River Series 3 Flashcards
In topographically low parts of the landscape ______, ________, and _________ sediments are deposited
valleys, pidemonts, coastal plains
Fluvial sediments are laid down, and depositional landforms are created at:
- Dominantly or partially subaerial conditions
- Topographic discontinuities
- Valleys and the margin of water bodies
Restricted lateral movement and confined within the valley
upstream channel
Channels are able to shift positions over wide areas
Near baselevel
Arid to Semi-Arid Environments
Large Sediment Supply
Occurs in Fault fronts, Tectonic Mountain Fronts or Sudden Topographic Discontinuities
General gradient: 3-6°
Marginal screes: ~30°
Near the apex: ~10°
Alluvial Fans
Upper part is incised passing outward to braided streams. Generally poorly sorted sediments near the head due to torrential deposition. It becomes better sorted sediments outwards with finer grains
Fan structure
Conglomerates/unsorted sediments near the head, with bettering sorting and finer-ing of sediments outwards.
Sediment mechanics of alluvial fans
Exhibits complex cross-bedding and Flow Structures
Bedding
an size should be related to the size of the sediment source area AKA drainage basin.
Growth of fans
Have a smaller fan, compared to fans underlain by mudstone and shale
Geology influences the relation, in a drainage basin e.g underlain by sandstone
Fan surface is not dissected. Relatively smaller than fans of perennial streams
Dry fans
Streams flows through incision. Coarser materials are rarely carried downstream. River widens downstream, braided. Progressively shifting channel
Wet fans
Where alluvial fans are large enough, or sufficiently closely spaced, they may coalesce laterally to produce a depositional bed. To produce a __________.
Peidmont zone or Bajada
Mainly are terraces and floodplains of a river. Normally finer upward. Channels are mainly braided to meandering though some other rivers
Valley fills
Change in positive baselevel change: resulting into the backfilling of the
Uplift or Change in climate: resulting into increased sedimentation, sediment load progressively moves down the system
Uniform filling of the valley: increase in production of sediments from the numerous tributaries
Valley Filling
Enumerate the channel types in valley fills
Meandering
Anastomosing
Braided
Straight
Sinuous Channels
Near Baselevel
Low Gradient/Flat
Fine Sediments (Mainly suspended loads and dissolved loads)
Shifts laterally
Meandering channels
Low Bedload discharge
Low sinuousity
Low bedload unless due to incision
Straight
Relatively permanent
Cohesive banks, Sinuous channels
Diverges/Converges around stable, vegetated bars
Anastomosing stream
High sediment supply
High Gradient changes
Low stability
Braided
Enumerate the causes of braiding
Overloading of sediments discharge
Steep slopes which produce a wide shallow channel where bars and islands form readily
Easily erodible banks which permit widening at high flow
Large bed material load, in comparison with its suspended load where large material is immobile except during high flood stages
Alluvial surface adjacent to a channel that is frequently inundated. Highest elevations normally occurs on the abandoned levee and point bar ridges
Floodplains
Occasionally be formed by the incision of bedrock surfaces, but they are most likely the remnants of floodplains trenched by rivers. Caused by negative baselevel changes- resulting into headward surges of rejuvination
River terraces
______ may represent reworking and may contain heavy mineral concentrations
Terrace sediments
Formed when sediment-laden rivers flow into standing bodies of water.
Deltas
Geomorphic Characteristics of deltas are determined by the following factors:
Relative densities of the river water and standing water
River hydrology and variations of discharge
The amount and caliber of river load esp. bed load and suspended load
Intensity of coastal processes, particularly: wave action, longshore currents and tidal scour
Geometry of the slope
Vertical tectonic stability
Climate
The dynamics of the delta morphology is strongly influenced by the relative densities of the inflowing and standing waters
Delta morphology
Carries large proportions of fine suspended load
Usually occurs to the snout of tide water glacier
Submarine slumps produce turbidity currents: Graded beddings
Hyperpycnal
Typically is when rivers flow into freshwater lakes.
Devoid of significant tidal or wave forces
Bedload settles immediately
Homopycnal
Rivers flow into saltwater (most common)
Coarse bedload is deposited near the mouth as leeves
Fine suspended load carried seaward in the surface which spreads over the sea
Produces gently sloping submarine delta front
Hypopycnal
Floodplain sands and silts; marsh organic deposits; platform sands.
Topset
Cross-bedded coarser sands and silts of the delta slope, together with channel fingers- grading laterally into clay and marsh deposits and sand splays
Foreset
Offshore clays, toeslope turbidity silt deposits
Bottomset
Deltas are dominated by the removal of river debris by wave or tidal currents
Wave Influenced/Dominated
Tide Influenced/Dominated
High destructive
Deltas are dominated by large debris supply from the river
Lobate/Fan
Elongate/Bird’s foot
High constructive
Flattened in a pan
Rapid removal of debris by strong longshore currents
Wave dominated
Funnel Shaped distributaries
Open and straight distributaries
Tide dominated
Much of the debris is coarse
Distributaries change in position similar to alluvial fans
Compact distributaries
Lobate fans
Delivers large quantity of fine sediment
Builds out
Constantly shifting
Elongate/ Birds foot delta
