| Agent | Amount |
|---|---|
| 1011 kg/year | |
| rivers | 225 |
| glaciers | 20 |
| wind | 0.6 |
| groundwater | 4.8 |
| runoff | 320,000 |
- Mechanically weathered sediments, consisting of rock particles of various sizes, called clastic sediments.
- Names given to different size ranges (from big to small):
- boulders
- cobbles
- gravel 2 - 15 mm
- sand 0.06 - 2 mm
- silt 0.004 - 0.06 mm
- clay < 0.004 mm
- Transport processes by air and water often sort sediments according to size. (Glaciers do not generally sort sediments because of the high viscosity of the ice).
- Hjulstrom curve: Different combinations of particle size and river
velocity divided into three fields (Fig. 15.14):
- Transportation: material being moved by river
- Depositon: material falling out of river and collecting on bottom
- Erosion: material being scoured from bottom and added to river's sediment load
- Sand-sized particles most easily eroded. (Clay is cohesive because of electrostatic attraction between platy grains, and boulders are heavy).
- Types of transportation
- Dissolved load: material in solution
- Suspended load: particles held in water by turbulent (compare to laminar) flow (Fig. 15.6)
- Bed load: particles rolled along bottom or moving by saltation (Fig. 15.13)
- Ways of describing a river's ability to carry sediment:
- Competance: ability of stream to move large particles - depends on velocity
- Capacity: how much load a stream can carry - depends on discharge of stream
- Lower Mississippi: large capacity but small competance
- Effect of transport on sediments
- Rounding of grains
- Sorting by size: suspended clays take longer time to settle out than sands which are transported along the streambed.
- Floods may often carry the bulk of sediment - other times of year may carry very little sediment
- Graded profile: longitudinal profile - steeper at upper end (head), flatter at lower end (mouth)
- Base level: level at which water stops flowing due to lack of further
elevation change - ultimate base level is the ocean
- lakes and reservoirs provide local base levels to which a river must adjust
- deposition in reservoir; erosion below dam
- fastest flow near middle of stream in straight stretches
- fastest flow on outsides of bends - erosion of cut bank
- deposition on insides of bends - formation of point bars
- Depositon occurs when water velocity decreases:
- Bends in rivers may grow, due to faster erosion on outer bank, forming a meander. After flood, velocity of water in flood plain drops, river deposits sediments.
- Rivers emptying into still water (lake or ocean) form delta
- Cementation of grains by CaCO3 or SiO2
- Names of clastic rocks depend on size of grains (from big to small)
- Conglomerates - tend to be poorly sorted
- Sandstones - often well-sorted
- Shales - usually made of clay minerals
- Non-clastic rocks: from dissolved load
- Limestones - CaCO3, usually removed from water by a biological process (e.g. corals and sea-shells)
- Evaporites - NaCl and CaSO4 from evaporation of seawater in enclosed basins (e.g. Utah's Great Salt Lake and the Mid-East's Dead Sea).