competence, and sediment settles out. This can happen in either a lake or the sea.

Delta shape varies depending on current activity, waves, and vegetation.
Geologists refer to any wedge of sediment formed at a river mouth as a delta, even though relatively few have the triangular shape of the Nile Delta (figure above a–c). Some deltas curve smoothly outward, whereas others consist of many elongate lobes formed at different times. Bird’s-foot deltas, so-named because they resemble the scrawny toes of a bird, develop where several distributaries extend far out into relatively calm water; the end of the Mississippi’s active channel ends in a bird’s-foot delta  (figure above a–c). 

 A map showing ancient lobes of the Mississippi Delta. A major flood could divert water from the Mississippi into the channel of the Atchafalaya.
The existence of several overlapping deltas indicates that the main course of the river in the delta has shifted on several occasions. These shifts occur when a toe builds so far out into the sea that the slope of the stream becomes too gentle to allow the river to flow. At this point, the river overflows a natural levee upstream and begins to flow in a new direction, an event called an avulsion. The distinct lobes of the Mississippi Delta suggest that avulsions have happened several times during the past 9,000 years (figure above). New Orleans, built along one of the Mississippi’s distributaries, may eventually lose its riverfront, for a break in a levee upstream of the city could divert the Mississippi into the Atchafalaya River channel. 
The shape of a delta depends on many factors. Deltas that form where the strength of the river current exceeds that of ocean currents have a bird’s-foot shape, since the sediment can be carried far offshore. In contrast, deltas that form where the ocean currents are strong have a $ shape, for the ocean currents redistribute sediment in bars running parallel to the shore. And in places where waves and currents are strong enough to remove sediment as fast as it arrives, a river has no delta at all.
With time, the sediment of a larger delta compacts, and the weight of the delta pushes down the crust below. As a consequence, the surface of a delta slowly sinks. Distributaries can provide sediment that fills the resulting space so that the delta’s surface remains at or just above sea level, forming a broad, flat area called a delta plain. But if people build up artificial levees to constrain the river to its channel, sediment gets carried directly to the seaward edge of the delta and the delta’s interior “starves” (does not receive sediment). When this happens, the delta’s surface drops below sea level. Because of this process, much of New Orleans lies below sea level.
Figures credited to Stephen Marshak.

Running Water

The Work of Running Water

How Do Streams Erode? 

The energy that makes running water move comes from gravity. As water flows downslope from a higher to a lower elevation, the gravitational potential energy stored in water transforms into kinetic energy. About 3% of this energy goes into the work of eroding the walls and beds of stream channels. Running water causes erosion in four ways:
  • Scouring: Running water can remove loose fragments of sediment, a process called scouring. 
  • Breaking and lifting: In some cases, the push of flowing water can break chunks of solid rock off the channel floor or walls. In addition, the flow of a current over a clast can cause the clast to rise, or lift off the substrate. 
  • Abrasion: Clean water has little erosive effect, but sedimentladen water acts like sandpaper and grinds or rasps away at the channel floor and walls, a process called abrasion. In places where turbulence produces long-lived whirlpools, abrasion by sand or gravel carves a bowl-shaped depression, called a pothole, into the floor of the stream (figure below a, b). 
  • Dissolution: Running water dissolves soluble minerals as it passes, and carries the minerals away in solution. 

Erosion and transportation in streams.
The efficiency of erosion depends on the velocity and volume of water and on its sediment content. A large volume of fast moving, turbulent, sandy water causes more erosion than does a trickle of quiet, clear water. Thus, most erosion takes place during floods, when a stream carries a large volume of fast moving, sediment-laden water.

How Do Streams Transport Sediment?

The Mississippi River received the nickname “Big Muddy” for a reason its water can become chocolate brown because of all the clay and silt it carries. Geologists refer to the total volume of sediment carried by a stream as its sediment load. The sediment load consists of three components (figure above c): 
  • Dissolved load: Running water dissolves soluble minerals in the sediment or rock that it flows over, and groundwater seeping into a stream brings dissolved minerals with it. The ions of these dissolved minerals constitute a stream’s dissolved load. 
  • Suspended load: The suspended load of a stream usually consists of tiny solid grains (silt or clay size) that swirl along with the water without settling to the floor of the channel. 
  • Bed load: The bed load of a stream consists of large particles (such as sand, pebbles, or cobbles) that bounce or roll along the stream floor. Bed-load movement commonly involves saltation. During saltation, a multitude of grains bounce along in the direction of flow, within a zone that extends up from the surface of the stream bed for a distance of several centimetres to several tens of centimetres. Each saltating grain in this zone follows a curved trajectory up through the water and then back down to the bed. When it strikes the bed, it knocks other grains upward, and thus supplies grains to the saltation zone.
When describing a stream’s ability to carry sediment, geologists specify its competence and capacity. The competence of a stream refers to the maximum particle size it carries; a stream with high competence can carry large particles, whereas one with low competence can carry only small particles. Competence depends on water velocity. Thus, a fast-moving, turbulent stream has greater competence (it can carry bigger particles) than a slow-moving stream, and a stream in flood has greater competence than a stream with normal flow. In fact, the huge boulders that litter the bed of a mountain creek move only during floods. The capacity of a stream refers to the total quantity of sediment it can carry. A stream’s capacity depends on its competence and discharge. So a large river has more capacity than a small creek.

Depositional Processes 

A raging torrent of water can carry coarse and fine sediment the finer clasts rush along with the water as suspended load, whereas the coarser clasts may bounce and tumble as bed load. If the flow velocity decreases, either because the slope of the stream bed becomes shallower or because the channel broadens out and friction between the bed and the water increases, then the competence of the stream decreases and sediment settles out. The size of the clasts that settle at a particular locality depends on the decrease in flow velocity at the locality. For example, if the stream slows by a small amount, only large clasts settle; if the stream slows by a greater amount, medium-sized clasts settle; and if the stream slows almost to a standstill, the fine grains settle. Because of this process of sediment sorting, stream deposits tend to be segregated by size gravel accumulates in one location and mud in another.

Sediments, carried and deposited by streams. The clast size depends upon the stream velocity.
Geologists refer to sediments transported by a stream as fluvial deposits (from the Latin fluvius, meaning river) or alluvium. Fluvial deposits may accumulate along the stream bed in elongate mounds, called bars (figure above a, b). In cases where the stream channel makes a broad curve, water slows along the inner edge of a curve, so a crescent-shaped point bar bordering the shoreline of the inner curve develops. During floods, a stream may over-top the banks of its channel and spread out over its floodplain, a broad flat area bordering the stream. Friction slows the water on the floodplain, so a sheet of silt and mud settles out to comprise floodplain deposits. Where a stream empties at its mouth into a standing body of water, the water slows and a wedge of sediment, called a delta, accumulates (figure above c).
Figures credited to Stephen Marshak.

Land drainage

Draining the Land 

Water that drains the land has a series of streams network which is filled from either the ground water or the water from the atmosphere, hydrologic cycle.

Forming Streams and Drainage Networks

Excess surface water (runoff) comes from rain, melting ice or snow, and ground water springs. On flat round, water accumulates in puddles ow swamps, but no slopes, it flows downslope in streams.
Where does the water in a stream come from? Recall that water enters the hydrologic cycle by evaporating from the Earth’s surface and rising into the atmosphere. After a relatively short residence time, atmospheric water condenses and falls back to the Earth’s surface as rain or snow that accumulates in various reservoirs. Some rain or snow remains on the land as surface water (in puddles, swamps, lakes, snowfields, and glaciers), some flows downslope as a thin film called sheetwash, and some sinks into the ground, where it either becomes trapped in soil (as soil moisture) or descends below the water table to become groundwater. (the water table is the level below which groundwater fills all the pores and cracks in subsurface rock or sediment. Above the water table, air partially or entirely fills the pores and cracks.) Streams can receive input of water from all of these reservoirs (figure above). Specifically, gravity pulls surface water (including meltwater) downhill into stream channels, the pressure exerted by the weight of new rainfall squeezes existing soil moisture back out of the ground, and groundwater seeps out of the channel walls into the channel, if the floor of the channel lies below the water table. 
Running water collects in stream channels, because a channel is lower than the surrounding area and gravity always moves material from higher to lower elevation. How does a stream channel form in the first place? The process of channel formation begins when sheetwash starts flowing downslope. Like any flowing fluid, sheetwash erodes its substrate (the material it flows over). The efficiency of such erosion depends on the velocity of the flow faster flows erode more rapidly. In nature, the ground is not perfectly planar, not all substrate has the same resistance to erosion, and the amount of vegetation that covers and protects the ground varies with location. Thus, the velocity of sheetwash also varies with location. Where the flow happens to be a bit faster, or the substrate is a little weaker, erosion scours (digs) a channel. Since the channel is lower than the surrounding ground, sheetwash in adjacent areas starts to head toward it. With time, the extra flow deepens the channel relative to its surroundings, a process called downcutting, and a stream forms. 

 An example of headward erosion. The main stream flows in a deep valley. Side streams are cutting into the bordering plateau.
As its flow increases, a stream channel begins to lengthen at its origin, a process called headward erosion (figure above). Headward erosion occurs for two reasons. First, it happens when the surface flow converging at the entrance to a channel has sufficient erosive power to downcut. Second, it happens at locations where groundwater seeps out of the ground and enters the entrance to the stream channel. Such seepage, called “groundwater sapping,” gradually weakens and undermines the soil or rock just upstream of the channel’s endpoint until the material collapses into the channel; the collapsed debris eventually washes away during a flood. Each increment of collapse makes the channel longer.
As downcutting deepens the main channel, the surrounding land surfaces start to slope toward the channel. Thus, new side channels, or tributaries, begin to form, and these flow into the main channel. Eventually, an array of linked streams evolves, with the smaller tributaries flowing into a trunk stream. The array of interconnecting streams together constitute the drainage network. Like transportation networks of roads, drainage networks of streams reach into all corners of a region, providing conduits for the removal of runoff. 

 Block diagrams illustrating five types of drainage networks.
The configuration of tributaries and trunk streams defines the map pattern of a drainage network. This pattern depends on the shape of the landscape and the composition of the substrate. Geologists recognize several types of networks on the basis of the network’s map pattern (figure above).
  • Dendritic: When rivers flow over a fairly uniform substrate with a fairly uniform initial slope, they develop a dendritic network, which looks like the pattern of branches connecting to the trunk of a deciduous tree. 
  • Radial: Drainage networks forming on the surface of a cone shaped mountain flow outward from the mountain peak, like spokes on a wheel. Such a pattern defines a radial network. 
  • Rectangular: In places where a rectangular grid of fractures (vertical joints) breaks up the ground, channels form along the preexisting fractures, and streams join each other at right angles, creating a rectangular network. 
  • Trellis: In places where a drainage network develops across a landscape of parallel valleys and ridges, major tributaries flow down a valley and join a trunk stream that cuts across the ridges. The resulting map pattern resembles a garden trellis, so the arrangement of streams constitutes a trellis network. 
  • Parallel: On a uniform slope, several streams with parallel courses develop simultaneously. The group comprises a parallel network.

Drainage Basins and Divides

Drainage divides and basins.
A drainage network collects water from a broad region, variously called a drainage basin, catchment, or watershed, and feeds it into the trunk stream, which carries the water away. The highland, or ridge, that separates one watershed from another is a drainage divide (figure above a, b). A continental divide separates drainage that flows into one ocean from drainage that flows into another. For example, if you straddle the continental divide where it runs along the crest of the Rocky Mountains in the western United States, and pour a cup of water out of each hand, the water in one hand flows to the Atlantic, and the water in the other flows to the Pacific. Three divides bound part of the Mississippi drainage basin, which drains the interior of the United States.

Streams That Last, Streams That Don’t

The contact between permanent and ephemeral streams.
Permanent streams flow all year long, whereas ephemeral streams flow only for part of the year. Some ephemeral streams flow only for tens of minutes to a few hours, following a heavy rain. Most permanent streams exist where the floor (or bed) of the stream channel lies below the water table (figure above a). In these streams, which occur in humid or temperate climates, water comes not only from upstream or from surface runoff, but also from springs through which groundwater seeps. If the bed of a stream lies above the water table, then the stream can be permanent only when the rate at which water  arrives from upstream exceeds the rate at which water infiltrates into the ground below. For example, the downstream portion of the Colorado River in the dry Sonoran Desert of Arizona flows all year, because enough water enters it from the river’s wet headwaters upstream in Colorado; hardly any water enters the stream from the desert itself. 
Streams that do not have a sufficient upstream source, and whose beds lie above the water table, are ephemeral, because the water that fills a channel due to a heavy rain or a spring thaw eventually sinks into the ground and/or evaporates, and the stream dries up (figure above b). Streams whose watersheds lie entirely within an arid region tend to be ephemeral. The dry bed of an ephemeral stream is variously called a dry wash, an arroyo, or a wadi.
Credits: Stephen Marshak (Essentials of Geology)