Wasting
Mass Mass wasting: the down slope (hill) movement of rock, soil, or sediment under the influence of gravity. Mass wasting events occur in different ways and at different rates, depending on the types of materials involved (such as rock, soil or earth, mud, debris) and the motion involved (fall, slide, flow).
Factors that effect mass wasting include:
Nature of the materials (unconsolidated sediment-soil or consolidated rock) Steepness of slope Water: groundwater can create pressures that destabilize a slope, ALSO, flowing water saturates the soil, thus weakening the slope Vegetation Presence and orientation of planes of weakness (bedding, joints, metamorphic foliations (i.e. potential slide surfaces) heights Climate (in addition to precipitation, freeze-thaw cycles are important)
Some natural processes that over-steepen slopes include:
Stream erosion Wave erosion Tectonic uplift Volcanic activity
Landslide is the general term referring to all slides, flows (even falls) that occur at a moderately fast rate. They cost the United States nearly 6 billion dollars per year.
Humans by can cause landslides:
Deforestation Blasting and earthwork Vibrations from traffic Any man-made changes in water flow
Fall (Fastest)
EXAMPLES: prehistoric Blackhawk Slide, Frank (Alberta) slide in 1902 Air travels beneath the slide, allowing in to spread across massive distances Often leaves minor vegetation relatively undisturbed
Debris Flow (Fast)
EXAMPLES: Mount St. Helens, Glenwood Springs (Colorado), 1999 in Venezuela, and the 1992 Mount Mayuyama slide that created a tsunami Slope material becomes saturated and develops into an debris/mud flow Picks up rocks, soil, mud, trees, cars, houses, etc. Often mistaken for a flash floods because of the rapid and unannounced occurance Most dangerous debris flows accompany volcanoes
Avalanche (Fast)
EXAMPLE: Hauscaran Mountain avalanche in Yungay, Peru Ice and snow picks up debris, rapidly charging downhill
Slumps: (Slow-Fast)
EXAMPLE: The 1994 La Conchita Slide began as a slump Material moves as coherent mass along curved surfaces. This motion results in a slightly backward rotation of blocks. Note that the ground surfaces on the intermediate blocks dip slightly toward the master sole of the slump. The distal end (toe) of the slump moves along a less steep surface. Often the toe of slumps moves more as a flow than as a slide.
Earth Flow (Slower)
EXAMPLES: The 1994 La Conchita Slide (moderately fast), Slumgullion Earthflow (slow, long-lasting), Down-slope, viscous, flows of saturated materials Carried along by flow from within More water = higher velocity
Begin when the pore pressures in a fine-grained mass increase until enough of the weight of the material is supported by pore water to significantly decrease the internal shearing strength of the material. This thereby creates a bulging lobe, which advances with a slow, rolling motion. As these lobes spread out, drainage of the mass increases and the margins dry out, thereby lowering the overall velocity of the flow. This process causes the flow to thicken. The bulbous variety of earth flows are not that spectacular, but they are much more common than their rapid counterparts. They develop sag at their heads and are usually derived from the slumping at the source.
Creeps (Slowest)
The slowest rate of mass wasting processes Can be a "grain by grain" process which are facilitated by freeze-thaw cycles Small influences such as bugs and worms cause slight, net downhill movement Can be recognized by curves in the bottom of trees
Streams
Sea level can be considered the ultimate base level for all streams.
Most water falling on the land as precipitation either infiltrates into the ground to be stored as groundwater or runs off the surface into the ocean (or inland lakes). Some of the factors that influence the infiltration versus runoff include:
The permeability of the surficial material The slope of the land Amount and type of vegetation Recent amounts of precipitation (degree of saturation of the land)
Runoff
Occurs either as sheet flow, or it occurs in channels. Channelized runoff is referred to as a stream (anything from a small creek to the Amazon River).
Streams
Carry water and sediment. Deposit sediments or erode and pick up sediment. Relative importance or dominance of these two processes can vary along the length of a stream and locally with time as conditions change.
Capacity
The amount of sediment that a stream can transport (its depends on its discharge)
Discharge
The amount of water flowing past a point on a stream per unit time.
Competency
The largest particle a stream can transport; depends on its velocity. Streams transport some material in solution. Depending on the competency of a stream, sediment is carried either in suspension in the water column, or the sediment is dragged, rolls, or hops along the bottom (bed) of the stream channel.
The brown color of the stream to the left is due to the suspended load of mud and silt.
The stream in the photo at left is carrying a light load of suspended mud and probably moving fine sand as bedload. During certain times when the stream's velocity and discharge are greater, this stream can carry suspended sand and move the cobbles (seen in left fore ground) as bedload.
Velocity of Streams Depend on:
Stream gradient (slope of surface over which it flows) The size and shape of the channel Channel roughness
Discharge
In a straight stream having a channel with a symmetrical, U-shaped cross-section, the fastest current would be in the center of the stream, slightly below the water surface (close to the bed and channel walls, velocity is slowed by drag effects). But streams are never perfectly straight, nor symmetrical in cross-section, so within streams are currents of different velocities.
How do these characteristics affect velocity?
Because the stream velocity and discharge increases along the outside of the meander, a stream will begin to pick up more sediment, by eroding the bank and deepening the channel there. In cross section, a stream channel can become asymmetrical - deeper near the cut-bank and shallower near the point bar.
It is a common misconception that the velocities of streams with steep gradients are faster than velocities with lesser gradients, or that the velocity of the upstream reaches of a river, where its gradient is steep (think of the "long profile") is greater than its velocity downstream, near its mouth.
Other velocity controlling factors are important:
1. Down stream, a river generally has a wider, deeper, smoother channel, and a greater discharge. 2. It is carrying the waters supplied by tributaries. It is true, however, that along a given segment or reach of a stream, if its gradient is increased (such as by construction of a meander cutoff of comparable channel size), the velocity will be increased.
Point Bars
When the stream velocity on the inside of a meander slows down, this loss of energy results in the stream depositing some of the sediment it is carrying. This deposition begins to fill in the channel along the inside of the curve forming point bars (generally sand or gravel).
Meanders
Streams develop curves or bends called meanders. Water flowing a into meander tends to "pile up" on the outside of the meander. The stream velocity is higher there on the outside of the bend, than it is on the inside.
Cutoffs
Forms from abandoned meanders. The widest part of the stream channel is the cutoff. Often these abandoned parts of meanders contain water and are referred to as oxbow lakes.
Oxbow Lakes
An oxbow is a crescent-shaped lake lying alongside a winding river. It is created over time as erosion and deposits of soil change the river's course.
Rejuvenated Streams
Possess a complex and broad meander belt (a characteristic of stream close to base level). These features are evidence that the stream was once close to base level (when it developed the meanders across a broad flood plain) and then is brought far above their base level.
Now the stream is far above base level and therefore spending its energy down cutting into the rock. Rejuvenation can occur either by changes in base level (such as sea level changes between ice ages and warmer periods) or tectonic changes in the elevation of the land (uplift or subsidence).
Entrenched (incised) Meanders
The meanders that cut down into deep and steep V-shaped valleys are referred to as
River Terraces
Are valley walls that are what is left of the old flood plain of the stream. After the stream had evolved close enough to base level to have developed a wide floodplain.
Levees
Formed when a stream floods Thickest and coarsest sediments deposited at channel edges Thin and fine sediment deposited over outer parts of floodplain After many floods, the natural levees are built up enough to be visibly seen
The sketch above is a cross section of the valley of another rejuvenated stream.
Characteristics of streams far above base level
V-shaped valleys Steep valley walls Actively downcutting channel and valley Absent to narrow flood plain Fairly straight channels (no well developed meander belt) Steep stream gradients
Characteristics of streams near base level
They are contained in very broad valleys (often with only very low divided between tributaries) Very low stream gradients (nearly flat long profile) Broad and complex meander belts with such features as cutoffs and oxbow lakes Flood plains are many times wider than meander belt
Braided Streams
A braided stream has many different channels that merge together. Looks like a `braid'.
Things that reduce the infiltration capacity of the ground results in more runoff and greater flooding potential. Think of some natural and human activities that can increase the flooding potential of streams?
When excess discharge is present in a river or stream, at first the water moves more quickly and perhaps some erosion of the channel takes place. If discharge increases too rapidly, however, water must move out of the channel and out onto the surrounding area, known as the floodplain. The floodplain is the area that floods first Some obvious causes of floods are heavy rains, melting snow and ice, and frequent storms within a short time duration. The common practice of humans to build homes and towns near rivers and other bodies of water (i.e., within natural floodplains) has contributed to the disastrous consequences of floods.
Erosional and Depositional Landforms produced by streams
Weathering, erosion, and deposition are a main force behind landscape types. They are not the only reason for a plain, plateau, and mountain region to form. Other factors such as glaciation and tectonic activity lead to landscape building. The water cycle or hydrologic cycle is the driving force behind W.E.D (weathering, erosion, and deposition)
River Delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of sediment carried by the river as the flow leaves the mouth of the river
Alluvial Fan
A fan-shaped accumulation of alluvium deposited at the mouth of a ravine or at the juncture of a tributary stream with the main stream.
Pediment
Occurs at the base of a mountain or as a plain having no associated mountain. Pediments, sometimes are mistaken for groups of merged alluvial fans, and are most conspicuous in basin-and-range-type desert areas throughout the world.
Peneplain
Is a low-relief plain representing the final stage of fluvial erosion during times of extended tectonic stability.
Terrace
A terrace is a geological term for a step-like landform that borders a shoreline or river floodplain and represents the former position of either a floodplain or the shoreline of a lake, sea, or ocean.
Lag time
The time between the peak rainfall and the peak water discharge from the river
Headward Erosion
Erosion caused by water flowing at the head of a valley.
Base Level
The lowest level to which a land surface can be reduced by the action of running water
Graded Stream
Is a system in equilibrium, one in which there exists a balance of the erosional and depositional processes. It is a stream that is transporting just that amount of sediment it is capable of, for its velocity and discharge. While a graded stream might be both picking up new sediment by erosion and depositing sediment, there is no net erosion or net deposition.
It might be argued that a graded stream is a concept, and that a truely graded stream does not exist, it is a concept that is certainly approached in nature, and that one that is powerful in predicting stream behavior. Indeed accepting the premise that all streams are either graded or are striving toward that condition, will provide you with a way anticipate results of external stimuli on a stream. Consider what factors influence a streams capacity to transport sediment (and pick them up by erosion) and its deposition of sediment. Obviously stream velocity is one important factor. If something is done to affect a stream, you should be able to reason how that might affect the stream velocity (since you know what determines the velocity (gradient, discharge, channel characteristics)) and therefore make a logical conclusion regarding how the stream will adjust in order to restore a graded condition (e.g. will it tend to erode and pick up more sediment, or will it begin to deposit sediment?)
Groundwater
Precipitation - evaporation - infiltration - runoff: what factors are important in determining whether rain will infiltrate and become part of the ground water, or runoff as sheet or channel flow? terms to know and understand: porosity, permeability, hydraulic gradient, hydraulic head, aquifer, aquiclude, artesian, water table, zone of saturation, drawdown, cone of depression, recharge, discharge. Understand Darcy's law - i.e. what controls the velocity of ground water flow, and understand factors that effect the rate of recharge of an aquifer. With this knowledge you can understand the relationship between topography and the geometry of the water table, how the table is effected by drawdown, and what might be its recovery rate. Be familar with those ground water problems that face some areas of the country and world; problems such as depletion, contamination, saltwater intrusion, subsidence (and land rising) Karst features such as sinkholes and caverns, hydrothermal waters