• fluvius (L.): river
• the work of rivers, but also the erosion of soil and rock on hillslopes by running water, particularly in semiarid environments (badlands)
• requires understanding of stream and hillslope hydrology and hydraulics
• hydrological cycle: orderly scheme to systematically examine and analyze the movement of water through the landscape
• flowing water is the result of net precipitation = total precipitation (input) - evapotranspiration (output or loss)

Erosional processes on slopes

1. raindrop impact
   
   
   • force of raindrops on bare soil causing disaggregation of surface soil
   
   
2. rainsplash erosion
   
   
   • displacement of wet soil by raindrops creating small craters in bare soil
   • on slopes the splash is asymmetrical resulting in the progressive downslope displacement of wet soil during intense rain on bare slopes
   
   
3. sheet wash (rain wash)
   
   
   • entrainment of loose particles in overland flow
     
     

     overland flow

     movement of water over slopes when precipitation intensity exceeds the soil infiltration capacity according to soil porosity and permeability, vegetation, slope gradient, antecedent moisture and seasonal factors (e.g. ice)

     
     
   • shallow overland flow (sheet flow) on smooth slopes is laminar (layered), so particles can only be displaced but not suspended
   • erosion is accentuated by raindrop impact, rainsplash erosion, surging of the overland flow as small vegetation or soil dams break, and by turbulence
   
   
4. rill erosion
   
   
   • overland flow deepens downslope, reaching a critical depth where laminar flow cannot be maintain and turbulence begins to develop
   • turbulent eddies suspend soil particles, creating rills
   
   
5. subsurface erosion
   
   

   piping

   formation of natural pipes as interflow and baseflow erode macropores and fractures in fine sediments

   sapping

   collapse of the roof of a pipe to form a gully

Fluvial landforms on slopes

• rill
  • shallow channels eroded by threads of turbulent flow developed in the sheet flow where turbulence and thus entrainment of soil is concentrated
  • during storms rills erode headward on the steepest local gradient at cm/minute or faster
  • on open slopes they tend to form parallel to one another, converging in hillside hollows to form dendritic patterns
  • ephemeral, that is, can be destroyed and recreated during major storms
  • terminate at the base of slopes and thus are not part of the regional drainage network
  
  
• gully
  • first-order stream channels that develop on slopes at the upper reaches of watersheds
  • carry ephemeral stream flow
  • narrow and steep sided
  • persist for years or decades, so more persistent than rills but still not "permanent" features
  • agricultural definition: farm machinery can pass through rills but not gullies

Stream Erosion and Sediment Transport

• streamflow accounts for 85-90% of total sediment transport to the ocean basins (glaciers account for 7%)
• 2-4% of the total potential energy of running water is converted to mechanical energy for geomorphic work

stream competence

• the maximum particle size transported
• increases with velocity because competence is a function of boundary (bed) shear stress
• the relationship between particle size and stream velocity is given by the Hjulstrom curves
  • critical erosional velocities determined for uniform bed materials and for average (versus bed) velocity
  • small particles are cohesive and thus have a high erosional velocity but remain in suspension in running water (low depositional threshold)
  • large particles are continuously transported and deposited because the erosion threshold is only slightly > the transport threshold
  • intermediate particle sizes (coarse silt/fine sand) and most easily eroded by running water

stream capacity

• the theoretical maximum mass of suspended sediment transported by a stream
• difficult to determine because a sediment laden stream is transitional to a debris flow
• increases with the 2-3rd power of discharge (i.e. faster than the increase of channel width or depth with discharge) as mass wasting and slope erosion in headwaters deliver sediment to tributary streams

types of sediment load

1. dissolved
   • sediment in solution (ionic)
   • transport requires no mechanical energy
   • reflects solubility of rocks in the watershed, rates of weathering and proportion of groundwater input versus softer surface water
   
   
2. traction (bedload)
   • coarse fraction that rolls and slides along bed or moves in long low paths by saltation
   • common where coarse materials are delivered to the channel with high velocities (flood flows or steep channels)
   
   
3. suspended
   • accounts for most stream sediment and most of the work performed by streams, because suspendable (fine) sediments are always available and all streams are capable of suspending fine sediment
   • measured and expressed as load (mass) or concentration (mass/unit volume)
   • concentration decreases downstream as the number of small tributaries (sources of much sediment) decrease and the proportion of baseflow (groundwater) increases

Fluvial Morphology

• the spatial expression of fluvial geomorphic processes as channel, network and basin morphologies

Channel geometry

• significant difference between bedrock (structurally controlled) versus alluvial(adjustable) channels

1. plan view
   
   
   1. meandering
      
      • sinuous single thread, the most stable and efficient channel geometry (least variable energy distribution) to conduct water and sediment over any surface (e.g. supraglacial streams
      • formed and maintained by erosion of banks and deposition on point bars
      
      
   2. braided
      
      • multiple thread, superimposed meandering channels as discharge and sediment load vary seasonally and diurnally, e.g. semiarid and proglacial streams
      • bars reforms during flood stage, deposition during falling stage that splits subsequent flow
      • different hydraulic geometry at different stages
      
      
   3. anatomosing
      
      • permanent multiple channels and mid-channel bars
      • channel width increases and depth decreases below a threshold for sediment transport and flow splits into deeper more narrow channels
      
      
   4. straight
      
      • either artificial or structurally controlled
      
      
2. longitudinal profile
   
   
   • concave upward, i.e. slope is an inverse exponential function of discharge (Q^0.5 to ^-1.0)
   • because flow is more efficient in larger channels, i.e. requires less slope to maintain velocity and sediment transport
   • thus in an influent stream, where Q decreases with distance downstream (e.g. an irrigation canal) slope must increase downstream to maintain flow
   • straight or convex segment of the long profile (e.g. bedrock outcrops, waterfalls, other knickpoints represent local deviations from the gross concave upward profile
   • along the profile potential energy (mgh) is converted to kinetic energy (1/2mv²)according to he rate of decrease in h (i.e. the slope) and increase in velocity, until PE reaches 0 at base level: sea level (ultimate base level) or a lake or trunk stream (local base level)
   
   
3. cross-sectional (hydraulic) geometry
   
   
   • power functions of discharge:
     
     • width = aQ^b, depth = cQ^f, velocity = kQ^m
     • for 20 stations in the central and SW US, for rising stage and units of feet (Leopold and Maddock, 1953, USGS professional paper 252):
       
       
       • b = .26 (i.e. 4th root of Q)
       • f = .4  (i.e. ~ square root of Q)
       • m = .34 (i.e. cube root of Q)
       
       
     • change downstream
       
       
       • b = .5
       • f = .4
       • m = .1
       
       
     • that is, width changes most rapidly and the increase in depth results in a slight increase in velocity (the first empirical evidence that velocity increases downstream)
     • note: Q = Av = wdv = aQ^b * cQ^f * kQ^m
       so Q = ackQ^b+f+m  or  b+f+m = 1
     • at flood stage m = 0, that is, there is no increase in velocity in the flooded reach (slope of stream is very small), so most of the increase in Q is increase in depth and especially width

Graded river (G.K. Gilbert)

• over geologic time, slope and channel characteristics adjust to provide, with the available discharge, just the velocity required to transport the sediment supplied from the basin
• analogous to a railway grade
• no excess, erosion or deposition, i.e. only to maintain the channel morphology
• a change in any of the controlling factors will cause a displacement of the system in a direction that will tend to absorb the effect of the change (dynamic equilibrium)
  
  
• independent factors
  
  
  • discharge, sediment from the basin and base level (potential energy)
  • determined by climate and geology, i.e. external to the fluvial system
  • a change in any one of these controlling factors results in an adjustment of the stream channel by degradation or aggradation towards a new longitudinal profile, also manifest in plan view by a change in meander pattern and locally in terms of cross-sectional (hydraulic) geometry
  
  
• semi-dependent factors
  
  
  • channel width, depth, roughness, velocity, pattern and load grain size
  • depend on the controlling factors but also some self-regulation (dependence on each other)
  
  
• dependent factors
  
  
  • slope of the water surface
  • the final adjustment, responds to the semi-dependent variable, cannot change abruptly like the other factors
  
  
• rate of adjustment
  
  
  • depends on resistance of the bed materials and amount of energy, i.e. mass (Q) and relief (base level)
  • thus fastest adjustments with large Q and adjustable materials
  • grade can exist locally in alluvial channels bounded by barriers such as resistant rock (waterfalls) or a landslide (e.g. Battle Creek valley)
  • grade extends to the entire profile as the barriers are eliminated

Network geometry

• all streams in adjustable materials will from a dendritic (tree-like, branching) network
• all other channel networks (radial, trellis, rectangular, distributary, annular) result from structural control
  
  
• deterministic explanation
  • represents the movement of water with the least expenditure of energy; least path length
  • governed by conservation of energy in an open system
  • acute junctions involve least rate of work (power) expended and therefore neither erosion (excess energy) of deposition (insufficient energy to transport the load)
  
  
• probabilistic explanation
  • branching networks are high probable random structures
  • they can be produced from a random walk model (using random number, dice or a bingo machine)
  • this, however, is only a explanation of the form and not the origin (e.g. headward erosion or progressive intersection of channels)

Basin morphology: fluvial landforms

river valley

• V-shaped while river degrades
• flat floor with aggradation
• most common landform in the world
• genetic classification (i.e. involves interpretation of drainage network evolution)
  
  

  consequent

  channel is the consequence of the initial topography and drainage; i.e. on a newly exposed (deglaciation) or created (tectonism) surface
  
  

  subsequent

  rivers in structurally-controlled valleys that evolve subsequent to the consequent drainage (e.g. annular drainage in an eroded structural dome)
  
  

  resequent (renewed consequent)

  in the same direction and usually parallel to the consequent drainage
  
  

  obsequent

  opposite to the consequent drainage
  
  

  antecedent

  preceded but not defeated by tectonism, e.g. a gorge eroded into a rising land mass
  
  

  superposed

  superimposed on underlying strata exposed by denudation, thus often not controlled by underlying structure because river course established according to structure of overlying strata

stream capture

• drainage progressively or abruptly diverted from one basin to another as a stream is beheaded by headward erosion
• scenarios:
  • progressive adjustment of drainage network to geologic structure exposed by erosion
  • parallel streams at different elevation and with different base levels (drainage captures by lower stream)
  • one stream has a structural advantage, e.g. degrading more rapidly in softer rock
  
  

  water gap

  gorge cut in an interfluve, become wind gaps as drainage is captured and streams incise to lower elevations

  elbow of capture

  a sharp change in direction reflecting capture of a low order stream, tends to have anonymously steep gradient

floodplain

• the surface of low relief developed on the alluvium adjacent to a stream
• becomes the stream bed during flood
• an efficient hydraulic geometry during flood stage (peak annual discharge); constant velocity and steep shallow and wide relative to the meandering channel
• floodplain features: point bars (lateral accretion), overbank sediments (horizontal accretion), levees, levee crevasses, splay deposit, meanders, neck cutoff, oxbow lake

terraces

• elevated sections of former floodplain
  
  

  unpaired terraces

  fragment of former floodplain “accidentally” preserved (e.g. by a rock buttress) as a meandering stream slowly degrades it floodplain
  
  

  paired terraces

  occur at same elevations on opposite valley sides; produced by intermittent downcutting with changes in Q, load or base level, i.e. the independent factors in the fluvial system

  
  
• on the Canadian plains, paired terraces usually reflect downcutting to a lower base level (drained glacial lake or the floor of meltwater channel) or response to climate change
• elsewhere paired terraces can be record of tectonism

alluvial fan

• segment of a low-angle cone with its apex at the mouth of a canyon
• convex in cross-section, slightly concave in long-profile
• as a stream leaves a canyon, drainage becomes distributary; these wider, shallower, lower-gradient streams have less transport capacity
• also water infiltrates the coarse bed materials, losing transport capacity
• debris flows may occur with increased sediment concentration
• adjacent alluvial fans coalesce to produce a peidmont plain at the base of mountain fronts

delta

• similar morphology to an alluvial fan but deposition results from sharp reduction in velocity as a stream enters standing water
• also tends to include finer sediments and turbidity currents