A stream is a body of water that carries rock particles and dissolved ions and flows down slope along a clearly defined path, called a channel. Thus streams may vary in width from a few centimeters to several kilometers. Streams are important for several reasons:
The stream channel is the conduit for water being carried by the stream. The stream can continually adjust its channel shape and path as the amount of water passing through the channel changes. The volume of water passing any point on a stream is called the discharge. Discharge is measured in units of volume/time (m3/sec).
The cross sectional shape varies with position in the stream, and discharge. The deepest part of channel occurs where the stream velocity is the highest. Both width and depth increase downstream because discharge increases downstream. As discharge increases the cross sectional shape will change, with the stream becoming deeper and wider.
plot of elevation versus distance. Usually shows a steep gradient near the
source of the stream and a gentle gradient as the stream approaches its mouth.
Base level is defined as the limiting level below which a stream cannot erode its channel. For streams that empty into the oceans, base level is sea level. Local base levels can occur where the stream meets a resistant body of rock, where a natural or artificial dam impedes further channel erosion, or where the stream empties into a lake. When a natural or artificial dam impedes stream flow, the stream adjusts to the new base level by adjusting its long profile. In the example here, the long profile above and below the dam are adjusted. Erosion takes place downstream from the dam (especially if it is a natural dam and water can flow over the top). Just upstream from the dam the velocity of the stream is lowered so that deposition of sediment occurs causing the gradient to become lower.
A stream's velocity depends on position in the stream channel, irregularities in the stream channel caused by resistant rock, and stream gradient. The average velocity is the time it takes a given particle of water to traverse a given distance. Stream flow can be either laminar, in which all water molecules travel along similar parallel paths, or turbulent, in which individual particles take irregular paths. Turbulent flow can keep sediment in suspension longer than laminar flow and aids in erosion of the stream bottom. Average linear velocity is generally greater in laminar flow than in turbulent flow.
The discharge of a stream is the amount of water passing any point in a given time.
Discharge (m3/sec) = Cross-sectional Area (width x average depth) (m2) x Average Velocity (m/sec). As the amount of water in a stream increases, the stream must adjust its velocity and cross sectional area in order to form a balance. Discharge increases as more water is added through rainfall, tributary streams, or from groundwater seeping into the stream. As discharge increases, generally width, depth, and velocity of the stream also increase.
The rock particles and dissolved ions carried by the stream are the called the stream's load. Stream load is divided into three parts. Suspended Load - particles that are carried along with the water in the main part of the streams. The size of these particles depends on their density and the velocity of the stream. Higher velocity currents in the stream can carry larger and denser particles.
Coarser and denser particles that remain on the bed of the stream most of the time but move by a process of saltation (jumping) as a result of collisions between particles, and turbulent eddies. Note that sediment can move between bed load and suspended load as the velocity of the stream changes.
Ions that have been introduced into the water by chemical weathering of rocks. This load is invisible because the ions are dissolved in the water. The dissolved load consists mainly of HCO3- (bicarbonate ions), Ca+2, SO4-2, Cl-, Na+2, Mg+2, and K+. These ions are eventually carried to the oceans and give the oceans their salty character. Streams that have a deep underground source generally have higher dissolved load than those whose source is on the Earth's surface.
As one moves along a stream in the downstream direction: Discharge increases,
as noted above, because water is added to the stream from tributary streams
and groundwater. As discharge increases, the width, depth, and average velocity
of the stream increase.
The gradient of the stream, however, will decrease. It may seem to be counter to your observations that velocity increases in the downstream direction, since when one observes a mountain stream near the headwaters where the gradient is high, it appears to have a higher velocity than a stream flowing along a gentle gradient. But, the water in the mountain stream is likely flowing in a turbulent manner, due to the large boulders and cobbles which make up the streambed. If the flow is turbulent, then it takes longer for the water to travel the same linear distance, and thus the average velocity is lower. Also as one moves in the downstream direction, The size of particles that make up the bed load of the stream tends to decrease. Even though the velocity of the stream increases downstream, the bed load particle size decreases mainly because the larger particles are left in the bed load at higher elevations and abrasion of particles tends to reduce their size.
The composition of the particles in the bed load tends to change along the stream as different bedrock is eroded and added to the stream's load. Floods Floods occur when the discharge of the stream becomes too high to be accommodated in the normal stream channel. When the discharge becomes too high, the stream widens its channel by overtopping its banks and flooding the low-lying areas surrounding the stream. The areas that become flooded are called floodplains.
Straight stream channels are rare. Where they do occur, the channel is usually controlled by a linear zone of weakness in the underlying rock, like a fault or joint system. Even in straight channel segments water flows in a sinuous fashion, with the deepest part of the channel changing from near one bank to near the other. Velocity is highest in the zone overlying the deepest part of the stream. In these areas, sediment is transported readily resulting in pools. Where the velocity of the stream is low, sediment is deposited to form bars. The bank closest to the zone of highest velocity is usually eroded and results in a cutbank.
Because of the velocity structure of a stream, and especially in streams flowing over low gradients with easily eroded banks, straight channels will eventually erode into meandering channels. Erosion will take place on the outer parts of the meander bends where the velocity of the stream is highest. Sediment deposition will occur along the inner meander bends where the velocity is low. Such deposition of sediment results in exposed bars, called point bars. Because meandering streams are continually eroding on the outer meander bends and depositing sediment along the inner meander bends, meandering stream channels tend to migrate back and forth across their flood plain.
If erosion on the outside meander bends continues to take place, eventually a meander bend can become cut off from the rest of the stream. When this occurs, the cutoff meander bend, because it is still a depression, will collect water and form a type of lake called an oxbow lake.
streams having highly variable discharge and easily eroded banks, sediment
gets deposited to form bars and islands that are exposed during periods of
low discharge. In such a stream the water flows in a braided pattern around
the islands and bars, dividing and reuniting as it flows downstream. Such
a channel is termed a braided channel. During periods of high discharge,
the entire stream channel may contain water and the islands are covered to
become submerged bars. During such high discharge, some of the islands could
erode, but the sediment would be re-deposited as the discharge decreases,
forming new islands or submerged bars. Islands may become resistant to erosion
if they become inhabited by vegetation.
Streams erode because they have the ability to pick up rock fragments and transport them to a new location. The size of the fragments that can be transported depends on the velocity of the stream and whether the flow is laminar or turbulent. Turbulent flow can keep fragments in suspension longer than laminar flow. Streams can also eroded by undercutting their banks resulting in mass-wasting processes like slumps or slides. When the undercut material falls into the stream, the fragments can be transported away by the stream. Streams can cut deeper into their channels if the region is uplifted or if there is a local change in base level. As they cut deeper into their channels the stream removes the material that once made up the channel bottom and sides.
Sudden changes in velocity can result in deposition by streams. Within a stream we have seen that the velocity varies with position, and, if sediment gets moved to the lower velocity part of the stream the sediment will come out of suspension and be deposited. Other sudden changes in velocity that affect the whole stream can also occur. For example if the discharge is suddenly increased, as it might be during a flood, the stream will overtop its banks and flow onto the floodplain where the velocity will then suddenly decrease. This results in deposition of such features as levees and floodplains. If the gradient of the stream suddenly changes by emptying into a flat-floored basin, an ocean basin, or a lake, the velocity of the stream will suddenly decrease resulting in deposition of sediment that can no longer be transported. This can result in deposition of such features as alluvial fans and deltas. Floodplains and Levees - As a stream overtops its banks during a flood, the velocity of the flood will first be high, but will suddenly decrease as the water flows out over the gentle gradient of the floodplain. Because of the sudden decrease in velocity, the coarser grained suspended sediment will be deposited along the riverbank, eventually building up a natural levee. Natural levees provide some protection from flooding because with each flood the levee is built higher and therefore discharge must be higher for the next flood to occur.
Terraces are exposed former floodplain deposits that result when the stream begins down cutting into its flood plain (this is usually caused by regional uplift or by lowering the regional base level, such as a drop in sea level).
a steep mountain stream enters a flat valley, there is a sudden decrease
in gradient and velocity. Sediment transported in the stream will suddenly
become deposited along the valley walls in an alluvial fan. As the velocity
of the mountain stream slows it becomes choked with sediment and breaks up
into numerous distributary channels.
When a stream enters a standing body of water such as a lake or ocean, again there is a sudden decrease in velocity and the stream deposits its sediment in a deposit called a delta. Deltas build outward from the coastline, but will only survive if the ocean currents are not strong enough to remove the sediment. As the velocity of a stream decreases on entering the delta, the stream becomes choked with sediment and conditions become favorable to those of a braided stream channel, but instead of braiding, the stream breaks into many smaller streams called distributary streams.
Drainage Basins and Divides - Drainage systems develop in such a way as to efficiently move water off the land. Each stream in a drainage system drains a certain area, called a drainage basin. In a single drainage basin, all water falling in the basin drains into the same stream. Drainage basins can range in size from a few km2, for small streams, to extremely large areas, such as the Mississippi River drainage basin which covers about 40% of the contiguous United States . A divide separates each drainage basin from other drainage basins.
smallest streams in a drainage network have no tributary streams. These are
called first order streams. Two first order streams unite to form a second
order stream. Second order streams only have first-order streams as tributaries.
Third order streams only have second and first order streams as tributaries,
etc. As the order of the stream increases, the discharge increases, the gradient
decreases, the velocity increases, and the channel dimensions (width and
depth) increase to accommodate the increased discharge.
Drainages tend to develop along zones where rock type and structure are most easily eroded. Thus various types of drainage patterns develop in a region and these drainage patterns reflect the structure of the rock. Continental Divides - Continents can be divided into large drainage basins that empty into different ocean basins.
In Eastern Australia a coastal mountain range creates the Great Divide; rivers to the east flow into the Coral Sea and rivers to the west flow inland to the site of a sea that no longer exists or to the Gulf of Carpenteria.
In North America can be divided into several basins west of the Rocky Mountains that empty into the Pacific Ocean. Streams in the northern part of North America empty into the Arctic Ocean, and streams East of the Rocky Mountains empty into the Atlantic Ocean or Gulf of Mexico. Lines separating these major drainage basins are termed Continental Divides.
Such divides usually run along high mountain crests that formed recently enough that they have not been eroded. Thus major continental divides and the drainage patterns in the major basins reflect the recent geologic history of the continents.
Mass-wasting is the down-slope movement of Regolith (loose uncemented mixture of soil and rock particles that covers the Earth's surface) by the force of gravity without the aid of a transporting medium such as water, ice, or wind. Still, as we shall see, water plays a key role. Mass-wasting is part of a continuum of erosional processes between weathering and stream transport. Mass-wasting causes regolith to move down-slope where sooner or later the loose particles will be picked up by another transporting agent and eventually moved to a site of deposition such as an ocean basin or lake bed. In order for regolith to move in a mass wasting process it must be on a slope, since gravity will only cause motion if the material is on a slope.
is a force that acts everywhere on the Earth's surface, pulling everything
in a direction toward the center of the Earth. On a flat surface, parallel
to the Earth's surface the force of gravity acts downward. So long as the
material remains on the flat surface it will not move under the force of
gravity. On a slope, the force of gravity can be resolved into two components:
a component acting perpendicular to the slope, and a component acting tangential
to the slope.
The perpendicular component of gravity, gp, helps to hold the object in place on the slope.
The tangential component of gravity, gt, causes a shear stress parallel to
the slope and helps to move the object in the down-slope direction.
On a steeper slope, the shear stress or tangential component of gravity, gt, increases, and the perpendicular component of gravity, gp, decreases.
Another force resisting movement down the slope is grouped under the term shear strength and includes frictional resistance and cohesion among the particles that make up the object.
When the sheer stress becomes greater than the combination of forces holding the object on the slope, the object will move down-slope.
Thus, down-slope movement is favored by steeper slope angles (increasing the shear stress) and anything that reduces the shear strength (such as lowering the cohesion among the particles or lowering the frictional resistance.
Although water is not directly involved as the transporting medium in mass-wasting processes, it does play an important role. Think about building a sandcastle on the beach. If the sand is totally dry, it is impossible to build a pile of sand with a steep face like a castle wall. If the sand is somewhat wet, however, one can build a vertical wall. If the sand is too wet, then it flows like a fluid and cannot remain in position as a wall.
Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose. The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general, for dry materials the angle of repose increases with increasing grain size, but usually lies between about 30 and 37o.
Slightly wet unconsolidated materials exhibit a very high angle of repose
because surface tension between the water and the grains tends to hold the
grains in place.
When the material becomes saturated with water, the angle of repose is reduced to very small values and the material tends to flow like a fluid. This is because the water gets between the grains and eliminates grain to grain frictional contact.
The down-slope movement of material, whether it be bedrock, regolith, or a mixture of these, is commonly referred to as a landslide. All of these processes generally grade into one another, so classification of mass-wasting processes is somewhat difficult. We will use the classification used by your textbook, which divides mass wasting processes into two broad categories and further subdivides these categories.
A sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping.
Debris flows down hill mixed with water or air.
of slides wherein downward rotation of rock or regolith occurs along a curved
surface. The upper surface of each slump block remains relatively undisturbed,
as do the individual blocks. Slumps leave arcuate scars or depressions on
the hill slope. Heavy rains or earthquakes usually trigger slumps.
Rock falls occur when a piece of rock on a steep slope becomes dislodged and falls down the slope. Debris falls are similar, except they involve a mixture of soil, regolith, and rocks. A rock fall may be a single rock, or a mass of rocks, and the falling rocks can dislodge other rocks as they collide with the cliff. At the base of most cliffs is an accumulation of fallen material termed talus. The slope of the talus is controlled by the angle of repose for the size of the material. Since talus results from falling large rocks or masses of debris the angle of repose is usually greater than it would be for sand.
slides and debris slides result when rocks or debris slide down a pre-existing
surface, such as a bedding plane or joint surface. Piles of talus are common
at the base of a rock slide or debris slide.
Sediment flows occur when sufficient force is applied to rocks and regolith that they begin to flow down slope. A sediment flow is a mixture of rock, regolith with some water. They can be broken into two types depending on the amount of water present.
Slurry flows are sediment flows that contain between about 20 and 40% water.
As the water content increases above about 40% slurry flows grade into streams.
Granular flows are sediment flows that contain between 20 and 0% water. Note that granular flows are possible with little or no water. Fluid-like behavior is given these flows by mixing with air. Each of these classes of sediment flows can be further subdivided on the basis of the velocity at which flowage occurs.
Flowage at rates measured on the order of centimeters per year of regolith containing water. Solifluction produces distinctive lobes on hill slopes . These occur in areas where the soil remains saturated with water for long periods of time.
Devris flows occur at higher velocities than solifluction, and often result from heavy rains causing saturation of the soil and regolith with water. They sometimes start with slumps and then flow down hill forming to lobes with an irregular surface consisting of ridges and furrows.
A highly fluid, high velocity mixture of sediment and water that has a consistency of wet concrete. These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams. Thus after a heavy rain streams can turn into mudflows as they pick up more and more loose sediment. Mudflows can travel for long distances over gently sloping stream beds. Because of their high velocity and long distance of travel they are potentially very dangerous.
The very slow, usually continuous movement of regolith down slope. Creep occurs on almost all slopes, but the rates vary. Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles .
Earthflows are usually associated with heavy rains and move at velocities between several cm/yr and 110s of m/day. They usually remain active for long periods of time. They generally tend to be narrow tongue-like features that begin at a scarp or small cliff
Usually form in relatively dry material, such as a sand dune, on a steep slope. A small disturbance sends the dry unconsolidated grains moving rapidly down slope.
are very high velocity flows of large volume mixtures of rock and regolith
that result from complete collapse of a mountainous slope. They move down
slope and then can travel for considerable distances along relatively gentle
slopes. They are often triggered by earthquakes and volcanic eruptions.
Mass-wasting in cold climates is governed by the fact that water is frozen as ice during long periods of the year. Ice, although it is solid, does have the ability to flow, and freezing and thawing cycles can also contribute to movement. Frost Heaving - this process is large contributor to creep in cold climates. When water saturated soils freeze, they expand, pushing rocks and boulders on the surface upward perpendicular to the slope. When the soil thaws, the boulders move down vertically resulting in a net down slope movement.
Similar to solifluction, this process occurs when the upper layers of soil thaw during the warmer months resulting in water saturated soil that moves down slope.
A lobe of ice-cemented rock debris (mostly rocks with ice between the blocks) that slowly moves downhill Subaqueous Mass-Wasting Mass wasting processes also occur on steep slopes in the ocean basins. A slope failure can occur due to over-accumulation of sediment on slope or in a submarine canyon, or could occur as a result of a shock like an earthquake. Slumps, debris flows, and landslides are common.
A mass-wasting event can occur any time a slope becomes unstable. Sometimes, as in the case of creep or solifluction, the slope is unstable all of the time, and the process is continuous. But other times, triggering events can occur that cause a sudden instability to occur.
A sudden shock, such as an earthquake may trigger a slope instability. Minor shocks like heavy trucks rambling down the road, trees blowing in the wind, or man made explosions can also trigger mass-wasting events.
Modification of slope either by humans or by natural causes can result in changing the slope angle so that it is no longer at the angle of repose. A mass-wasting event can then restore the slope to its angle of repose.
eroding their banks or surf action along a coast can undercut a slope making
Heavy rains can saturate regolith reducing grain to grain contact and reducing the angle of repose, thus triggering a mass-wasting event.
produce shocks like explosions and earthquakes. They can also cause snow to melt or empty crater lakes, rapidly releasing large amounts of water that can be mixed with regolith to reduce grain to grain contact and result in debris flows, mudflows, and landslides.
These can be caused by rapid deposition of sediment that does not allow water trapped between grains to escape, or by generation of methane gas from the decay of organic material, which increases pressure between unconsolidated grains and thus reduces grain to grain contact.
Image courtesy of USGS
A rapid downslope movement of rock or soil as a more or less coherent mass.
Landslides are characterized by a slippage plane that is clearly defined.
A landslide may turn into a flow at the bottom as the blocks become tumbled
over. Usually the material moves as a large block known as a slump block.
The scar above a landslide is easily visible. Steep slopes of shale are susceptible
to landslide activity. But landslides occur everywhere on large or small
scales. They can occur after earthquakes or removal of part of the slope
due to construction, particularly in the construction of roads.
Diagram of the apparatus used to monitor Pore Water Pressure
Pore Water Pressure is the key to monitoring landslides. Pore water pressure is the pressure that develops as water fills in the pore spaces inbetween particles. Shear strength, a resisting force, decreases and the weight, a driving force, increases. The safety factor becomes less than one and a movement becomes possible.
Equipment such as this measures the Deep Pore Pressure and Shallow Pore Pressure. Data received from one of these monitors may look like this.
"Steady change in the extensometer output (vertical axis) indicates downslope
movement, brief changes that return to a constant value, such as the spikes
in the graph of E-4 (yellow), usually result from physical or electrical
disturbances and do not indicate movement."
Soil creep is a very, very slow form of mass wasting. It's just a slow adjustment of soil and rocks that is so hard to notice unless you can see the effects of the movement. These effects would be things like fenceposts shifted out of alignment, or telephone poles tipping downslope. Another effect is the way a grass covered slope seems to ooze downhill forming little bulges in the soil. This heaving of the soil occurs in regions subjected to freeze-thaw conditions. The freeze lifts particles of soil and rocks and when there is a thaw, the particles are set back down, but not in the same place as before.
Gravity always causes the rocks and soil to settle just a little farther downslope than where they started from. This is the slow movement that defines creep. Creep can also be seen in areas that experience a constant alternation of wetting and drying periods which work in the same way as the freeze/thaw.
Monitoring is essentially done through observation of the effects of creep. Since the process is so slow, it can only be monitored in terms of flow over long periods of time.
An ancient debris flow showing just how big they can be.
Debris flows are one of the most dangerous of all mass wasting events. They can occur suddenly and inundate entire towns in a matter of minutes. Debris flows are made of exactly what the name suggests: debris. This debris can include anything from the smallest mud particles to boulders, trees, cars, and parts of buildings. Debris flows occur when rain water begins to wash material from a slope or when water sheets off of a freshly burned stretch of land. Chapparral land is especially susceptible to debris flows after a fire. The rapidly moving water cascades down the slopes, and into the canyons and valleys below. It picks up speed and some debris as it descends the valley walls. In the valley itself, months of dry ravel, loose soil and rocks that have rolled or slid off the slope, begins to move with the water. As the system gradually picks up speed, the flow takes on the characteristics of a basic river system. The faster the water flow, the more the water can pick up. As the water picks up more mud and rocks, it begins to resemble a fast flowing river of concrete. This wall of debris can move so rapidly that it can pluck boulders the size of cars from the floors of the canyons and hurl them along the path of the flow. It's the speed and enormity of carried particles that makes a debris flow so dangerous. Boulders crash through homes and the mud-water mix fills in the rooms sometime totally overtaking the house.
Catch basin in British Columbia, Canada
Debris Chute in British Columbia, Canada
People have tried many methods for stopping or diverting debris flows. In California, catch basins have been constructed to "catch" the debris. Some basins have special overflow ducts with screens to remove the water from the flow and allow more room for the bigger items that may be washed in and take up needed space.
Debris flows happen so rapidly that there is really no way to monitor one until it is on top of you. Instruments in catch basins and flow channels can measure the rate or discharge of the flow by calculating the amount (volume) of material per unit of time (usually seconds).
Mud flows and Lahars are special forms of debris flows that are mainly made of the smallest mud and silt particles. Extremely heavy rain, or a sudden thaw can trigger these types of flows. In the case of lahars, a sudden thaw of mountain snow due to a volcanic eruption can send a torrent of mud, ash, and hot water down the slope of the volcano and over neighboring towns.As can be seen from this photograph of the unfortunate town of Armero, a lahar can overtake a town far from a volcano. This lahar rushed down stream and river valleys into the town and killed over 23,000 people. They had no warning. The town was quickly buried by mud that later, as rescuers attempted to find victims, dried and hardened like cement.
This map shows how the lahar found it's way to Armero and the extent of the flow.
In parts of Canada and Scandinavia, a special type of mudflow can occur. Marine mud at the margins of a receding glacier can have a property known as quick clay. There is a high water content in these marine muds due to their relatively low compaction. These clays can change into a viscous fluid with only the slightest disturbance. They become flows that can move very rapidly even on a slight grade. Since these flows also give no warning, they can be very destructive.
A major type of mass movement in cold polar regions and some high mountains. Solifluction is a special type of creep that occurs in areas of permafrost. Permafrost refers to the layer of groundwater that fills in the pore spaces of soil and rock that is permanently frozen. The permafrost layer can be anywhere from a meter to several hundred meters thick. It takes up about 20% of the world's land. In times of warm weather, the ground will begin to thaw from the surface downward. All of the freshly melted water cannot absorb into or move through the permafrost layer. This causes the upper layer of soil and regolith to become saturated and flow down the slightest of slopes as it slips over the frozen ground underneath.
Another visible aspect of solifluction areas is cryoturbation.
cryoturbation, "small ridges and mounds of bare soil are produced by
the processes of frost churning (cryoturbation) and soil flow (solifluction).
Freeze-thaw generates a circular motion in the surface material, heaving
the soil to the surface (the light- coloured areas) and dragging it down
at the margins to form gutters (the darker, vegetated areas). The process
creates a network of circular patches which, on slopes, are stretched into
long stripes by an additional creeping movement. Flowing water then deepens
the gutters." (Terrain Sciences Division)
Falls are usually the free-fall of pieces of rock from a mountain or cliff face. The size of the piece(s) can range from tiny grains to blocks weighing a ton or more. The debris and rock fragements from rock falls collect at the base of the slope as talus. This talus protects the base of the mountain from erosion. On mountains, ice wedging is the main contributor. As water from snowmelt finds its way into the cracks and joints of the rock face, it may refreeze and being to expand. This expansion widens the cracks in the rock. Over time, the cracks have been widened enough so that they are a point of structural weakness. Gravity takes over and the pieces of rock fall from the face of the mountain.
Rock slides can travel a long way from the source of the initial fall
Slides are rapid downslope movements of blocks along a bedding plane, joint,
or area of weakness. The blocks tend to break up into smaller pieces as the
slide moves downslope and large pieces collide with each other. These pieces
can travel a great distance due to the force of the falling rock. Road cuts
are susceptible to rock falls and slides when the base of a mountain is removed
for the roadbed. So to make the roads safer for motorists, some protective
barriers have been designed to catch the falling rocks.