Rivers and Landslides

Path of desire or path of least resistance? Desire Path: A term in landscape architecture used to describe a path that isn t designed but rather is worn casually away by people finding the shortest distance between two points. http://www.yourdailyawesome.com/

Running Water Running water is the major agent of erosion Stream networks dominate the Earth s surface Humid as well as arid environments Along with mountain ranges rivers are the most prevalent landform

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Hydrologic Cycle Hydrologic cycle - the movement and interchange of water between the sea, air, and land Evaporation Solar radiation provides energy Precipitation Rain or snow Transpiration Evaporation from plants Runoff Water flowing over land surface Infiltration Water soaking into the ground

River Systems A river or stream is any body of water that is flowing downhill in a well defined channel River systems consist of a main channel and all its tributaries

River terminology Headwaters - upper part of stream near its source in the mountains Mouth - place where a stream enters sea, lake or larger stream Channel - a long, narrow depression eroded by a stream into rock or sediment Stream banks - sides of channel Streambed - bottom of the channel Floodplain - flat valley floor composed of sediment deposited by the stream

River Systems Drainage basin The area of land that is drained by a certain river drainage system All the water in that area eventually finds its way into one of the rivers in the system Drainage divide Boundaries that separate one drainage basin from another

Drainage Basins Drainage basin - the total area drained by a stream and its tributaries Tributary - a small stream flowing into a larger one Divide - ridge or high ground that divides one drainage basin from another Continental Divide separates the streams that flow into the Pacific from those that flow into the Atlantic and Gulf of Mexico

Mississippi River Drainage Basin

Powder River Drainage Basin

Yellowstone River Drainage Basin

Missouri River Drainage Basin

Mississippi River Drainage Basin

River Systems May be divided into 3 subsystems Collecting system Transporting system Dispersing system

A conceptual river system

Dendritic = tree-like or root-like http://honolulu.hawaii.edu/distance/gg101/programs/program1%20introduction/usashade.jpg

Stream Flow Dynamics Factors affecting stream behavior Discharge (Q) Gradient Average velocity (V) Channel cross-sectional area (A) Load Base level

Stream Flow Dynamics Gradient How steep is the slope of the stream channel Change in elevation divided by the distance traveled (m/km) Longitudinal profile is a concave upward Steep at headwaters Nearly flat at discharge area

Stream Flow Dynamics Load Material carried along by the stream Suspended load (fine particles) or Bed load (coarse particles) Dissolved load (ions in solution)

Stream Flow Dynamics Suspended Load - fine particles carried long distances suspended in the stream water Carried along at approximately the same rate as the stream water velocity Usually the largest fraction of transported sediment

Stream Flow Dynamics Bed Load - particles moved along the stream bed by rolling, sliding, or saltation Saltation involves coarse particles jumping forward Bed load moves slower than water velocity Velocity controls the grain size of the bed load

Relationship between flow velocity & sediment transport

Stream Flow Dynamics Dissolved Load is a product of chemical weathering Primarily composed of seven common ions Cations - Ca +2, Na +1, Mg +2, K +1 Anions - HCO 3-1, SO 4-2, Cl -1 Streams that receive large amounts of groundwater have higher dissolved loads Streams that receive mostly surface run-off have lower dissolved loads

Stream Flow Dynamics Base Level - level to which a stream will erode Velocity of stream decreases as it approaches base level Energy to erode decreases with stream velocity Base level represented by large bodies of water Sea Level - ultimate base level Lakes & Reservoirs - local base level

Equilibrium adjustments in stream profile

Downcutting of Channels Bottom of stream channel abraded by transported sand & gravel May create steep narrow gorges or potholes May occur only during high flow due to size of particles

Headward Erosion Erosion rates are greatest where stream gradients are high Steep gradients increase erosion New channels develop at stream head Extends the network of collecting streams As a network expands, other streams may be captured - stream piracy

Headward erosion and stream capture by the Pecos River

Drainage Patterns Drainage pattern - the arrangement, in map view, of a stream and its tributaries Most tributaries join the main stream at an acute angle, forming a V or Y pointing downstream Dendritic - drainage pattern resembling the branches of a tree Radial pattern - streams diverge outward like the spokes of a wheel Typically form on conical mountains (volcanoes) Rectangular pattern - tributaries have frequent 90 bends and join other streams at right angles Trellis pattern - parallel streams with short tributaries meeting at right angles

Meandering Streams All rivers flow in a sinuous pattern Turbulent flow & velocity changes eroded one bank, deposit on the other Erosion changes course of river Deposition of point bars fills in old channel: meander cutoffs Oxbow lakes form in abandoned channel

Floodplains & meander evolution

The Big Sandy: a meandering stream

Deposition of sand bars, formation of cross bedding and Ox-bows (not related)

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Floodplains Floods Normal stage - water level below the bank Bankfull stage - water level even with the bank Flood stage - water level above the bank Described by recurrence intervals A 100-year flood is, on average, the size of the largest flood within a 100-year period of time Floodplains Areas that will be submerged when a river is at flood stage

Floodplains Rivers have associated floodplains Channels may meander across the floodplain Channel position changes with time River may contain multiple channels Unique geomorphic features develop as a result

A braided stream in its floodplain: Alaska

A braided stream in its floodplain: Laramie River

Oxbow lakes in a floodplain

Riparian foliage in a floodplain

Floodplain Features Natural levees Embankments at river s edge Flood waters flow over bank Rapid drop in velocity produces sand deposit Backswamps Low lying areas behind levees May be below river level Poorly drained

Urban flooding Paved areas and storm sewers increase runoff by inhibiting infiltration Rapid delivery of water to streams increases peak discharge and hastens occurrence of flood Flooding

Mississippi River flood: Iowa

Stream Valley Development Stream terraces Step-like landforms found above a stream and its floodplain Occurs when river rapidly cuts downward into its own floodplain Represents relatively sudden change in rate of erosion Can be caused by rapid uplift, drops in base level, changes in underlying lithology or climate changes

The Colorado: an incised meandering stream

The Colorado: downcutting due to tectonic uplift

Entrenched or incised river in its floodplain: Virgin River

Stream Valley Development Incised meanders Retain sinuous pattern as they cut vertically downward May be produced by profound base level changes, as when rapid tectonic uplift occurs

Sediment Deposition Sediments are temporarily deposited along stream course as bars and floodplain deposits, and at/near its end as deltas or alluvial fans Bars - ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a stream Braided streams contain sediment deposited as numerous bars around which water flows in highly interconnected rivulets

Stream Deposition Deposition occurs primarily in the transporting and dispersing systems Flow velocity is reduced allowing for deposition to create: Floodplains Deltas Alluvial valleys and fans

Braided Streams Multiple channel system Interlaced channels with islands High sediment load Common in arid & semi-arid regions Seasonal high flows Common in front of glaciers Large sediment load, fluctuating water flow

The North Platte River: A Braided Stream

A Braided River: typical of rivers just disgorging from mountainous areas. A rapid drop in current velocity and too much stuff to carry

Deltas Formed by sudden drop in velocity Stream enters ocean or lake Stream drops sediment load Evolve by three mechanisms Growth may be influenced by ocean waves and longshore currents

Delta Growth Distributaries Levee and in-stream deposit formation Channel splits into two smaller channels Splays Crevasse allow water to leave main channel Water spreads out forming a splay deposit Avulsion Shift in main course to follow steeper slope

Fig. 12.32. Historic flow of the Mississippi

The Ganges River Delta

Alluvial Fans Accumulation of sediment in a dry basin Deposition due to rapid velocity drop Usually arid climate High sediment load - braided streams Form fan-shaped deposit

Alluvial fans in Death Valley

Mass Movement: landslides, debris flows, avalanches, lahars, mudflows and creep

Types of Mass Movement Includes all types of slope failures Classified by Type of motion Type of material Rate of movement

Types of Mass Movement Creep Extremely slow movement of soil and regolith - 1 to 10 mm/yr Combination of factors cause creep Heaving of soil - expansion & contraction is the primary cause Wet-dry cycles Freeze-thaw cycles

Types of Mass Movement Creep Other factors adding to creep Plant root growth - displaces soil Burrowing animals - displaces and loosens soil Undercutting - streams, earthquakes, human activity

Types of Mass Movement Debris Flows Mixtures of water, mud and rock Liquified soil flows downhill Up to 50 km/hr Water lubricates mass of soil and rock Large boulders, building etc. may be carried by viscous fluid

Types of Mass Movement Lahars Volcanic debris flows Common is loose pyroclastic material Rain or melting snow saturates soil and rock Explosive eruptions and associated earthquakes can be triggers Very fast flowing - up to 150 km/hr

Types of Mass Movement Mudflows Debris flows consisting of slit and clay sized particles Usually associated with heavy rain High water content - up to 30% Usually follow stream valleys

Types of Mass Movement Landslides Mass movement along well defined slippage plane Landslide block moves as a single or group of units Rock type, orientation and water content influence events

Yellowstone River valley

DebrisFlow_Moscardo.mov A landslide A debris flow DebrisFlow_simualted.mov A flash flood Banda Aceh Tsunami Simulated debris flow: graded bedding http://www.searchles.com/links/show/youtube.com%2fwatch%3fv%3dyoqkplxjalk&feature%3dplaylist&p%3d9b1d99fb0c4dd21b&index%3d4